OmaPhoto 1.0.0: the Linux port of Compositor

A layered image editor for Linux and Omarchy, ported from Compositor
(github.com/robbietilton/Compositor), whose Swift source is its
specification. It reads and writes Compositor's .comp projects.

Packages for Ubuntu, Fedora and Arch come from the release workflow;
scripts/install/ holds one install script per distribution family,
and flake.nix serves NixOS.

Co-Authored-By: Claude <noreply@anthropic.com>

src/Document/AdjustmentEditing+Session.h

@@ -0,0 +1,12 @@
+// Swift's AdjustmentEditing extension: the colour editors write layer settings.
+public:
+    // Draws what lies beneath off the UI thread; `done` follows.
+    void beginAdjustmentEditing(QUuid id, std::function<void()> done = {});
+    // Writes the edited settings, or the original with Preview off.
+    bool previewAdjustmentEditing(bool preview);
+    // OK keeps the edited settings; Cancel puts the original back.
+    bool finishAdjustmentEditing(bool commit);
+
+private:
+    LayerAdjustment editedAdjustment() const;
+    void openAdjustmentEditor(QUuid id, const LayerAdjustment &original, const AdjustmentInput &made);

src/Document/AdjustmentEditing.cpp

@@ -0,0 +1,147 @@
+#include "Document/EditorSession.h"
+#include "Document/LayerGroups.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include <QtConcurrent>
+
+// Swift's AdjustmentEditing.swift: pixels beneath feed histogram and sampling.
+
+void EditorSession::beginAdjustmentEditing(QUuid id, std::function<void()> done)
+{
+    const int index = m_document ? indexOf(m_document->layers, id) : -1;
+    if (m_adjustmentEditingID != id || m_adjustmentOriginal || m_levels || m_hueSaturation || m_filterEdit || index < 0
+        || !m_document->layers[size_t(index)].adjustment) {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+        return;
+    }
+    const LayerAdjustment original = *m_document->layers[size_t(index)].adjustment;
+    ProjectSnapshot source = projectSnapshot().value();
+    // Records stay for live masks and folders; beneath alone shows.
+    QSet<QUuid> underneath;
+    for (const LayerHierarchy::Entry &entry : LayerHierarchy::entries(source.manifest.layers)) {
+        if (entry.layer.id == id)
+            break;
+        underneath.insert(entry.layer.id);
+    }
+    for (ProjectLayerRecord &layer : source.manifest.layers) {
+        if (!underneath.contains(layer.id))
+            layer.isVisible = false;
+    }
+    // A watcher per call, as Swift's calls are tasks.
+    auto *watcher = new QFutureWatcher<AdjustmentInput>(this);
+    connect(watcher, &QFutureWatcherBase::finished, this, [this, watcher, id, original, done = std::move(done)] {
+        watcher->deleteLater();
+        openAdjustmentEditor(id, original, watcher->result());
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    });
+    // A painted layer flattens there, where a failure is caught.
+    watcher->setFuture(QtConcurrent::run([source]() -> AdjustmentInput {
+        try {
+            const QImage image = ImageExporter::render(source).image;
+            return {ImportedImage(image, PixelAdjust::thumbnail(image), QStringLiteral("Adjustment input")), std::nullopt};
+        } catch (const ExportError &error) {
+            return {std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::openAdjustmentEditor(QUuid id, const LayerAdjustment &original, const AdjustmentInput &made)
+{
+    // Swift's guard after the await; an open editor wins.
+    if (m_adjustmentEditingID != id || m_adjustmentOriginal)
+        return;
+    std::optional<QString> failure = made.failure;
+    // Beyond Swift: an edit begun meanwhile keeps its editor.
+    if (!failure && (m_levels || m_hueSaturation || m_filterEdit)) {
+        qCWarning(lcApp) << "an adjustment's editor gave way to an edit begun meanwhile";
+        m_adjustmentEditingID.reset();
+        resumeFileRequests();
+        notify();
+        return;
+    }
+    if (!failure) {
+        try {
+            const ImageLayer layer(made.asset.value(), QPointF(0, 0));
+            switch (original.kind) {
+            case AdjustmentKind::levels:
+                m_levels.emplace(layer, std::nullopt);
+                m_levels->settings = original.levels;
+                countLevelsHistogram();
+                break;
+            case AdjustmentKind::hsv:
+                m_hueSaturation.emplace(layer.id, made.asset.value(), std::nullopt, layer.transform);
+                m_hueSaturation->settings = original.resolvedHSV();
+                break;
+            case AdjustmentKind::curves:
+            case AdjustmentKind::exposure:
+            case AdjustmentKind::gradientMap:
+            case AdjustmentKind::grain: {
+                FilterSettings settings;
+                settings.curves = original.curves;
+                settings.exposure = original.exposure();
+                settings.gradientMap = original.gradientMap();
+                settings.grain = original.grain();
+                m_filterEdit.emplace(filterKind(original.kind).value(), layer, std::nullopt, settings, std::nullopt);
+                break;
+            }
+            }
+        } catch (const ExportError &error) {
+            failure = QString::fromUtf8(error.what());
+        }
+    }
+    if (failure) {
+        m_adjustmentEditingID.reset();
+        resumeFileRequests();
+        setBrushError(failure);
+        return;
+    }
+    m_adjustmentOriginal = original;
+    beginEdit(QStringLiteral("Edit %1 Adjustment").arg(rawValue(original.kind)));
+}
+
+// The kind's edit lives as long as the original does.
+LayerAdjustment EditorSession::editedAdjustment() const
+{
+    LayerAdjustment value = m_adjustmentOriginal.value();
+    switch (value.kind) {
+    case AdjustmentKind::levels: value.levels = m_levels.value().settings; break;
+    case AdjustmentKind::hsv: value.hsvSettings = m_hueSaturation.value().settings; break;
+    case AdjustmentKind::curves: value.curves = m_filterEdit.value().settings.curves; break;
+    case AdjustmentKind::exposure: value.setExposure(m_filterEdit.value().settings.exposure); break;
+    case AdjustmentKind::gradientMap: value.setGradientMap(m_filterEdit.value().settings.gradientMap); break;
+    case AdjustmentKind::grain: value.setGrain(m_filterEdit.value().settings.grain); break;
+    }
+    return value;
+}
+
+bool EditorSession::previewAdjustmentEditing(bool preview)
+{
+    if (!m_adjustmentEditingID || !m_adjustmentOriginal)
+        return false;
+    updateAdjustment(*m_adjustmentEditingID, preview ? editedAdjustment() : *m_adjustmentOriginal);
+    return true;
+}
+
+bool EditorSession::finishAdjustmentEditing(bool commit)
+{
+    if (!m_adjustmentEditingID || !m_adjustmentOriginal)
+        return false;
+    updateAdjustment(*m_adjustmentEditingID, commit ? editedAdjustment() : *m_adjustmentOriginal);
+    m_hueSampleMode.reset();
+    m_hueTargeting = false;
+    m_hueTargetDrag.reset();
+    m_levels.reset();
+    m_hueSaturation.reset();
+    m_filterEdit.reset();
+    endEdit();
+    m_adjustmentOriginal.reset();
+    m_adjustmentEditingID.reset();
+    resumeFileRequests();
+    ++m_canvasFocusRequest;
+    ++m_brushRevision;
+    notify();
+    return true;
+}

src/Document/BlurTool+Session.h

@@ -0,0 +1,4 @@
+// Swift's BlurTool extension: what a Blur stroke paints.
+public:
+    // The active layer, or its mask, softened at document size.
+    std::optional<QImage> blurSample(const CanvasDocument &document, bool mask = false) const;

src/Document/BlurTool.cpp

@@ -0,0 +1,52 @@
+#include "Document/EditorSession.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/LayerRenderer.h"
+#include <cmath>
+
+// Swift's BlurTool extension: the layer, or its mask, softened.
+std::optional<QImage> EditorSession::blurSample(const CanvasDocument &document, bool mask) const
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    if (!layer)
+        return std::nullopt;
+    // The softening follows the brush's size.
+    const double sigma = std::min(30.0, std::max(1.5, m_brushSettings.diameter / 10));
+    try {
+        if (mask) {
+            if (!layer->mask)
+                return std::nullopt;
+            QImage context = BrushRaster::context(document.width, document.height, true);
+            // Past its pixels a mask keeps its edge tone.
+            const int tone = int(std::round(LayerMask::background(layer->mask->asset.thumbnail) * 255));
+            context.fill(QColor(tone, tone, tone));
+            QPainter painter(&context);
+            // Coverage only adds white: the mask's area starts black.
+            const LayerTransform placement = layer->maskTransform();
+            painter.save();
+            QTransform box;
+            box.translate(placement.center().x(), placement.center().y());
+            box.rotateRadians(placement.radians());
+            painter.setTransform(box, true);
+            painter.fillRect(QRectF(-placement.size.width() / 2, -placement.size.height() / 2, placement.size.width(), placement.size.height()),
+                             Qt::black);
+            painter.restore();
+            LayerRenderer::drawCoverage(layer->mask->asset.image(), placement, painter);
+            painter.end();
+            return PixelAdjust::gaussianBlur(context, sigma, true);
+        }
+        if (!layer->asset)
+            return std::nullopt;
+        QImage context = BrushRaster::context(document.width, document.height, false);
+        QPainter painter(&context);
+        // A painted asset flattens here; the catch covers it too.
+        const LayerTransform transform = displayedTransform(*layer);
+        LayerRenderer::draw(layer->asset->image(), transform, transform.center(), painter, {});
+        painter.end();
+        return PixelAdjust::gaussianBlur(context, sigma, false);
+    } catch (const ExportError &error) {
+        qCWarning(lcApp) << "the Blur tool cannot sample the layer:" << error.what();
+        return std::nullopt;
+    }
+}

src/Document/BrushStroke+Commit.cpp

@@ -0,0 +1,180 @@
+#include "Document/BrushStroke.h"
+#include "Document/LayerMask.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include "Rendering/RasterSnapshot.h"
+#include <QRandomGenerator>
+#include <cstring>
+#include <new>
+
+extern "C" {
+#include "BrushPixels.h"
+#include "HealPixels.h"
+}
+
+// The stroke as a layer: bounds, placement, the commit's image.
+namespace {
+QRectF integral(const QRectF &rect)
+{
+    return QRectF(QPointF(std::floor(rect.left()), std::floor(rect.top())), QPointF(std::ceil(rect.right()), std::ceil(rect.bottom())));
+}
+
+// Half-open bounds of the pixels that are there, or empty.
+QRectF alphaBounds(const QImage &pixels)
+{
+    size_t edges[4] = {0, 0, 0, 0};
+    brush_alpha_bounds(pixels.constBits(), size_t(pixels.width()), size_t(pixels.height()), size_t(pixels.bytesPerLine()), edges);
+    if (edges[2] <= edges[0] || edges[3] <= edges[1])
+        return QRectF();
+    return QRectF(edges[0], edges[1], edges[2] - edges[0], edges[3] - edges[1]);
+}
+
+void drawSource(const ImportedImage &source, const QRectF &rect, QPainter &painter)
+{
+    if (source.raster)
+        source.raster->draw(rect, painter);
+    else
+        BrushRaster::draw(source.image(), rect, painter);
+}
+}
+
+QRectF BrushStroke::committedBounds() const
+{
+    return integral(m_allocatedBounds.value_or(sourceRect));
+}
+
+LayerTransform BrushStroke::committedTransform() const
+{
+    return transform(committedBounds());
+}
+
+LayerTransform BrushStroke::transform(const QRectF &bounds) const
+{
+    const QPointF center = pixelToDocument.map(bounds.center());
+    LayerTransform result = paintTransform;
+    result.size = {bounds.width() * paintTransform.size.width() / width, bounds.height() * paintTransform.size.height() / height};
+    result.origin = {center.x() - result.size.width() / 2, center.y() - result.size.height() / 2};
+    return result;
+}
+
+// Swift's heal: the painted spot rebuilt from nearby texture.
+void BrushStroke::heal()
+{
+    if (!settings.healing || isMask)
+        return;
+    std::optional<QRectF> painted;
+    for (const auto &[key, coverage] : m_coverage) {
+        long edges[4];
+        heal_coverage_bounds(coverage.constBits(), size_t(coverage.width()), size_t(coverage.height()), size_t(coverage.bytesPerLine()), edges);
+        if (edges[2] <= edges[0] || edges[3] <= edges[1])
+            continue;
+        const QRectF rect = QRectF(edges[0], edges[1], edges[2] - edges[0], edges[3] - edges[1]).translated(m_tiles.at(key).rect.topLeft());
+        painted = painted ? painted->united(rect) : rect;
+    }
+    if (!painted)
+        return;
+    // Room for the kernel's search, about three spots away.
+    const double reach = (std::max(painted->width(), painted->height()) + 32) * 3.2;
+    const QRect region = painted->adjusted(-reach, -reach, reach, reach).intersected(QRectF(0, 0, width, height)).toAlignedRect();
+    QImage pixels = BrushRaster::context(region.width(), region.height(), false);
+    {
+        QPainter painter(&pixels);
+        const QRectF placed = sourceRect.translated(-region.topLeft());
+        if (m_source && m_source->raster)
+            m_source->raster->draw(placed, painter);
+        else if (m_source)
+            BrushRaster::draw(m_source->image(), placed, painter);
+    }
+    // The kernel reads coverage rows packed, as Swift's contexts are.
+    const std::unique_ptr<uint8_t[]> painting(new (std::nothrow) uint8_t[size_t(region.width()) * size_t(region.height())]());
+    if (!painting)
+        throw ExportError(ExportError::Kind::render);
+    for (const auto &[key, coverage] : m_coverage) {
+        const QRect tile = m_tiles.at(key).rect.toRect().translated(-region.topLeft());
+        const QRect within = tile.intersected(QRect(QPoint(0, 0), region.size()));
+        for (int y = within.top(); y <= within.bottom(); ++y)
+            std::memcpy(painting.get() + size_t(y) * size_t(region.width()) + size_t(within.left()),
+                        coverage.constScanLine(y - tile.top()) + (within.left() - tile.left()), size_t(within.width()));
+    }
+    if (spot_heal(pixels.bits(), painting.get(), size_t(region.width()), size_t(region.height()), size_t(pixels.bytesPerLine()),
+                  float(settings.opacity), int(settings.healingMode), QRandomGenerator::global()->generate()) != 0)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    for (const auto &[key, coverage] : m_coverage) {
+        Tile &tile = m_tiles.at(key);
+        QImage healed = tile.base;
+        healed.detach();
+        if (healed.isNull())
+            throw ExportError(ExportError::Kind::render);
+        {
+            QPainter painter(&healed);
+            BrushRaster::draw(pixels, QRectF(region).translated(-tile.rect.topLeft()), painter);
+        }
+        // Swift clips the context; the selection blends here instead.
+        tile.context = selectionClip ? PixelAdjust::blend(healed, tile.base, *selectionClip,
+                                                          QTransform::fromTranslate(tile.rect.left(), tile.rect.top()) * pixelToDocument, false)
+                                     : healed;
+        tile.image = tile.context;
+    }
+}
+
+// Painting only adds alpha: the touched tiles alone are inspected.
+PaintSnapshot BrushStroke::paintSnapshot() const
+{
+    std::optional<QRectF> bounds = m_source ? std::optional(sourceRect) : std::nullopt;
+    for (const auto &[key, tile] : m_tiles) {
+        if (isMask)
+            break;
+        const QRectF rect = alphaBounds(tile.context);
+        if (rect.isEmpty())
+            continue;
+        const QRectF placed = rect.translated(tile.rect.topLeft());
+        bounds = bounds ? std::optional(bounds->united(placed)) : std::optional(placed);
+    }
+    const QRectF crop = bounds.value_or(committedBounds());
+    const std::shared_ptr<const RasterSnapshot> raster = RasterSnapshot::replacing(m_source, sourceRect, patches(), crop, isMask);
+    return {ImportedImage(raster, raster->thumbnail(), layer.name), transform(crop), crop};
+}
+
+BrushCommit::Input BrushStroke::commitInput() const
+{
+    const QRectF bounds = committedBounds();
+    std::vector<BrushPatch> shifted;
+    for (const BrushPatch &patch : patches())
+        shifted.push_back({patch.rect.translated(-bounds.topLeft()), patch.image});
+    return {int(bounds.width()), int(bounds.height()), m_source, shifted, isMask, layer.name, sourceRect.translated(-bounds.topLeft())};
+}
+
+ImportedImage BrushCommit::expandMask(const ImportedImage &asset, const Input &input, const QRectF &crop)
+{
+    if (input.sourceRect == crop)
+        return asset;
+    QImage context = BrushRaster::context(int(crop.width()), int(crop.height()), true);
+    // New canvas area has no mask; old coverage stays aligned.
+    context.fill(Qt::white);
+    QPainter painter(&context);
+    drawSource(asset, input.sourceRect.translated(-crop.topLeft()), painter);
+    painter.end();
+    return LayerMask::assetFrom(context);
+}
+
+BrushCommit::Output BrushCommit::render(const Input &input)
+{
+    QImage context = BrushRaster::context(input.width, input.height, input.mask);
+    QPainter painter(&context);
+    if (input.source)
+        drawSource(*input.source, input.sourceRect, painter);
+    for (const BrushPatch &patch : input.patches)
+        BrushRaster::draw(patch.image, patch.rect, painter);
+    painter.end();
+    const QRectF fullBounds(0, 0, input.width, input.height);
+    if (input.mask)
+        return {LayerMask::assetFrom(context), fullBounds};
+    // Every pixel with alpha stays, the soft rim included.
+    const QRectF found = alphaBounds(context);
+    const QRectF crop = found.isEmpty() ? fullBounds : found;
+    const QImage image = crop == fullBounds ? context : context.copy(crop.toRect());
+    if (image.isNull())
+        throw ExportError(ExportError::Kind::render);
+    return {ImportedImage(image, PixelAdjust::thumbnail(image), input.name), crop};
+}

src/Document/BrushStroke+Stroke.cpp

@@ -0,0 +1,466 @@
+#include "Document/BrushStroke.h"
+#include "Rendering/PoolMap.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include "Rendering/RasterSnapshot.h"
+#include <QtConcurrent>
+#include <cmath>
+#include <functional>
+#include <stdexcept>
+
+// The stroke's geometry, samples, tiles and coverage.
+namespace {
+QRectF integral(const QRectF &rect)
+{
+    return QRectF(QPointF(std::floor(rect.left()), std::floor(rect.top())), QPointF(std::ceil(rect.right()), std::ceil(rect.bottom())));
+}
+
+// The clone through coverage and a clip: over, or copied.
+void fillClone(const QImage &sample, const QImage &coverage, double alpha, QImage &context, const QImage &clip, bool replaces)
+{
+    const bool gray = context.format() == QImage::Format_Grayscale8;
+    if (sample.format() != QImage::Format_RGBA8888_Premultiplied || (gray && replaces))
+        throw std::logic_error("a clone samples premultiplied colour, and a mask never copies");
+    const int channels = gray ? 1 : 4;
+    for (int y = 0; y < coverage.height(); ++y) {
+        const uchar *mask = coverage.constScanLine(y), *source = sample.constScanLine(y);
+        const uchar *within = clip.isNull() ? nullptr : clip.constScanLine(y);
+        uchar *row = context.scanLine(y);
+        for (int x = 0; x < coverage.width(); ++x) {
+            const double k = mask[x] / 255.0 * alpha * (within ? within[x] / 255.0 : 1.0);
+            // Copying replaces by coverage alone, so clear clears.
+            const double m = mask[x] / 255.0 * (within ? within[x] / 255.0 : 1.0);
+            if (replaces ? m <= 0 : k <= 0)
+                continue;
+            // A gray sample's edge keeps its coverage, as CoreGraphics draws.
+            const uchar *from = source + x * 4;
+            uchar *pixel = row + x * channels;
+            const double keep = replaces ? 1 - m : 1 - from[3] / 255.0 * k;
+            for (int c = 0; c < channels; ++c)
+                pixel[c] = uchar(pixel[c] * keep + from[c] * k + 0.5);
+        }
+    }
+}
+
+// The tiles a pixel rect touches, by key.
+template <typename Body> void eachTile(const QRectF &affected, int width, const Body &body)
+{
+    if (affected.isEmpty())
+        return;
+    const qint64 columns = (width + BrushStroke::tileSize - 1) / BrushStroke::tileSize;
+    const qint64 firstY = qint64(affected.top()) / BrushStroke::tileSize, lastY = (qint64(std::ceil(affected.bottom())) - 1) / BrushStroke::tileSize;
+    const qint64 firstX = qint64(affected.left()) / BrushStroke::tileSize, lastX = (qint64(std::ceil(affected.right())) - 1) / BrushStroke::tileSize;
+    for (qint64 y = firstY; y <= lastY; ++y) {
+        for (qint64 x = firstX; x <= lastX; ++x)
+            body(y * columns + x, x, y);
+    }
+}
+}
+
+double BrushStroke::spacingFraction(double hardness)
+{
+    return hardness >= 1 ? 0.015 : 0.025;
+}
+
+BrushStroke::Grid BrushStroke::grid(const ImageLayer &layer, bool mask, const BrushSettings &settings, const QRectF &canvas)
+{
+    // A placed mask paints in its grid; else the layer's.
+    const bool placedMask = mask && layer.mask && layer.mask->placement;
+    const LayerTransform base = placedMask ? *layer.mask->placement : layer.transform;
+    const int originalWidth = placedMask ? layer.mask->asset.size().width()
+        : layer.asset ? layer.asset->size().width() : int(std::round(layer.size().width()));
+    const int originalHeight = placedMask ? layer.mask->asset.size().height()
+        : layer.asset ? layer.asset->size().height() : int(std::round(layer.size().height()));
+    if (originalWidth < 1 || originalWidth > 30'000 || originalHeight < 1 || originalHeight > 30'000)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    const QTransform originalMapping = BrushRaster::pixelToDocument(base, originalWidth, originalHeight);
+    const QRectF originalBounds(0, 0, originalWidth, originalHeight);
+    const QRectF extent = mask ? originalBounds : originalBounds.united(integral(originalMapping.inverted().mapRect(canvas)));
+    // Judged as a double: past int the conversion is undefined.
+    if (extent.width() < 1 || extent.width() > 1'000'000'000 || extent.height() < 1 || extent.height() > 1'000'000'000)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    Grid grid{int(extent.width()), int(extent.height()), originalBounds.translated(-extent.topLeft()),
+              QTransform::fromTranslate(extent.left(), extent.top()) * originalMapping, base};
+    grid.paintTransform.size = {grid.width * base.size.width() / originalWidth, grid.height * base.size.height() / originalHeight};
+    const QPointF center = originalMapping.map(extent.center());
+    grid.paintTransform.origin = {center.x() - grid.paintTransform.size.width() / 2, center.y() - grid.paintTransform.size.height() / 2};
+    if (!std::isfinite(settings.diameter) || settings.diameter < 1 || settings.diameter > 2000
+        || !std::isfinite(settings.hardness) || settings.hardness < 0 || settings.hardness > 1
+        || !std::isfinite(settings.opacity) || settings.opacity < 0.01 || settings.opacity > 1)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    return grid;
+}
+
+BrushStroke::BrushStroke(const ImageLayer &layer, bool mask, const BrushSettings &settings, QSizeF canvas)
+    : BrushStroke(layer, mask, settings, QRectF(QPointF(0, 0), canvas), grid(layer, mask, settings, QRectF(QPointF(0, 0), canvas)))
+{
+}
+
+BrushStroke::BrushStroke(const ImageLayer &layer, bool mask, const BrushSettings &settings, const QRectF &canvas, const Grid &grid)
+    : layer(layer), isMask(mask), settings(settings), canvas(canvas), width(grid.width), height(grid.height), sourceRect(grid.sourceRect),
+      pixelToDocument(grid.pixelToDocument), paintTransform(grid.paintTransform),
+      m_source(mask ? (layer.mask ? std::optional(layer.mask->asset) : std::nullopt) : layer.asset),
+      m_paintColor(mask ? QColor::fromRgbF(settings.red, settings.red, settings.red) : QColor::fromRgbF(settings.red, settings.green, settings.blue))
+{
+}
+
+void BrushStroke::append(QPointF point)
+{
+    if (!std::isfinite(point.x()) || !std::isfinite(point.y()) || std::abs(point.x()) > 10'000'000 || std::abs(point.y()) > 10'000'000)
+        return;
+    if (!m_samples.empty() && m_samples.back() == point)
+        return;
+    m_samples.push_back(point);
+    if (m_samples.size() > 4)
+        m_samples.erase(m_samples.begin());
+    const size_t n = m_samples.size();
+    std::vector<BrushSegment> settled;
+    if (n == 1)
+        settled = {BrushSegment{float(point.x()), float(point.y()), float(point.x()), float(point.y())}};
+    else if (n >= 3)
+        settled = continuousCurve(m_samples[n - 3], m_samples[n - 2], m_samples[n >= 4 ? n - 4 : 0], point);
+    std::vector<BrushSegment> tail;
+    if (n >= 2)
+        tail = {BrushSegment{float(m_samples[n - 2].x()), float(m_samples[n - 2].y()), float(point.x()), float(point.y())}};
+    renderContinuous(settled, tail);
+}
+
+void BrushStroke::flush()
+{
+    const size_t n = m_samples.size();
+    if (n < 2)
+        return;
+    const std::vector<BrushSegment> settled = continuousCurve(m_samples[n - 2], m_samples[n - 1], m_samples[n >= 3 ? n - 3 : 0], m_samples[n - 1]);
+    renderContinuous(settled, {});
+    m_samples = {m_samples[n - 1]};
+}
+
+// Centripetal Catmull–Rom, subdivided within 0.2 document pixels.
+std::vector<BrushSegment> BrushStroke::continuousCurve(QPointF start, QPointF end, QPointF before, QPointF after) const
+{
+    const auto knot = [](double t, QPointF a, QPointF b) { return t + std::max(0.0001, std::sqrt(std::hypot(b.x() - a.x(), b.y() - a.y()))); };
+    const auto mix = [](QPointF a, QPointF b, double ta, double tb, double t) {
+        const double wa = (tb - t) / (tb - ta), wb = (t - ta) / (tb - ta);
+        return QPointF(a.x() * wa + b.x() * wb, a.y() * wa + b.y() * wb);
+    };
+    const double t0 = 0, t1 = knot(t0, before, start), t2 = knot(t1, start, end), t3 = knot(t2, end, after);
+    const auto point = [&](double u) {
+        if (u == 0)
+            return start;
+        if (u == 1)
+            return end;
+        const double t = t1 + (t2 - t1) * u;
+        const QPointF a = mix(before, start, t0, t1, t), b = mix(start, end, t1, t2, t), c = mix(end, after, t2, t3, t);
+        return mix(mix(a, b, t0, t2, t), mix(b, c, t1, t3, t), t1, t2, t);
+    };
+    std::vector<BrushSegment> result;
+    const std::function<void(QPointF, QPointF, double, double, int)> subdivide = [&](QPointF a, QPointF b, double lo, double hi, int depth) {
+        const double dx = b.x() - a.x(), dy = b.y() - a.y(), lengthSquared = dx * dx + dy * dy;
+        const auto error = [&](QPointF p) {
+            const double t = lengthSquared > 0 ? std::min(1.0, std::max(0.0, ((p.x() - a.x()) * dx + (p.y() - a.y()) * dy) / lengthSquared)) : 0;
+            return std::hypot(p.x() - a.x() - t * dx, p.y() - a.y() - t * dy);
+        };
+        const double mid = (lo + hi) / 2;
+        const QPointF m = point(mid);
+        const double deviation = std::max({error(m), error(point((lo + mid) / 2)), error(point((mid + hi) / 2))});
+        if (deviation <= 0.2 || depth >= 10) {
+            result.push_back(BrushSegment{float(a.x()), float(a.y()), float(b.x()), float(b.y())});
+            return;
+        }
+        subdivide(a, m, lo, mid, depth + 1);
+        subdivide(m, b, mid, hi, depth + 1);
+    };
+    subdivide(start, end, 0, 1, 0);
+    return result;
+}
+
+std::set<qint64> BrushStroke::continuousKeys(const std::vector<BrushSegment> &segments) const
+{
+    std::set<qint64> keys;
+    const double reach = settings.diameter / 2 + 2;
+    const QTransform inverse = pixelToDocument.inverted();
+    for (const BrushSegment &s : segments) {
+        const QRectF box = QRectF(QPointF(std::min(s.x0, s.x1), std::min(s.y0, s.y1)), QPointF(std::max(s.x0, s.x1), std::max(s.y0, s.y1)))
+                               .adjusted(-reach, -reach, reach, reach).intersected(canvas);
+        if (box.isEmpty())
+            continue;
+        const QRectF affected = integral(inverse.mapRect(box)).intersected(QRectF(0, 0, width, height));
+        eachTile(affected, width, [&](qint64 key, qint64, qint64) { keys.insert(key); });
+    }
+    return keys;
+}
+
+void BrushStroke::renderContinuous(const std::vector<BrushSegment> &settled, const std::vector<BrushSegment> &tail)
+{
+    const std::set<qint64> tailKeys = continuousKeys(tail);
+    std::set<qint64> changed = continuousKeys(settled);
+    changed.insert(tailKeys.begin(), tailKeys.end());
+    changed.insert(m_tailKeys.begin(), m_tailKeys.end());
+    const qint64 columns = (width + tileSize - 1) / tileSize;
+    std::vector<BrushCoverage::Work> work;
+    for (qint64 key : changed) {
+        allocateTile(key, key % columns, key / columns);
+        const Tile &tile = m_tiles.at(key);
+        if (!m_coverage.contains(key)) {
+            m_coverage.emplace(key, BrushRaster::context(int(tile.rect.width()), int(tile.rect.height()), true));
+            m_coverageTiles.emplace(key, BrushCoverage::tile(int(tile.rect.width()), int(tile.rect.height())));
+        }
+        work.push_back({&m_coverageTiles.at(key), tile.rect, &m_coverage.at(key)});
+    }
+    BrushCoverage::render(work, settled, tail, pixelToDocument, settings, canvas.size());
+    m_tailKeys = tailKeys;
+    publish(changed);
+}
+
+// Each tile anew: original, then colour × coverage × opacity.
+void BrushStroke::publish(const std::set<qint64> &changed)
+{
+    struct Fill {
+        Tile *tile;
+        const QImage *coverage;
+        QImage clip;
+        QImage sample;
+    };
+    // Copies here, where a failure throws; fills side by side.
+    std::vector<Fill> work;
+    m_dirtyDocumentRect = std::nullopt;
+    for (qint64 key : changed) {
+        Tile &tile = m_tiles.at(key);
+        tile.context = tile.base;
+        tile.context.detach();
+        if (tile.context.isNull())
+            throw ExportError(ExportError::Kind::render);
+        work.push_back({&tile, &m_coverage.at(key), selectionCoverage(tile), clonePixels(tile)});
+        const QRectF rect = pixelToDocument.mapRect(tile.rect).intersected(canvas);
+        m_dirtyDocumentRect = m_dirtyDocumentRect ? std::optional(m_dirtyDocumentRect->united(rect)) : std::optional(rect);
+    }
+    // Healing shows Swift's dark wash until heal() rebuilds the spot.
+    const bool healing = settings.healing && !isMask;
+    const QPainter::CompositionMode mode = !healing && settings.erasing && !isMask ? QPainter::CompositionMode_DestinationOut : QPainter::CompositionMode_SourceOver;
+    const QColor colour = healing ? QColor::fromRgbF(0.12, 0.12, 0.12) : m_paintColor;
+    const double alpha = healing ? 0.45 : settings.opacity;
+    PoolMap::blocking(work, [&](const Fill &item) {
+        Tile &tile = *item.tile;
+        if (clone && (!isMask || isBlur))
+            fillClone(item.sample, *item.coverage, settings.opacity, tile.context, item.clip, replacesWithClone);
+        else
+            BrushRaster::fill(colour, *item.coverage, QRectF(QPointF(0, 0), tile.rect.size()), alpha, tile.context, mode, item.clip);
+        tile.image = tile.context;
+    });
+}
+
+// Swift clips the context; the fill takes the selection instead.
+QImage BrushStroke::selectionCoverage(const Tile &tile) const
+{
+    if (!selectionClip)
+        return QImage();
+    return PixelAdjust::coverage(*selectionClip, int(tile.rect.width()), int(tile.rect.height()),
+                                 QTransform::fromTranslate(tile.rect.left(), tile.rect.top()) * pixelToDocument);
+}
+
+// The clone as this tile sees it, before the fill.
+QImage BrushStroke::clonePixels(const Tile &tile) const
+{
+    if (!clone)
+        return QImage();
+    const int width = int(tile.rect.width()), height = int(tile.rect.height());
+    // Tile pixels to clone pixels: grid, document, then offset.
+    const QTransform toClone = QTransform::fromTranslate(tile.rect.left(), tile.rect.top()) * pixelToDocument
+        * QTransform::fromTranslate(clone->offset.width(), clone->offset.height());
+    // Colour even for gray clones: edges carry their coverage.
+    const QImage &image = clone->image;
+    if (toClone.type() > QTransform::TxTranslate) {
+        // Swift's medium quality: bilinear under a scale or turn.
+        QImage sample = BrushRaster::context(width, height, false);
+        QPainter painter(&sample);
+        painter.setRenderHint(QPainter::SmoothPixmapTransform);
+        painter.setTransform(toClone.inverted());
+        painter.drawImage(QRectF(QPointF(0, 0), image.size()), image);
+        painter.end();
+        return sample;
+    }
+    // QPainter snaps a translation; a fraction is resampled by hand.
+    const double left = std::floor(toClone.dx()), top = std::floor(toClone.dy());
+    const bool whole = left == toClone.dx() && top == toClone.dy();
+    QImage sample = BrushRaster::context(whole ? width : width + 1, whole ? height : height + 1, false);
+    QPainter painter(&sample);
+    BrushRaster::draw(image, QRectF(-left, -top, image.width(), image.height()), painter);
+    painter.end();
+    if (whole)
+        return sample;
+    const QImage part = BrushRaster::shiftedByFraction(sample, 1 - (toClone.dx() - left), 1 - (toClone.dy() - top)).copy(1, 1, width, height);
+    if (part.isNull())
+        throw ExportError(ExportError::Kind::render);
+    return part;
+}
+
+void BrushStroke::allocateTile(qint64 key, qint64 x, qint64 y)
+{
+    if (m_tiles.contains(key))
+        return;
+    const QRectF rect(x * tileSize, y * tileSize, std::min<qint64>(tileSize, width - x * tileSize), std::min<qint64>(tileSize, height - y * tileSize));
+    const QRectF nextBounds = m_allocatedBounds ? m_allocatedBounds->united(rect) : m_source ? sourceRect.united(rect) : rect;
+    if (nextBounds.width() > 30'000 || nextBounds.height() > 30'000 || nextBounds.width() * nextBounds.height() > pixelLimit)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    m_allocatedBounds = nextBounds;
+    QImage context = BrushRaster::context(int(rect.width()), int(rect.height()), isMask);
+    QPainter painter(&context);
+    const QRectF initialRect = sourceRect.translated(-rect.topLeft());
+    if (m_source && m_source->raster) {
+        m_source->raster->draw(initialRect, painter);
+    } else if (m_source) {
+        // This tile's share of the source; a 1×1 stretches.
+        const QImage source = m_source->image();
+        const QRectF overlap = rect.intersected(sourceRect);
+        if (source.width() > 2 && source.height() > 2 && !overlap.isEmpty()) {
+            const double scaleX = source.width() / sourceRect.width(), scaleY = source.height() / sourceRect.height();
+            const QRectF crop = integral(QRectF((overlap.left() - sourceRect.left()) * scaleX, (overlap.top() - sourceRect.top()) * scaleY,
+                                                overlap.width() * scaleX, overlap.height() * scaleY));
+            BrushRaster::draw(source, overlap.translated(-rect.topLeft()), painter, crop);
+        } else {
+            BrushRaster::draw(source, initialRect, painter);
+        }
+    }
+    painter.end();
+    m_tiles.emplace(key, Tile{rect, context, std::nullopt, context});
+}
+
+std::vector<BrushPatch> BrushStroke::patches() const
+{
+    std::vector<BrushPatch> result;
+    for (const auto &[key, tile] : m_tiles) {
+        if (tile.image)
+            result.push_back({tile.rect, *tile.image});
+    }
+    return result;
+}
+
+// Redrawing restarts from each tile's original content.
+void BrushStroke::paintCanvas(bool withinSource, const std::function<void(QPainter &)> &draw)
+{
+    QRectF area = canvas;
+    if (selectionClip)
+        area = area.intersected(selectionClip->rect);
+    if (area.isEmpty())
+        return;
+    const QTransform inverse = pixelToDocument.inverted();
+    QRectF affected = integral(inverse.mapRect(area)).intersected(QRectF(0, 0, width, height));
+    if (withinSource)
+        affected = affected.intersected(sourceRect);
+    if (affected.isEmpty())
+        return;
+    eachTile(affected, width, [&](qint64 key, qint64 x, qint64 y) {
+        allocateTile(key, x, y);
+        Tile &tile = m_tiles.at(key);
+        QImage drawn = m_source ? tile.base : BrushRaster::context(int(tile.rect.width()), int(tile.rect.height()), isMask);
+        drawn.detach();
+        if (drawn.isNull())
+            throw ExportError(ExportError::Kind::render);
+        QPainter painter(&drawn);
+        painter.setTransform(inverse * QTransform::fromTranslate(-tile.rect.left(), -tile.rect.top()));
+        painter.setClipRect(canvas);
+        draw(painter);
+        painter.end();
+        // Swift clips to the selection; the twin mixes through it.
+        if (selectionClip)
+            drawn = PixelAdjust::blend(drawn, tile.base, *selectionClip, QTransform::fromTranslate(tile.rect.left(), tile.rect.top()) * pixelToDocument, isMask);
+        tile.context = drawn;
+        tile.image = drawn;
+    });
+    m_dirtyDocumentRect = canvas;
+}
+
+void BrushStroke::fillGradient(GradientShape shape, QPointF start, QPointF end, const std::array<QColor, 2> &colors, double opacity)
+{
+    // Padded both ways, as Swift's before and after locations.
+    QGradient gradient = shape == GradientShape::linear
+                             ? QGradient(QLinearGradient(start, end))
+                             : QGradient(QRadialGradient(start, std::hypot(end.x() - start.x(), end.y() - start.y())));
+    gradient.setColorAt(0, colors[0]);
+    gradient.setColorAt(1, colors[1]);
+    paintCanvas(false, [&](QPainter &painter) {
+        painter.setOpacity(std::min(1.0, std::max(0.0, opacity)));
+        painter.fillRect(canvas, gradient);
+    });
+}
+
+void BrushStroke::fill(const QColor &color)
+{
+    paintCanvas(false, [&](QPainter &painter) { painter.fillRect(canvas, color); });
+}
+
+void BrushStroke::clearPixels()
+{
+    paintCanvas(true, [&](QPainter &painter) {
+        painter.setCompositionMode(QPainter::CompositionMode_DestinationOut);
+        painter.fillRect(canvas, Qt::black);
+    });
+}
+
+// Cuts the selected pixels from the source; false with none.
+bool BrushStroke::liftSelection()
+{
+    if (isMask || !m_source || !selectionClip || !selectionClip->coverage)
+        return false;
+    const QRectF region = integral(pixelToDocument.inverted().mapRect(selectionClip->rect)).intersected(sourceRect);
+    if (region.width() < 1 || region.height() < 1)
+        return false;
+    QImage lifted = BrushRaster::context(int(region.width()), int(region.height()), false);
+    QPainter painter(&lifted);
+    painter.translate(-region.left(), -region.top());
+    if (m_source->raster)
+        m_source->raster->draw(sourceRect, painter);
+    else
+        BrushRaster::draw(m_source->image(), sourceRect, painter);
+    painter.end();
+    // Swift clips the context; the selection's coverage multiplies in.
+    const QImage clip = PixelAdjust::coverage(*selectionClip, lifted.width(), lifted.height(), QTransform::fromTranslate(region.left(), region.top()) * pixelToDocument);
+    QPainter multiplying(&lifted);
+    multiplying.setCompositionMode(QPainter::CompositionMode_DestinationIn);
+    multiplying.drawImage(QRectF(lifted.rect()), BrushRaster::alphaView(clip));
+    multiplying.end();
+    m_lifted = Lifted{lifted, region};
+    return true;
+}
+
+// Tiles anew: a hole, and the pixels `offset` away.
+void BrushStroke::moveLifted(QSizeF offset, bool duplicate)
+{
+    if (!m_lifted || !selectionClip)
+        return;
+    const QTransform inverse = pixelToDocument.inverted();
+    const QPointF zero = inverse.map(QPointF(0, 0)), moved = inverse.map(QPointF(offset.width(), offset.height()));
+    const QRectF target = m_lifted->rect.translated(moved.x() - zero.x(), moved.y() - zero.y());
+    const bool whole = target.left() == std::round(target.left()) && target.top() == std::round(target.top());
+    // QPainter snaps a fraction; Swift's high quality resamples it.
+    const QPointF place(std::floor(target.left()), std::floor(target.top()));
+    const QImage lifted = whole ? m_lifted->image : BrushRaster::shiftedByFraction(m_lifted->image, target.left() - place.x(), target.top() - place.y());
+    const QRectF needed = integral(m_lifted->rect.united(target)).intersected(QRectF(0, 0, width, height));
+    std::set<qint64> keys = m_moveTiles;
+    eachTile(needed, width, [&](qint64 key, qint64 x, qint64 y) {
+        allocateTile(key, x, y);
+        keys.insert(key);
+    });
+    for (qint64 key : keys) {
+        Tile &tile = m_tiles.at(key);
+        QImage drawn = tile.base;
+        drawn.detach();
+        if (drawn.isNull())
+            throw ExportError(ExportError::Kind::render);
+        QPainter painter(&drawn);
+        if (!duplicate) {
+            const QImage clip = PixelAdjust::coverage(*selectionClip, drawn.width(), drawn.height(),
+                                                      QTransform::fromTranslate(tile.rect.left(), tile.rect.top()) * pixelToDocument);
+            painter.setCompositionMode(QPainter::CompositionMode_DestinationOut);
+            painter.drawImage(QRectF(drawn.rect()), BrushRaster::alphaView(clip));
+            painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
+        }
+        painter.drawImage(QRectF(place - tile.rect.topLeft(), lifted.size()), lifted);
+        painter.end();
+        tile.context = drawn;
+        tile.image = drawn;
+    }
+    m_moveTiles = keys;
+    m_dirtyDocumentRect = canvas;
+}

src/Document/BrushStroke.cpp

@@ -0,0 +1,131 @@
+#include "Document/BrushStroke.h"
+#include "Rendering/PoolMap.h"
+#include "IO/ImageExporter.h"
+#include <QtConcurrent>
+#include <cmath>
+#include <numeric>
+#include <stdexcept>
+
+QString rawValue(SpotHealingMode mode)
+{
+    switch (mode) {
+    case SpotHealingMode::contentAware:
+        return QStringLiteral("Content-Aware");
+    case SpotHealingMode::createTexture:
+        return QStringLiteral("Create Texture");
+    case SpotHealingMode::proximityMatch:
+        return QStringLiteral("Proximity Match");
+    }
+    throw std::logic_error("unknown spot healing mode");
+}
+
+QImage BrushRaster::context(int width, int height, bool mask)
+{
+    QImage surface(width, height, mask ? QImage::Format_Grayscale8 : QImage::Format_RGBA8888_Premultiplied);
+    if (surface.isNull())
+        throw ExportError(ExportError::Kind::render);
+    surface.fill(0);
+    return surface;
+}
+
+void BrushRaster::draw(const QImage &image, const QRectF &rect, QPainter &context, const QRectF &source)
+{
+    context.save();
+    context.setRenderHint(QPainter::SmoothPixmapTransform, false);
+    context.setCompositionMode(QPainter::CompositionMode_Source);
+    context.drawImage(rect, image, source.isNull() ? QRectF(image.rect()) : source);
+    context.restore();
+}
+
+QImage BrushRaster::alphaView(const QImage &mask)
+{
+    if (mask.format() != QImage::Format_Grayscale8)
+        throw std::logic_error("a layer mask must be 8-bit gray");
+    return QImage(mask.constBits(), mask.width(), mask.height(), mask.bytesPerLine(), QImage::Format_Alpha8);
+}
+
+QRectF BrushRaster::visibleRect(const QPainter &context)
+{
+    const QPaintDevice &device = *context.device();
+    // A widget's size is logical; an image's is device pixels.
+    QTransform toDevice = context.combinedTransform();
+    if (device.devType() == QInternal::Widget)
+        toDevice *= QTransform::fromScale(1 / device.devicePixelRatio(), 1 / device.devicePixelRatio());
+    const QRectF bounds = toDevice.inverted().mapRect(QRectF(0, 0, device.width(), device.height()));
+    return context.hasClipping() ? bounds.intersected(context.clipBoundingRect()) : bounds;
+}
+
+QTransform BrushRaster::pixelToDocument(const LayerTransform &transform, int width, int height)
+{
+    const QPointF center = transform.center();
+    QTransform map;
+    map.translate(center.x(), center.y());
+    map.rotateRadians(transform.radians());
+    map.scale(transform.size.width() / width * (transform.flipX ? -1 : 1),
+              transform.size.height() / height * (transform.flipY ? -1 : 1));
+    map.translate(-width / 2.0, -height / 2.0);
+    return map;
+}
+
+// Source-over of the colour at coverage × alpha, bytewise.
+void BrushRaster::fill(const QColor &color, const QImage &coverage, const QRectF &rect, double alpha, QImage &context,
+                       QPainter::CompositionMode mode, const QImage &clip)
+{
+    if (coverage.format() != QImage::Format_Grayscale8 || coverage.size() != rect.size().toSize() || rect.topLeft() != rect.topLeft().toPoint())
+        throw std::logic_error("coverage must be gray and 1:1 with its rect");
+    if (!clip.isNull() && (clip.format() != QImage::Format_Grayscale8 || clip.size() != coverage.size()))
+        throw std::logic_error("a clip must be gray and sized as the coverage");
+    // Erasing takes coverage out; a mask never erases.
+    const bool erasing = mode == QPainter::CompositionMode_DestinationOut;
+    if ((!erasing && mode != QPainter::CompositionMode_SourceOver) || (erasing && context.format() == QImage::Format_Grayscale8))
+        throw std::logic_error("a brush fills source-over, or erases pixels");
+    const int left = int(rect.left()), top = int(rect.top());
+    const bool gray = context.format() == QImage::Format_Grayscale8;
+    if (!gray && context.format() != QImage::Format_RGBA8888_Premultiplied)
+        throw std::logic_error("a brush context is premultiplied RGBA or gray");
+    // What each channel moves towards; erasing moves towards nothing.
+    const double target[4] = {erasing ? 0 : color.redF() * 255, erasing ? 0 : color.greenF() * 255, erasing ? 0 : color.blueF() * 255,
+                              erasing ? 0 : 255.0};
+    const int channels = gray ? 1 : 4;
+    for (int y = 0; y < coverage.height(); ++y) {
+        const uchar *mask = coverage.constScanLine(y);
+        // A clip scales the coverage, rounded once, as CoreGraphics composites.
+        const uchar *within = clip.isNull() ? nullptr : clip.constScanLine(y);
+        uchar *row = context.scanLine(top + y) + left * channels;
+        for (int x = 0; x < coverage.width(); ++x) {
+            const double k = mask[x] / 255.0 * alpha * (within ? within[x] / 255.0 : 1.0);
+            if (k <= 0)
+                continue;
+            // Values are never negative: adding a half rounds them.
+            uchar *pixel = row + x * channels;
+            for (int c = 0; c < channels; ++c)
+                pixel[c] = uchar(pixel[c] * (1 - k) + target[c] * k + 0.5);
+        }
+    }
+}
+
+QImage BrushRaster::shiftedByFraction(const QImage &image, double fx, double fy)
+{
+    QImage shifted = BrushRaster::context(image.width() + 1, image.height() + 1, false);
+    // Weights in 1/65536: the pixel, left, above, above left.
+    const qint64 here = std::lround((1 - fx) * (1 - fy) * 65536), left = std::lround(fx * (1 - fy) * 65536);
+    const qint64 above = std::lround((1 - fx) * fy * 65536), corner = 65536 - here - left - above;
+    uchar *const out = shifted.bits();
+    const uchar *const in = image.constBits();
+    const qsizetype outStride = shifted.bytesPerLine(), inStride = image.bytesPerLine();
+    const int width = image.width(), height = image.height();
+    std::vector<int> rows(size_t(shifted.height()));
+    std::iota(rows.begin(), rows.end(), 0);
+    PoolMap::blocking(rows, [&](int y) {
+        const uchar *row = y < height ? in + y * inStride : nullptr, *up = y > 0 ? in + (y - 1) * inStride : nullptr;
+        uchar *line = out + y * outStride;
+        for (int x = 0; x <= width; ++x) {
+            for (int c = 0; c < 4; ++c) {
+                const qint64 sum = (row && x < width ? here * row[x * 4 + c] : 0) + (row && x > 0 ? left * row[(x - 1) * 4 + c] : 0)
+                    + (up && x < width ? above * up[x * 4 + c] : 0) + (up && x > 0 ? corner * up[(x - 1) * 4 + c] : 0);
+                line[x * 4 + c] = uchar((sum + 32768) >> 16);
+            }
+        }
+    });
+    return shifted;
+}

src/Document/BrushStroke.h

@@ -0,0 +1,190 @@
+#pragma once
+#include "Document/EditorSession+Model.h"
+#include "Document/Gradient.h"
+#include "Document/LayerTransform.h"
+#include "Document/Selection.h"
+#include "IO/ImageImporter.h"
+#include "Rendering/BrushCoverage.h"
+#include <QImage>
+#include <QPainter>
+#include <array>
+#include <functional>
+#include <map>
+#include <set>
+
+struct BrushPatch {
+    QRectF rect;
+    QImage image;
+};
+
+// Swift's three ways of healing a spot, in its order.
+enum class SpotHealingMode { contentAware, createTexture, proximityMatch };
+inline constexpr std::array allSpotHealingModes{SpotHealingMode::contentAware, SpotHealingMode::createTexture, SpotHealingMode::proximityMatch};
+QString rawValue(SpotHealingMode mode);
+
+// Tip, colour, and what the stroke does with them.
+struct BrushSettings {
+    double diameter = 40;
+    double hardness = 1;
+    double red = 0;
+    double green = 0;
+    double blue = 0;
+    // Caps the whole stroke: overlapping dabs never exceed it.
+    double opacity = 1;
+    // Erase clears the layer's pixels instead of painting on them.
+    bool erasing = false;
+    // Spot healing takes nearby pixels instead of the colour.
+    bool healing = false;
+    SpotHealingMode healingMode = SpotHealingMode::contentAware;
+};
+
+// Shared raster drawing; coverage is copied without color conversion.
+namespace BrushRaster {
+QImage context(int width, int height, bool mask);
+// Copies `image`, or its `source` part, into `rect`.
+void draw(const QImage &image, const QRectF &rect, QPainter &context, const QRectF &source = QRectF());
+// A solid colour through gray coverage, and a clip, 1:1.
+void fill(const QColor &color, const QImage &coverage, const QRectF &rect, double alpha, QImage &context,
+          QPainter::CompositionMode mode = QPainter::CompositionMode_SourceOver, const QImage &clip = QImage());
+// Gray mask bytes read as alpha, without a copy.
+QImage alphaView(const QImage &mask);
+// CoreGraphics boundingBoxOfClipPath: the clip within the device.
+QRectF visibleRect(const QPainter &context);
+// Premultiplied pixels shifted right and down by fractions, bilinear.
+QImage shiftedByFraction(const QImage &image, double fx, double fy);
+QTransform pixelToDocument(const LayerTransform &transform, int width, int height);
+}
+
+// A stroke's pixels as a painted layer, placed and bounded.
+struct PaintSnapshot {
+    ImportedImage asset;
+    LayerTransform transform;
+    QRectF bounds;
+};
+
+// Swift's BrushCommit actor: the stroke's tiles made one image.
+namespace BrushCommit {
+struct Input {
+    int width;
+    int height;
+    std::optional<ImportedImage> source;
+    std::vector<BrushPatch> patches;
+    bool mask;
+    QString name;
+    QRectF sourceRect;
+};
+struct Output {
+    ImportedImage asset;
+    QRectF pixelBounds;
+};
+ImportedImage expandMask(const ImportedImage &asset, const Input &input, const QRectF &crop);
+Output render(const Input &input);
+}
+
+// Only touched 256 px tiles hold pixels; snapshots copy those.
+class BrushStroke {
+public:
+    BrushStroke(const ImageLayer &layer, bool mask, const BrushSettings &settings, QSizeF canvas);
+
+    const ImageLayer layer;
+    const bool isMask;
+    const BrushSettings settings;
+    const QRectF canvas;
+    const int width;
+    const int height;
+    const QRectF sourceRect;
+    const QTransform pixelToDocument;
+    const LayerTransform paintTransform;
+    // Swift's Int: the budget left may run below zero.
+    qint64 pixelLimit = 100'000'000;
+    // Limits every edit to the document selection; nil when none.
+    std::optional<SelectionClip> selectionClip;
+    // Clone Stamp: a document-sized image, and each point's offset.
+    struct Clone {
+        QImage image;
+        QSizeF offset;
+    };
+    std::optional<Clone> clone;
+    // Blur: the clone is the layer softened, in place.
+    bool isBlur = false;
+    // The clone replaces what is under the tip, clearing too.
+    bool replacesWithClone = false;
+    // The undo name when the stroke's kind says none.
+    std::optional<QString> editName;
+    static constexpr int tileSize = 256;
+    // Soft-tip deposition rate, shared with the continuous integral.
+    static double spacingFraction(double hardness);
+
+    // A sample; the newest piece shows as a straight tail.
+    void append(QPointF point);
+    // Replaces the tail with the curve's last piece; repeatable.
+    void flush();
+    std::vector<BrushPatch> patches() const;
+    const std::optional<QRectF> &dirtyDocumentRect() const { return m_dirtyDocumentRect; }
+    // The selection, or the canvas, filled with one colour.
+    void fill(const QColor &color);
+    // Swift's fillGradient: across the canvas, linear or radial.
+    void fillGradient(GradientShape shape, QPointF start, QPointF end, const std::array<QColor, 2> &colors, double opacity);
+    // Pixels inside the selection made transparent, where pixels are.
+    void clearPixels();
+    // Cuts the selected pixels from the source; false with none.
+    bool liftSelection();
+    // The tiles anew: a hole, and the pixels `offset` away.
+    void moveLifted(QSizeF offset, bool duplicate);
+    // A placed mask as the stroke leaves it (LayerMask.cpp).
+    std::optional<QImage> placedMaskPreview(const LayerTransform &placement) const;
+    QRectF committedBounds() const;
+    LayerTransform committedTransform() const;
+    LayerTransform transform(const QRectF &bounds) const;
+    // Spot Healing's end: the painted spot rebuilt from around it.
+    void heal();
+    PaintSnapshot paintSnapshot() const;
+    BrushCommit::Input commitInput() const;
+
+private:
+    struct Tile {
+        QRectF rect;
+        QImage context;
+        std::optional<QImage> image;
+        QImage base;
+    };
+    struct Lifted {
+        QImage image;
+        QRectF rect;
+    };
+    // The grid the stroke paints in, made before the members.
+    struct Grid {
+        int width;
+        int height;
+        QRectF sourceRect;
+        QTransform pixelToDocument;
+        LayerTransform paintTransform;
+    };
+    static Grid grid(const ImageLayer &layer, bool mask, const BrushSettings &settings, const QRectF &canvas);
+    BrushStroke(const ImageLayer &layer, bool mask, const BrushSettings &settings, const QRectF &canvas, const Grid &grid);
+    std::vector<BrushSegment> continuousCurve(QPointF start, QPointF end, QPointF before, QPointF after) const;
+    std::set<qint64> continuousKeys(const std::vector<BrushSegment> &segments) const;
+    void renderContinuous(const std::vector<BrushSegment> &settled, const std::vector<BrushSegment> &tail);
+    void publish(const std::set<qint64> &changed);
+    QImage selectionCoverage(const Tile &tile) const;
+    QImage clonePixels(const Tile &tile) const;
+    void allocateTile(qint64 key, qint64 x, qint64 y);
+    // `draw` in document coordinates over every tile the canvas covers.
+    void paintCanvas(bool withinSource, const std::function<void(QPainter &)> &draw);
+
+    // The pixels the stroke starts from: mask's or layer's.
+    const std::optional<ImportedImage> m_source;
+    const QColor m_paintColor;
+    std::optional<QRectF> m_allocatedBounds;
+    std::vector<QPointF> m_samples;
+    std::optional<QRectF> m_dirtyDocumentRect;
+    // Tile keys, row × columns + column, pass int.
+    std::map<qint64, Tile> m_tiles;
+    std::map<qint64, BrushCoverage::Tile> m_coverageTiles;
+    std::set<qint64> m_tailKeys;
+    // Per-tile gray coverage; the tile is recomposed from it.
+    std::map<qint64, QImage> m_coverage;
+    // Selected pixels cut from the source, in layer pixels.
+    std::optional<Lifted> m_lifted;
+    std::set<qint64> m_moveTiles;
+};

src/Document/CanvasSize.cpp

@@ -0,0 +1,75 @@
+#include "Document/CanvasSize.h"
+#include <cmath>
+
+QString rawValue(CanvasUnit unit)
+{
+    switch (unit) {
+    case CanvasUnit::pixels:
+        return QStringLiteral("Pixels");
+    case CanvasUnit::percent:
+        return QStringLiteral("Percent");
+    case CanvasUnit::inches:
+        return QStringLiteral("Inches");
+    case CanvasUnit::centimeters:
+        return QStringLiteral("Centimeters");
+    }
+    Q_UNREACHABLE();
+}
+
+CanvasSizeDraft::CanvasSizeDraft(qint64 width, qint64 height, double resolution)
+    : originalWidth(width), originalHeight(height), resolution(resolution), width(double(width)), height(double(height))
+{
+}
+
+bool CanvasSizeDraft::valid() const
+{
+    // NaN and infinity fail both range tests.
+    return std::round(width) >= 1 && std::round(width) <= 30'000 && std::round(height) >= 1 && std::round(height) <= 30'000;
+}
+
+double CanvasSizeDraft::displayed(bool widthAxis) const
+{
+    const double original = double(widthAxis ? originalWidth : originalHeight);
+    const double pixels = (widthAxis ? width : height) - (relative ? original : 0);
+    switch (unit) {
+    case CanvasUnit::pixels:
+        return pixels;
+    case CanvasUnit::percent:
+        return pixels / original * 100;
+    case CanvasUnit::inches:
+        return pixels / resolution;
+    case CanvasUnit::centimeters:
+        return pixels / resolution * 2.54;
+    }
+    Q_UNREACHABLE();
+}
+
+void CanvasSizeDraft::set(double value, bool widthAxis)
+{
+    const double original = double(widthAxis ? originalWidth : originalHeight);
+    double pixels = value;
+    if (unit == CanvasUnit::percent)
+        pixels = value / 100 * original;
+    else if (unit == CanvasUnit::inches)
+        pixels = value * resolution;
+    else if (unit == CanvasUnit::centimeters)
+        pixels = value / 2.54 * resolution;
+    const double total = pixels + (relative ? original : 0);
+    if (widthAxis) {
+        width = total;
+        if (locked)
+            height = total * double(originalHeight) / double(originalWidth);
+    } else {
+        height = total;
+        if (locked)
+            width = total * double(originalWidth) / double(originalHeight);
+    }
+}
+
+QPointF CanvasSizeOptions::offset(qint64 fromWidth, qint64 oldHeight) const
+{
+    if (contentOffset)
+        return *contentOffset;
+    // Floor: growth lands right and bottom, cuts left and top.
+    return QPointF(std::floor(double(width - fromWidth) * double(anchor % 3) / 2), std::floor(double(height - oldHeight) * double(anchor / 3) / 2));
+}

src/Document/CanvasSize.h

@@ -0,0 +1,45 @@
+#pragma once
+#include <QPointF>
+#include <QString>
+#include <QtGlobal>
+#include <optional>
+
+enum class CanvasUnit { pixels, percent, inches, centimeters };
+// The name users see.
+QString rawValue(CanvasUnit unit);
+
+// What the canvas size sheet edits, in final pixels.
+struct CanvasSizeDraft {
+    CanvasSizeDraft(qint64 width, qint64 height, double resolution);
+
+    qint64 originalWidth;
+    qint64 originalHeight;
+    double resolution;
+    double width;
+    double height;
+    bool relative = false;
+    bool locked = false;
+    CanvasUnit unit = CanvasUnit::pixels;
+
+    bool valid() const;
+    double displayed(bool widthAxis) const;
+    void set(double value, bool widthAxis);
+};
+
+struct CanvasExtensionColor {
+    double red;
+    double green;
+    double blue;
+};
+
+struct CanvasSizeOptions {
+    qint64 width;
+    qint64 height;
+    // Row-major, top-left through bottom-right.
+    qint64 anchor = 4;
+    std::optional<CanvasExtensionColor> fill = std::nullopt;
+    // Crop supplies its own translation of the content.
+    std::optional<QPointF> contentOffset = std::nullopt;
+
+    QPointF offset(qint64 fromWidth, qint64 oldHeight) const;
+};

src/Document/CloneStamp+Session.h

@@ -0,0 +1,10 @@
+// Swift's CloneStamp extension: where Clone Stamp copies from.
+public:
+    // Alt-click: the source; a new source starts a new alignment.
+    void setCloneSource(QPointF point);
+    // The whole-pixel offset a stroke from `point` copies with.
+    std::optional<QSizeF> cloneStrokeOffset(QPointF point) const;
+    // The source for a brush at `point`: the canvas's crosshair.
+    std::optional<QPointF> cloneSamplePoint(QPointF point) const;
+    // What a stroke copies from, document-sized, taken as it starts.
+    std::optional<QImage> cloneSample(const CanvasDocument &document) const;

src/Document/CloneStamp.cpp

@@ -0,0 +1,61 @@
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/LayerRenderer.h"
+#include <cmath>
+
+// Swift's CloneStamp extension: the source, its offset, the sample.
+void EditorSession::setCloneSource(QPointF point)
+{
+    if (!std::isfinite(point.x()) || !std::isfinite(point.y()))
+        return;
+    m_cloneSource = point;
+    m_cloneOffset = std::nullopt;
+    notify();
+}
+
+void EditorSession::setCloneSettings(const CloneSettings &settings)
+{
+    m_cloneSettings = settings;
+    notify();
+}
+
+// Shared by stroke and preview: what a click stamps.
+std::optional<QSizeF> EditorSession::cloneStrokeOffset(QPointF point) const
+{
+    if (!m_cloneSource)
+        return std::nullopt;
+    if (m_cloneSettings.aligned && m_cloneOffset)
+        return m_cloneOffset;
+    return QSizeF(std::round(m_cloneSource->x() - point.x()), std::round(m_cloneSource->y() - point.y()));
+}
+
+// The source itself until a stroke fixes the offset.
+std::optional<QPointF> EditorSession::cloneSamplePoint(QPointF point) const
+{
+    if (!m_cloneSource)
+        return std::nullopt;
+    if (!m_cloneOffset || (!m_cloneSettings.aligned && !m_brushStroke))
+        return m_cloneSource;
+    return point + QPointF(m_cloneOffset->width(), m_cloneOffset->height());
+}
+
+std::optional<QImage> EditorSession::cloneSample(const CanvasDocument &document) const
+{
+    try {
+        QImage context = BrushRaster::context(document.width, document.height, false);
+        QPainter painter(&context);
+        // A painted asset flattens here; the catch covers it too.
+        if (m_cloneSettings.sampleAllLayers) {
+            drawLiveComposite(document, painter);
+        } else if (const std::optional<ImageLayer> layer = activeLayer(); layer && layer->asset) {
+            const LayerTransform transform = displayedTransform(*layer);
+            LayerRenderer::draw(layer->asset->image(), transform, transform.center(), painter, {});
+        }
+        painter.end();
+        return context;
+    } catch (const ExportError &error) {
+        qCWarning(lcApp) << "Clone Stamp cannot sample the canvas:" << error.what();
+        return std::nullopt;
+    }
+}

src/Document/CloneStamp.h

@@ -0,0 +1,10 @@
+#pragma once
+
+// Clone Stamp's options-bar settings.
+struct CloneSettings {
+    // The source follows the brush, keeping its offset between strokes.
+    bool aligned = true;
+    // Every visible layer as shown, not the active one alone.
+    bool sampleAllLayers = false;
+    friend bool operator==(const CloneSettings &, const CloneSettings &) = default;
+};

src/Document/ColorPalette+Session.h

@@ -0,0 +1,32 @@
+// Swift's ColorPalette extension: foreground and background colours.
+public:
+    // The brush's own colour, as Swift's computed property.
+    PaletteColor foregroundColor() const;
+    void setForegroundColor(const PaletteColor &color);
+    bool canEditPalette() const;
+    // A mask's palette is black and white: its paint.
+    PaletteColor paletteColor(bool background) const;
+    void setPaletteColor(const PaletteColor &color, bool background);
+    void swapPaletteColors();
+    void resetPaletteColors();
+    void openColorPicker(bool background);
+    // The Type bar's swatch: open text's colour, else the foreground.
+    PaletteColor typeColor() const;
+    void openTextColorPicker();
+    void closeColorPicker(bool commit);
+    // The picker on an end of the open Gradient Map.
+    void openGradientMapColorPicker(bool highlights);
+    // The Gradient Map follows the picker's working colour.
+    void previewGradientMapColor();
+    // The picker on an effect of the open panel.
+    void openEffectColorPicker(LayerEffectKind kind);
+    // The effect follows the picker's working colour.
+    void previewEffectColor();
+    // Swift writes the picker's hsb in place; here, the session.
+    void setColorPickerHSB(const PickerHSB &hsb);
+    void sampleIntoColorPicker(QPointF point);
+    // The visible layers' sRGB at a pixel; none where clear.
+    std::optional<PaletteColor> sampleCompositeColor(QPointF point) const;
+
+private:
+    void setGradientMapColor(const PaletteColor &color, bool highlights);

src/Document/ColorPalette.cpp

@@ -0,0 +1,338 @@
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include <QPainter>
+#include <algorithm>
+#include <cmath>
+
+PaletteColor PaletteColor::quantized() const
+{
+    const auto snap = [](double value) { return std::round(value * 255) / 255; };
+    return {snap(red), snap(green), snap(blue)};
+}
+
+QString PaletteColor::hex() const
+{
+    const auto byte = [](double value) { return int(std::round(value * 255)); };
+    return QString::asprintf("%02X%02X%02X", byte(red), byte(green), byte(blue));
+}
+
+std::optional<PaletteColor> PaletteColor::fromHex(const QString &hex)
+{
+    // Swift trims spaces and tabs, not line breaks.
+    const auto blank = [](QChar character) { return character.category() == QChar::Separator_Space || character == u'\t'; };
+    qsizetype start = 0, end = hex.size();
+    while (start < end && blank(hex.at(start)))
+        ++start;
+    while (end > start && blank(hex.at(end - 1)))
+        --end;
+    QString text = hex.mid(start, end - start);
+    if (text.startsWith(u'#'))
+        text.remove(0, 1);
+    if (text.size() == 3)
+        text = QString(text.at(0)) + text.at(0) + text.at(1) + text.at(1) + text.at(2) + text.at(2);
+    if (text.size() != 6)
+        return std::nullopt;
+    // Swift's UInt32(_:radix:): a sign may lead; minus holds zero alone.
+    const bool negative = text.front() == u'-';
+    uint value = 0;
+    for (const QChar character : QStringView(text).mid(negative || text.front() == u'+' ? 1 : 0)) {
+        const char16_t unit = character.unicode();
+        const int digit = unit >= u'0' && unit <= u'9' ? unit - u'0' : unit >= u'a' && unit <= u'f' ? unit - u'a' + 10 : unit >= u'A' && unit <= u'F' ? unit - u'A' + 10 : -1;
+        if (digit < 0)
+            return std::nullopt;
+        value = value * 16 + uint(digit);
+    }
+    if (negative && value != 0)
+        return std::nullopt;
+    return PaletteColor{double((value >> 16) & 0xFF) / 255, double((value >> 8) & 0xFF) / 255, double(value & 0xFF) / 255};
+}
+
+PickerHSB::PickerHSB(const PaletteColor &color) : hue(0), saturation(0), brightness(0)
+{
+    setRGB(color);
+}
+
+PaletteColor PickerHSB::rgb() const
+{
+    const double h = std::fmod(std::fmod(hue, 360) + 360, 360) / 60;
+    const double c = brightness * saturation;
+    const double x = c * (1 - std::abs(std::fmod(h, 2) - 1));
+    const double m = brightness - c;
+    switch (int(h)) {
+    case 0:
+        return {c + m, x + m, m};
+    case 1:
+        return {x + m, c + m, m};
+    case 2:
+        return {m, c + m, x + m};
+    case 3:
+        return {m, x + m, c + m};
+    case 4:
+        return {x + m, m, c + m};
+    default:
+        return {c + m, m, x + m};
+    }
+}
+
+void PickerHSB::setRGB(const PaletteColor &color)
+{
+    const double high = std::max({color.red, color.green, color.blue});
+    const double low = std::min({color.red, color.green, color.blue});
+    const double delta = high - low;
+    brightness = high;
+    if (high > 0)
+        saturation = delta / high;
+    if (delta <= 0)
+        return;
+    double h;
+    if (high == color.red)
+        h = (color.green - color.blue) / delta;
+    else if (high == color.green)
+        h = (color.blue - color.red) / delta + 2;
+    else
+        h = (color.red - color.green) / delta + 4;
+    h *= 60;
+    hue = h < 0 ? h + 360 : h;
+}
+
+QString ColorPickerTarget::title() const
+{
+    if (kind == Kind::text)
+        return QStringLiteral("Color Picker (Text Color)");
+    if (kind == Kind::gradientMap)
+        return highlights ? QStringLiteral("Color Picker (Gradient Map Highlights)") : QStringLiteral("Color Picker (Gradient Map Shadows)");
+    if (kind == Kind::effect)
+        return QStringLiteral("Color Picker (%1 Color)").arg(rawValue(effect));
+    return background ? QStringLiteral("Color Picker (Background Color)") : QStringLiteral("Color Picker (Foreground Color)");
+}
+
+// Swift's ColorPalette extension: the palette and its picker.
+PaletteColor EditorSession::foregroundColor() const
+{
+    return {m_brushSettings.red, m_brushSettings.green, m_brushSettings.blue};
+}
+
+void EditorSession::setForegroundColor(const PaletteColor &color)
+{
+    BrushSettings settings = m_brushSettings;
+    settings.red = color.red;
+    settings.green = color.green;
+    settings.blue = color.blue;
+    setBrushSettings(settings);
+}
+
+void EditorSession::setBackgroundColor(const PaletteColor &color)
+{
+    m_backgroundColor = color;
+    refreshGradient();
+    notify();
+}
+
+bool EditorSession::canEditPalette() const
+{
+    return !m_isProjectBusy && !m_brushStroke;
+}
+
+PaletteColor EditorSession::paletteColor(bool background) const
+{
+    if (m_isMaskSelected)
+        return (background ? !m_maskPaintWhite : m_maskPaintWhite) ? PaletteColor::white() : PaletteColor::black();
+    return background ? m_backgroundColor : foregroundColor();
+}
+
+void EditorSession::setPaletteColor(const PaletteColor &color, bool background)
+{
+    if (!canEditPalette())
+        return;
+    if (m_isMaskSelected) {
+        const bool white = color == PaletteColor::white();
+        setMaskPaintWhite(background ? !white : white);
+    } else if (background) {
+        setBackgroundColor(color);
+    } else {
+        setForegroundColor(color);
+        // Open text follows the swatch; drafts live under Type.
+        if (m_textDraft) {
+            changeTextStyle([&color](LayerTextStyle &style) {
+                style.red = color.red;
+                style.green = color.green;
+                style.blue = color.blue;
+            });
+        }
+    }
+}
+
+void EditorSession::swapPaletteColors()
+{
+    if (!canEditPalette())
+        return;
+    if (m_isMaskSelected) {
+        setMaskPaintWhite(!m_maskPaintWhite);
+    } else {
+        const PaletteColor old = foregroundColor();
+        setPaletteColor(m_backgroundColor, false);
+        setBackgroundColor(old);
+    }
+}
+
+void EditorSession::resetPaletteColors()
+{
+    if (!canEditPalette())
+        return;
+    if (m_isMaskSelected) {
+        setMaskPaintWhite(false);
+    } else {
+        setPaletteColor(PaletteColor::black(), false);
+        setBackgroundColor(PaletteColor::white());
+    }
+}
+
+void EditorSession::openColorPicker(bool background)
+{
+    if (!canEditPalette() || m_isMaskSelected)
+        return;
+    m_colorPicker = ColorPickerState({ColorPickerTarget::Kind::palette, background}, paletteColor(background));
+    notify();
+}
+
+PaletteColor EditorSession::typeColor() const
+{
+    if (!m_textDraft)
+        return foregroundColor();
+    return {m_textDraft->style.red, m_textDraft->style.green, m_textDraft->style.blue};
+}
+
+void EditorSession::openTextColorPicker()
+{
+    if (!canEditPalette() || m_colorPicker || m_tool != NavigationTool::type)
+        return;
+    const std::optional<QUuid> draftID = m_textDraft ? std::optional(m_textDraft->id) : std::nullopt;
+    m_colorPicker = ColorPickerState({ColorPickerTarget::Kind::text, false, draftID}, typeColor());
+    notify();
+}
+
+void EditorSession::closeColorPicker(bool commit)
+{
+    if (!m_colorPicker)
+        return;
+    // Closed first, so each signal sees it gone.
+    const ColorPickerState picker = *std::exchange(m_colorPicker, std::nullopt);
+    const PaletteColor color = picker.color();
+    const std::optional<QUuid> draftID = m_textDraft ? std::optional(m_textDraft->id) : std::nullopt;
+    if (commit && picker.target.kind == ColorPickerTarget::Kind::palette && !m_isMaskSelected) {
+        setPaletteColor(color, picker.target.background);
+    } else if (commit && picker.target.kind == ColorPickerTarget::Kind::text && m_tool == NavigationTool::type && draftID == picker.target.draftID) {
+        if (draftID) {
+            changeTextStyle([&color](LayerTextStyle &style) {
+                style.red = color.red;
+                style.green = color.green;
+                style.blue = color.blue;
+            });
+        } else {
+            m_textDefaults.red = color.red;
+            m_textDefaults.green = color.green;
+            m_textDefaults.blue = color.blue;
+        }
+        // The text colour is the foreground: the swatch follows.
+        if (!m_isMaskSelected)
+            setForegroundColor(color);
+    } else if (picker.target.kind == ColorPickerTarget::Kind::gradientMap) {
+        // The end previewed the working colour; Cancel puts it back.
+        setGradientMapColor(commit ? color : picker.original, picker.target.highlights);
+    } else if (picker.target.kind == ColorPickerTarget::Kind::effect) {
+        const PaletteColor chosen = commit ? color : picker.original;
+        changeEffects([&](LayerEffects &effects) { effects.setColor(chosen, picker.target.effect); });
+    }
+    notify();
+}
+
+void EditorSession::openGradientMapColorPicker(bool highlights)
+{
+    // A commit keeps the project busy: no `committing` term.
+    if (!canEditPalette() || m_colorPicker || !m_filterEdit || m_filterEdit->kind != FilterKind::gradientMap)
+        return;
+    const AdjustmentColor value = highlights ? m_filterEdit->settings.gradientMap.highlights : m_filterEdit->settings.gradientMap.shadows;
+    m_colorPicker = ColorPickerState({ColorPickerTarget::Kind::gradientMap, false, std::nullopt, highlights}, PaletteColor{value.red, value.green, value.blue});
+    notify();
+}
+
+void EditorSession::openEffectColorPicker(LayerEffectKind kind)
+{
+    if (!canEditPalette() || m_colorPicker || !m_effectsEditing)
+        return;
+    m_colorPicker = ColorPickerState({ColorPickerTarget::Kind::effect, false, std::nullopt, false, kind}, editingEffects().color(kind).value_or(PaletteColor::black()));
+    notify();
+}
+
+void EditorSession::previewEffectColor()
+{
+    if (!m_colorPicker || m_colorPicker->target.kind != ColorPickerTarget::Kind::effect)
+        return;
+    const PaletteColor color = m_colorPicker->color();
+    const LayerEffectKind kind = m_colorPicker->target.effect;
+    changeEffects([&](LayerEffects &effects) { effects.setColor(color, kind); });
+}
+
+void EditorSession::previewGradientMapColor()
+{
+    if (m_colorPicker && m_colorPicker->target.kind == ColorPickerTarget::Kind::gradientMap)
+        setGradientMapColor(m_colorPicker->color(), m_colorPicker->target.highlights);
+}
+
+void EditorSession::setGradientMapColor(const PaletteColor &color, bool highlights)
+{
+    // An adjustment's editor can close under its picker (9.5c).
+    if (!m_filterEdit || m_filterEdit->kind != FilterKind::gradientMap)
+        return;
+    const FilterEdit &edit = *m_filterEdit;
+    FilterSettings settings = edit.settings;
+    (highlights ? settings.gradientMap.highlights : settings.gradientMap.shadows) = AdjustmentColor(color);
+    if (settings == edit.settings)
+        return;
+    updateFilter(settings, edit.preview);
+}
+
+void EditorSession::setColorPickerHSB(const PickerHSB &hsb)
+{
+    if (!m_colorPicker || m_colorPicker->hsb == hsb)
+        return;
+    m_colorPicker->hsb = hsb;
+    notify();
+}
+
+void EditorSession::sampleIntoColorPicker(QPointF point)
+{
+    if (!m_colorPicker)
+        return;
+    const std::optional<PaletteColor> color = sampleCompositeColor(point);
+    if (!color)
+        return;
+    PickerHSB hsb = m_colorPicker->hsb;
+    hsb.setRGB(*color);
+    setColorPickerHSB(hsb);
+}
+
+std::optional<PaletteColor> EditorSession::sampleCompositeColor(QPointF point) const
+{
+    if (!m_document || !(point.x() >= 0 && point.y() >= 0 && point.x() < m_document->width && point.y() < m_document->height))
+        return std::nullopt;
+    try {
+        // The pixel under the point, drawn as the canvas shows.
+        QImage pixel = BrushRaster::context(1, 1, false);
+        QPainter painter(&pixel);
+        painter.translate(-std::floor(point.x()), -std::floor(point.y()));
+        drawLiveComposite(*m_document, painter);
+        painter.end();
+        const uchar *bytes = pixel.constBits();
+        const int alpha = bytes[3];
+        if (alpha == 0)
+            return std::nullopt;
+        const auto channel = [alpha](int value) { return std::round(double(std::min(alpha, value)) / alpha * 255) / 255; };
+        return PaletteColor{channel(bytes[0]), channel(bytes[1]), channel(bytes[2])};
+    } catch (const ExportError &error) {
+        qCWarning(lcApp) << "the colour under the pointer cannot be read:" << error.what();
+        return std::nullopt;
+    }
+}

src/Document/ColorPalette.h

@@ -0,0 +1,61 @@
+#pragma once
+#include <QColor>
+#include <QString>
+#include <QUuid>
+#include <optional>
+
+// Swift's PaletteColor: an sRGB colour without alpha.
+struct PaletteColor {
+    double red;
+    double green;
+    double blue;
+    static constexpr PaletteColor black() { return {0, 0, 0}; }
+    static constexpr PaletteColor white() { return {1, 1, 1}; }
+    QColor color(double alpha = 1) const { return QColor::fromRgbF(float(red), float(green), float(blue), float(alpha)); }
+    // The 8-bit values painting and export store.
+    PaletteColor quantized() const;
+    QString hex() const;
+    // Swift's init?(hex:): RRGGBB or RGB, with or without #.
+    static std::optional<PaletteColor> fromHex(const QString &text);
+    friend bool operator==(const PaletteColor &, const PaletteColor &) = default;
+};
+
+// The picker's own truth, so hue survives grays and black.
+struct PickerHSB {
+    PickerHSB(double hue, double saturation, double brightness) : hue(hue), saturation(saturation), brightness(brightness) {}
+    explicit PickerHSB(const PaletteColor &color);
+    double hue;
+    double saturation;
+    double brightness;
+    PaletteColor rgb() const;
+    // Grays keep the hue, black the saturation, as Photoshop's.
+    void setRGB(const PaletteColor &color);
+    friend bool operator==(const PickerHSB &, const PickerHSB &) = default;
+};
+
+enum class LayerEffectKind;
+
+// The picker's targets: palette, text, effect, Gradient Map end.
+struct ColorPickerTarget {
+    enum class Kind { palette, text, gradientMap, effect };
+    Kind kind;
+    bool background = false;
+    std::optional<QUuid> draftID = std::nullopt;
+    // A Gradient Map's end: highlights, else shadows.
+    bool highlights = false;
+    // An effect's colour: which effect.
+    LayerEffectKind effect{};
+    QString title() const;
+    friend bool operator==(const ColorPickerTarget &, const ColorPickerTarget &) = default;
+};
+
+// The open picker's working colour: nothing is written until OK.
+struct ColorPickerState {
+    ColorPickerState(const ColorPickerTarget &target, const PaletteColor &original) : target(target), original(original), hsb(original) {}
+    ColorPickerTarget target;
+    PaletteColor original;
+    PickerHSB hsb;
+    // Swift's object identity: a new picker opens its panel anew.
+    QUuid id = QUuid::createUuid();
+    PaletteColor color() const { return hsb.rgb().quantized(); }
+};

src/Document/ContentFill.cpp

@@ -0,0 +1,33 @@
+#include "Document/ContentFill.h"
+#include "Document/BrushStroke.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+
+// The kernel shares this file's name: angle brackets skip here.
+extern "C" {
+#include <ContentFill.h>
+}
+
+ContentFillError::ContentFillError(Kind kind)
+    : std::runtime_error("Not enough unselected, opaque image pixels to synthesize a fill. Use a smaller selection with some surrounding image."), kind(kind)
+{
+}
+
+QImage ContentFill::run(const FilterJob &job)
+{
+    if (!job.selection)
+        throw ContentFillError(ContentFillError::Kind::noSource);
+    const int width = job.image.width(), height = job.image.height();
+    QImage pixels = BrushRaster::context(width, height, false);
+    QPainter painter(&pixels);
+    BrushRaster::draw(job.image, QRectF(0, 0, width, height), painter);
+    painter.end();
+    // White through the selection: the pixels to make.
+    const QImage mask = PixelAdjust::coverage(*job.selection, width, height, job.mapping);
+    const int result = content_fill(pixels.bits(), size_t(pixels.bytesPerLine()), mask.constBits(), size_t(mask.bytesPerLine()), width, height);
+    if (result == 0)
+        throw ContentFillError(ContentFillError::Kind::noSource);
+    if (result != 1)
+        throw ExportError(ExportError::Kind::render);
+    return pixels;
+}

src/Document/ContentFill.h

@@ -0,0 +1,16 @@
+#pragma once
+#include "Document/Filters.h"
+#include <stdexcept>
+
+// Swift's ContentFill.Failure.
+class ContentFillError : public std::runtime_error {
+public:
+    enum class Kind { noSource };
+    explicit ContentFillError(Kind kind);
+    const Kind kind;
+};
+
+// Content-Aware Fill: the selection made from the pixels around it.
+namespace ContentFill {
+QImage run(const FilterJob &job);
+}

src/Document/Crop+Session.h

@@ -0,0 +1,15 @@
+// Swift's Crop extension: snapping, the frame, the crop itself.
+public:
+    SnapGuides transformSnapTargets(const QSet<QUuid> &moving) const;
+    LayerTransform snappedMove(const LayerTransform &draft, const QSet<QUuid> &moving, double tolerance);
+    SnapGuides cropSnapTargets() const;
+    // The Crop tool's frame: the canvas until one is drawn.
+    std::optional<QRectF> visibleCropRect() const;
+    std::optional<double> cropRatio() const;
+    void cancelCrop();
+    void changeCropRatio();
+    // Off the UI thread; `done` follows the step or refusal.
+    void commitCrop(std::function<void()> done = {});
+
+private:
+    void finishCropCommit();

src/Document/Crop.cpp

@@ -0,0 +1,293 @@
+#include "Document/Crop.h"
+#include "Document/Distort.h"
+#include "Document/EditorSession.h"
+#include "IO/CanvasResizer.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include <QtConcurrent>
+#include <cmath>
+#include <new>
+
+QRectF CropGeometry::snapped(const QRectF &rect)
+{
+    const QRectF standard = rect.normalized();
+    const double x = std::round(standard.left()), y = std::round(standard.top());
+    return QRectF(x, y, std::max(1.0, std::round(standard.right()) - x), std::max(1.0, std::round(standard.bottom()) - y));
+}
+
+// Swift's isFinite terms are the ranges' own: NaN passes none.
+bool CropGeometry::valid(const QRectF &rect)
+{
+    const QRectF standard = rect.normalized();
+    return standard.width() >= 1 && standard.width() <= 30'000 && standard.height() >= 1 && standard.height() <= 30'000
+        && std::abs(standard.left()) <= 1'000'000 && std::abs(standard.top()) <= 1'000'000;
+}
+
+QRectF CropGeometry::create(QPointF start, QPointF end, std::optional<double> ratio, bool symmetric)
+{
+    double dx = end.x() - start.x(), dy = end.y() - start.y();
+    if (ratio) {
+        if (std::abs(dx) > std::abs(dy) * *ratio)
+            dy = (dy < 0 ? -1 : 1) * std::abs(dx) / *ratio;
+        else
+            dx = (dx < 0 ? -1 : 1) * std::abs(dy) * *ratio;
+    }
+    if (symmetric)
+        return snapped(QRectF(start.x() - std::abs(dx), start.y() - std::abs(dy), std::abs(dx) * 2, std::abs(dy) * 2));
+    return snapped(QRectF(std::min(start.x(), start.x() + dx), std::min(start.y(), start.y() + dy), std::abs(dx), std::abs(dy)));
+}
+
+QRectF CropDrag::updated(QPointF point, std::optional<double> ratio, bool symmetric) const
+{
+    switch (mode.kind) {
+    case Kind::create: return CropGeometry::create(start, point, ratio, symmetric);
+    case Kind::move: return CropGeometry::snapped(original.translated(point - start));
+    case Kind::resize: {
+        const TransformDrag drag{.original = LayerTransform{.origin = original.topLeft(), .size = original.size()},
+                                 .start = start,
+                                 .mode = {TransformDrag::Kind::resize, mode.index},
+                                 .originalCorners = std::nullopt};
+        const LayerTransform next = drag.updated(point, ratio.has_value(), false, symmetric);
+        return CropGeometry::snapped(QRectF(next.origin, next.size));
+    }
+    }
+    throw std::logic_error("unknown crop drag");
+}
+
+namespace {
+std::optional<double> nearest(double value, const std::vector<double> &targets, double tolerance)
+{
+    std::optional<double> best;
+    for (const double target : targets) {
+        if (std::abs(target - value) > tolerance || (best && std::abs(*best - value) <= std::abs(target - value)))
+            continue;
+        best = target;
+    }
+    return best;
+}
+
+// The smallest shift that lands an edge on a target.
+double shift(std::initializer_list<double> edges, const std::vector<double> &targets, double tolerance)
+{
+    std::optional<double> best;
+    for (const double edge : edges) {
+        const std::optional<double> target = nearest(edge, targets, tolerance);
+        if (target && (!best || std::abs(*target - edge) < std::abs(*best)))
+            best = *target - edge;
+    }
+    return best.value_or(0);
+}
+}
+
+// Moves snap their closest edges; the rest, the dragged side's.
+QRectF CropSnap::apply(QRectF rect, const CropDrag &drag, QPointF point, std::optional<double> ratio, bool symmetric) const
+{
+    if (!(tolerance > 0))
+        return rect;
+    bool horizontal = true, vertical = true;
+    switch (drag.mode.kind) {
+    case CropDrag::Kind::move: return rect.translated(shift({rect.left(), rect.right()}, xs, tolerance), shift({rect.top(), rect.bottom()}, ys, tolerance));
+    // A fixed ratio snaps moves alone, so it stays exact.
+    case CropDrag::Kind::create:
+        if (ratio)
+            return rect;
+        break;
+    case CropDrag::Kind::resize:
+        if (ratio)
+            return rect;
+        horizontal = LayerTransform::handles[size_t(drag.mode.index)].x() != 0.5;
+        vertical = LayerTransform::handles[size_t(drag.mode.index)].y() != 0.5;
+        break;
+    }
+    QRectF result = rect;
+    if (horizontal) {
+        if (std::abs(point.x() - result.left()) <= std::abs(point.x() - result.right())) {
+            if (const std::optional<double> x = nearest(result.left(), xs, tolerance); x && *x < result.right())
+                result = QRectF(*x, result.top(), result.right() - *x, result.height());
+        } else if (const std::optional<double> x = nearest(result.right(), xs, tolerance); x && *x > result.left()) {
+            result.setWidth(*x - result.left());
+        }
+    }
+    if (vertical) {
+        if (std::abs(point.y() - result.top()) <= std::abs(point.y() - result.bottom())) {
+            if (const std::optional<double> y = nearest(result.top(), ys, tolerance); y && *y < result.bottom())
+                result = QRectF(result.left(), *y, result.width(), result.bottom() - *y);
+        } else if (const std::optional<double> y = nearest(result.bottom(), ys, tolerance); y && *y > result.top()) {
+            result.setHeight(*y - result.top());
+        }
+    }
+    if (symmetric) {
+        // The snapped edge sets the half; its opposite mirrors it.
+        const QPointF centre = drag.mode.kind == CropDrag::Kind::create ? drag.start : drag.original.center();
+        if (horizontal) {
+            const double half = point.x() >= centre.x() ? result.right() - centre.x() : centre.x() - result.left();
+            if (half >= 0.5)
+                result = QRectF(centre.x() - half, result.top(), half * 2, result.height());
+        }
+        if (vertical) {
+            const double half = point.y() >= centre.y() ? result.bottom() - centre.y() : centre.y() - result.top();
+            if (half >= 0.5)
+                result = QRectF(result.left(), centre.y() - half, result.width(), half * 2);
+        }
+    }
+    return result;
+}
+
+namespace {
+QRectF boxAround(const LayerTransform &transform)
+{
+    const Corners corners = DistortWarp::corners(transform);
+    QPointF low = corners[0], high = corners[0];
+    for (const QPointF corner : corners) {
+        low = {std::min(low.x(), corner.x()), std::min(low.y(), corner.y())};
+        high = {std::max(high.x(), corner.x()), std::max(high.y(), corner.y())};
+    }
+    return QRectF(low, high);
+}
+}
+
+// The canvas's edges and middle, and every other visible layer's.
+SnapGuides EditorSession::transformSnapTargets(const QSet<QUuid> &moving) const
+{
+    SnapGuides targets;
+    if (!m_document)
+        return targets;
+    const QSizeF size = m_document->size();
+    targets.xs = {0, size.width() / 2, size.width()};
+    targets.ys = {0, size.height() / 2, size.height()};
+    for (const ImageLayer &layer : m_document->renderLayers()) {
+        if (!layer.asset || moving.contains(layer.id))
+            continue;
+        const QRectF box = boxAround(displayedTransform(layer));
+        targets.xs.insert(targets.xs.end(), {std::round(box.left()), std::round(box.center().x()), std::round(box.right())});
+        targets.ys.insert(targets.ys.end(), {std::round(box.top()), std::round(box.center().y()), std::round(box.bottom())});
+    }
+    return targets;
+}
+
+// `draft` nudged onto a nearby edge or middle, within `tolerance`.
+LayerTransform EditorSession::snappedMove(const LayerTransform &draft, const QSet<QUuid> &moving, double tolerance)
+{
+    const SnapGuides targets = transformSnapTargets(moving);
+    const TransformSnap::Offset snap = TransformSnap::offset(boxAround(draft), targets.xs, targets.ys, tolerance);
+    snapGuides = {snap.x ? std::vector<double>{*snap.x} : std::vector<double>{}, snap.y ? std::vector<double>{*snap.y} : std::vector<double>{}};
+    if (snap.offset.isNull())
+        return draft;
+    LayerTransform snapped = draft;
+    snapped.origin += QPointF(snap.offset.width(), snap.offset.height());
+    return snapped;
+}
+
+// The canvas's edges and every visible layer's upright bounds.
+SnapGuides EditorSession::cropSnapTargets() const
+{
+    SnapGuides targets;
+    if (!m_document)
+        return targets;
+    const QSizeF size = m_document->size();
+    targets.xs = {0, size.width()};
+    targets.ys = {0, size.height()};
+    for (const ImageLayer &layer : m_document->renderLayers()) {
+        if (!layer.asset)
+            continue;
+        const QRectF box = boxAround(displayedTransform(layer));
+        targets.xs.insert(targets.xs.end(), {std::round(box.left()), std::round(box.right())});
+        targets.ys.insert(targets.ys.end(), {std::round(box.top()), std::round(box.bottom())});
+    }
+    return targets;
+}
+
+std::optional<QRectF> EditorSession::visibleCropRect() const
+{
+    if (m_tool != NavigationTool::crop || !m_document)
+        return std::nullopt;
+    return m_cropRect ? m_cropRect : QRectF(QPointF(0, 0), m_document->size());
+}
+
+std::optional<double> EditorSession::cropRatio() const
+{
+    if (m_cropRatioChoice == QStringLiteral("Original"))
+        return m_document ? std::optional(double(m_document->width) / double(m_document->height)) : std::nullopt;
+    if (m_cropRatioChoice == QStringLiteral("1:1"))
+        return 1;
+    if (m_cropRatioChoice == QStringLiteral("4:3"))
+        return 4.0 / 3;
+    if (m_cropRatioChoice == QStringLiteral("16:9"))
+        return 16.0 / 9;
+    return std::nullopt;
+}
+
+void EditorSession::cancelCrop()
+{
+    setCropRect(std::nullopt);
+}
+
+// The frame takes the ratio about its middle, width kept.
+void EditorSession::changeCropRatio()
+{
+    const std::optional<QRectF> rect = visibleCropRect();
+    const std::optional<double> ratio = cropRatio();
+    if (!rect || !ratio)
+        return;
+    const double height = rect->width() / *ratio;
+    const QRectF next = CropGeometry::snapped(QRectF(rect->left(), rect->center().y() - height / 2, rect->width(), height));
+    if (CropGeometry::valid(next))
+        setCropRect(next);
+}
+
+// The canvas cut to the frame off the UI thread.
+void EditorSession::commitCrop(std::function<void()> done)
+{
+    bool started = false;
+    if (canStartProjectOperation() && m_cropRect && CropGeometry::valid(*m_cropRect)) {
+        // Swift reads a CGRect's width and minX standardised.
+        const QRectF rect = m_cropRect->normalized();
+        const CanvasSizeOptions options{.width = qint64(rect.width()), .height = qint64(rect.height()), .contentOffset = QPointF(-rect.left(), -rect.top())};
+        // Taken before anything changes: running out changes nothing.
+        try {
+            std::optional<ProjectSnapshot> snapshot = projectSnapshot();
+            if (snapshot) {
+                m_cropCommit.setFuture(QtConcurrent::run([snapshot = std::move(*snapshot), options]() -> Cropped {
+                    try {
+                        return Cropped{CanvasResizer::resize(snapshot, options), std::nullopt};
+                    } catch (const ProjectError &error) {
+                        return Cropped{std::nullopt, QString::fromUtf8(error.what())};
+                    } catch (const ExportError &error) {
+                        return Cropped{std::nullopt, QString::fromUtf8(error.what())};
+                    }
+                }));
+                started = true;
+            }
+        } catch (const std::bad_alloc &) {
+            setCropError(QString::fromUtf8(ExportError(ExportError::Kind::render).what()));
+        }
+    }
+    if (!started) {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+        return;
+    }
+    m_committingCrop = std::move(done);
+    setIsProjectBusy(true);
+}
+
+void EditorSession::finishCropCommit()
+{
+    const std::function<void()> done = std::exchange(m_committingCrop, {});
+    try {
+        // Moved out: a large project's copy could fail.
+        const Cropped result = m_cropCommit.future().takeResult();
+        if (result.snapshot) {
+            applyDocumentSize(*result.snapshot, QStringLiteral("Crop"));
+            // Only a crop that landed lets its frame go.
+            m_cropRect.reset();
+        } else {
+            setCropError(result.failure);
+        }
+    } catch (const std::bad_alloc &) {
+        setCropError(QString::fromUtf8(ExportError(ExportError::Kind::render).what()));
+    }
+    setIsProjectBusy(false);
+    if (done)
+        QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+}

src/Document/Crop.h

@@ -0,0 +1,36 @@
+#pragma once
+#include "Document/LayerTransform.h"
+#include <QRectF>
+#include <optional>
+#include <vector>
+
+// Swift's CropGeometry: frames on whole pixels, within Swift's bounds.
+namespace CropGeometry {
+QRectF snapped(const QRectF &rect);
+bool valid(const QRectF &rect);
+// From `start` to `end`, or out from `start` when symmetric.
+QRectF create(QPointF start, QPointF end, std::optional<double> ratio, bool symmetric = false);
+}
+
+// Swift's CropDrag: a frame being made, moved or resized.
+struct CropDrag {
+    enum class Kind { create, move, resize };
+    struct Mode {
+        Kind kind;
+        int index = 0;
+    };
+    QPointF start;
+    QRectF original;
+    Mode mode;
+    // Symmetric (Alt) keeps the frame's middle still.
+    QRectF updated(QPointF point, std::optional<double> ratio, bool symmetric = false) const;
+};
+
+// Swift's CropSnap: dragged edges drawn to nearby edges.
+struct CropSnap {
+    std::vector<double> xs;
+    std::vector<double> ys;
+    // In document pixels.
+    double tolerance;
+    QRectF apply(QRectF rect, const CropDrag &drag, QPointF point, std::optional<double> ratio, bool symmetric = false) const;
+};

src/Document/Curves.cpp

@@ -0,0 +1,67 @@
+#include "Document/Curves.h"
+#include "Document/BrushStroke.h"
+#include "IO/ProjectStore.h"
+#include <QPainter>
+#include <algorithm>
+#include <array>
+#include <cmath>
+extern "C" {
+#include "LevelsPixels.h"
+}
+
+bool CurvesSettings::isValid() const
+{
+    // Fixed ends and rising x keep x in range.
+    return channels.size() == 4 && std::all_of(channels.begin(), channels.end(), [](const std::vector<CurvePoint> &points) {
+        return points.size() >= 2 && points.size() <= 32 && points.front().x == 0 && points.back().x == 255
+            && std::all_of(points.begin(), points.end(), [](const CurvePoint &point) { return point.y >= 0 && point.y <= 255; })
+            && std::adjacent_find(points.begin(), points.end(), [](const CurvePoint &a, const CurvePoint &b) { return !(a.x < b.x); }) == points.end();
+    });
+}
+
+double CurvesSettings::value(double x, size_t channel) const
+{
+    const std::vector<CurvePoint> &p = channels.at(channel);
+    // Swift's lastIndex(where:), else the first segment.
+    size_t last = 0;
+    for (size_t index = 0; index < p.size(); ++index) {
+        if (p[index].x <= x)
+            last = index;
+    }
+    const size_t i = std::min(p.size() - 2, last);
+    std::vector<double> d;
+    for (size_t index = 0; index + 1 < p.size(); ++index)
+        d.push_back((p[index + 1].y - p[index].y) / (p[index + 1].x - p[index].x));
+    const auto slope = [&](size_t j) {
+        if (j == 0)
+            return d.front();
+        if (j == p.size() - 1)
+            return d.back();
+        if (d[j - 1] * d[j] <= 0)
+            return 0.0;
+        return 2 / (1 / d[j - 1] + 1 / d[j]);
+    };
+    const double h = p[i + 1].x - p[i].x, t = std::min(1.0, std::max(0.0, (x - p[i].x) / h));
+    const double y = (2 * t * t * t - 3 * t * t + 1) * p[i].y + (t * t * t - 2 * t * t + t) * h * slope(i)
+        + (-2 * t * t * t + 3 * t * t) * p[i + 1].y + (t * t * t - t * t) * h * slope(i + 1);
+    return std::min(255.0, std::max(0.0, y));
+}
+
+QImage CurvesSettings::apply(const QImage &image) const
+{
+    if (!isValid())
+        throw ProjectError(ProjectError::Kind::invalid);
+    QImage context = BrushRaster::context(image.width(), image.height(), false);
+    {
+        QPainter painter(&context);
+        BrushRaster::draw(image, QRectF(0, 0, image.width(), image.height()), painter);
+    }
+    // Each colour's curve, then the composite's.
+    std::array<float, 3 * 256> tables;
+    for (size_t channel = 1; channel <= 3; ++channel) {
+        for (size_t x = 0; x < 256; ++x)
+            tables[(channel - 1) * 256 + x] = float(value(value(double(x), channel), 0) / 255);
+    }
+    levels_apply(context.bits(), size_t(image.width()) * size_t(image.height()), tables.data());
+    return context;
+}

src/Document/Curves.h

@@ -0,0 +1,22 @@
+#pragma once
+#include "Document/Levels.h"
+#include <QImage>
+#include <vector>
+
+struct CurvePoint {
+    double x;
+    double y;
+    friend bool operator==(const CurvePoint &, const CurvePoint &) = default;
+};
+
+// Swift's CurvesSettings: a curve a channel, the composite first.
+struct CurvesSettings {
+    LevelsChannel channel = LevelsChannel::rgb;
+    std::vector<std::vector<CurvePoint>> channels = std::vector<std::vector<CurvePoint>>(4, {{0, 0}, {255, 255}});
+    bool isValid() const;
+    // Shape-preserving cubic Hermite: no overshoot between handles.
+    double value(double x, size_t channel) const;
+    // Throws ProjectError when invalid, ExportError when out of memory.
+    QImage apply(const QImage &image) const;
+    friend bool operator==(const CurvesSettings &, const CurvesSettings &) = default;
+};

src/Document/Distort+Session.h

@@ -0,0 +1,14 @@
+// Swift's Distort extension: four corners dragged apart.
+public:
+    void beginDistort();
+    void previewCorners(const Corners &corners);
+    std::optional<DistortPreview> distortPreview(const ImageLayer &layer) const;
+    // A layer's effects warped with the pending distortion.
+    std::optional<DistortWarp::Warped> distortedEffects(const ImageLayer &layer, const QImage &image, double inset) const;
+    // For a known target: the commit's, once the edit ends.
+    std::optional<DistortWarp::Warped> distortedEffects(const ImageLayer &layer, const QImage &image, double inset, const DistortTarget &target) const;
+
+private:
+    std::optional<DistortTarget> distortTarget(const ImageLayer &layer, const TransformEdit &edit, const Corners &shape) const;
+    void commitDistort(const TransformEdit &edit, const Corners &shape);
+    void distort(int index, const LayerTransform &transform, const Corners &corners);

src/Document/Distort.cpp

@@ -0,0 +1,482 @@
+#include "Document/Distort.h"
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "Document/LayerEffects+Renderer.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include <QPainterPath>
+#include <QPolygonF>
+#include <QRegion>
+#include <cmath>
+#include <functional>
+
+extern "C" {
+#include "BrushPixels.h"
+}
+
+namespace {
+// Swift divides by w whatever its sign; Qt 6.11 clips.
+QPointF project(const QTransform &map, QPointF point)
+{
+    const double w = map.m13() * point.x() + map.m23() * point.y() + map.m33();
+    return QPointF((map.m11() * point.x() + map.m21() * point.y() + map.m31()) / w, (map.m12() * point.x() + map.m22() * point.y() + map.m32()) / w);
+}
+
+double area(QPointF a, QPointF b, QPointF c)
+{
+    return (b.x() - a.x()) * (c.y() - a.y()) - (b.y() - a.y()) * (c.x() - a.x());
+}
+
+using Triangle = std::array<QPointF, 3>;
+
+// The affine map taking three points to three others.
+std::optional<QTransform> affine(const Triangle &source, const Triangle &target)
+{
+    const QPointF u = source[1] - source[0], v = source[2] - source[0];
+    const QPointF uu = target[1] - target[0], vv = target[2] - target[0];
+    const double det = u.x() * v.y() - v.x() * u.y();
+    if (std::abs(det) <= 1e-9)
+        return std::nullopt;
+    const double a = (uu.x() * v.y() - vv.x() * u.y()) / det, c = (vv.x() * u.x() - uu.x() * v.x()) / det;
+    const double b = (uu.y() * v.y() - vv.y() * u.y()) / det, d = (vv.y() * u.x() - uu.y() * v.x()) / det;
+    return QTransform(a, b, c, d, target[0].x() - (a * source[0].x() + c * source[0].y()), target[0].y() - (b * source[0].x() + d * source[0].y()));
+}
+
+struct Target {
+    QPointF topLeft, topRight, bottomRight, bottomLeft;
+};
+
+// A flipped layer's pixels go to the opposite corners.
+Target imageCorners(const Corners &corners, bool flipX, bool flipY)
+{
+    const auto corner = [&](int x, int y) {
+        const int u = flipX ? 1 - x : x, v = flipY ? 1 - y : y;
+        return corners[size_t(std::array<int, 4>{0, 1, 3, 2}[size_t(v * 2 + u)])];
+    };
+    return {corner(0, 0), corner(1, 0), corner(1, 1), corner(0, 1)};
+}
+
+QRectF shapeBounds(const Corners &corners)
+{
+    QPointF low = corners[0], high = corners[0];
+    for (const QPointF corner : corners) {
+        low = {std::min(low.x(), corner.x()), std::min(low.y(), corner.y())};
+        high = {std::max(high.x(), corner.x()), std::max(high.y(), corner.y())};
+    }
+    return QRectF(QPointF(std::floor(low.x()), std::floor(low.y())), QPointF(std::ceil(high.x()), std::ceil(high.y())));
+}
+
+// Pixels whose centres a triangle holds, left bound inclusive.
+QRegion triangleRegion(const Triangle &triangle)
+{
+    QRegion region;
+    const double top = std::min({triangle[0].y(), triangle[1].y(), triangle[2].y()});
+    const double bottom = std::max({triangle[0].y(), triangle[1].y(), triangle[2].y()});
+    for (int row = int(std::ceil(top - 0.5)); row + 0.5 < bottom; ++row) {
+        const double y = row + 0.5;
+        std::optional<double> left, right;
+        for (size_t index = 0; index < 3; ++index) {
+            // Ends ordered by y: a shared edge gives one x.
+            const QPointF a = std::min(triangle[index], triangle[(index + 1) % 3], [](QPointF p, QPointF q) { return p.y() < q.y(); });
+            const QPointF b = a == triangle[index] ? triangle[(index + 1) % 3] : triangle[index];
+            // An edge holds its lower end, not its upper one.
+            if (a.y() == b.y() || y < a.y() || y >= b.y())
+                continue;
+            const double x = a.x() + (b.x() - a.x()) * (y - a.y()) / (b.y() - a.y());
+            left = std::min(left.value_or(x), x);
+            right = std::max(right.value_or(x), x);
+        }
+        if (!left)
+            continue;
+        const int first = int(std::ceil(*left - 0.5)), last = int(std::ceil(*right - 0.5)) - 1;
+        if (last >= first)
+            region += QRect(first, row, last - first + 1, 1);
+    }
+    return region;
+}
+
+// A folded shape: each half by its own affine map.
+void warpFolded(const QImage &image, const Target &target, const std::function<QPointF(QPointF)> &place, QPainter &painter)
+{
+    const QRectF source(0, 0, image.width(), image.height());
+    const std::array<std::pair<Triangle, Triangle>, 2> halves = {
+        std::pair{Triangle{source.topLeft(), source.topRight(), source.bottomRight()}, Triangle{place(target.topLeft), place(target.topRight), place(target.bottomRight)}},
+        std::pair{Triangle{source.topLeft(), source.bottomRight(), source.bottomLeft()}, Triangle{place(target.topLeft), place(target.bottomRight), place(target.bottomLeft)}}};
+    for (const auto &[from, to] : halves) {
+        const std::optional<QTransform> map = affine(from, to);
+        if (!map)
+            continue;
+        painter.save();
+        // A hard region along the diagonal: the halves meet exactly.
+        painter.setClipRegion(triangleRegion(to));
+        painter.setTransform(*map);
+        BrushRaster::draw(image, source, painter);
+        painter.restore();
+    }
+}
+}
+
+Corners DistortWarp::corners(const LayerTransform &transform)
+{
+    return {transform.point({0, 0}), transform.point({1, 0}), transform.point({1, 1}), transform.point({0, 1})};
+}
+
+bool DistortWarp::isUsable(const Corners &corners)
+{
+    for (const QPointF corner : corners) {
+        if (!std::isfinite(corner.x()) || !std::isfinite(corner.y()) || std::abs(corner.x()) > 1'000'000 || std::abs(corner.y()) > 1'000'000)
+            return false;
+    }
+    // Both halves need area, or one has nothing to draw.
+    return std::abs(area(corners[0], corners[1], corners[2])) > 0.01 && std::abs(area(corners[0], corners[2], corners[3])) > 0.01;
+}
+
+bool DistortWarp::isConvex(const Corners &corners)
+{
+    if (!isUsable(corners))
+        return false;
+    double sign = 0;
+    for (size_t index = 0; index < 4; ++index) {
+        const QPointF a = corners[index], b = corners[(index + 1) % 4], c = corners[(index + 2) % 4];
+        const double cross = (b.x() - a.x()) * (c.y() - b.y()) - (b.y() - a.y()) * (c.x() - b.x());
+        if (std::abs(cross) <= 0.01)
+            return false;
+        if (sign == 0)
+            sign = cross < 0 ? -1 : 1;
+        else if ((cross < 0) != (sign < 0))
+            return false;
+    }
+    return true;
+}
+
+QTransform DistortWarp::homography(const Corners &c)
+{
+    const double sx = c[0].x() - c[1].x() + c[2].x() - c[3].x(), sy = c[0].y() - c[1].y() + c[2].y() - c[3].y();
+    double g = 0, h = 0;
+    if (std::abs(sx) > 1e-9 || std::abs(sy) > 1e-9) {
+        const double dx1 = c[1].x() - c[2].x(), dx2 = c[3].x() - c[2].x(), dy1 = c[1].y() - c[2].y(), dy2 = c[3].y() - c[2].y();
+        const double den = dx1 * dy2 - dx2 * dy1;
+        if (std::abs(den) > 1e-12) {
+            g = (sx * dy2 - dx2 * sy) / den;
+            h = (dx1 * sy - sx * dy1) / den;
+        }
+    }
+    const double a = c[1].x() - c[0].x() + g * c[1].x(), b = c[3].x() - c[0].x() + h * c[3].x();
+    const double d = c[1].y() - c[0].y() + g * c[1].y(), e = c[3].y() - c[0].y() + h * c[3].y();
+    // Qt divides by the third row, as Swift by w.
+    return QTransform(a, d, g, b, e, h, c[0].x(), c[0].y(), 1);
+}
+
+DistortWarp::Warped DistortWarp::warp(const QImage &image, const LayerTransform &transform, const Corners &corners, bool isMask,
+                                      std::optional<double> limit)
+{
+    if (!isUsable(corners))
+        throw ProjectError(ProjectError::Kind::invalid);
+    const QRectF bounds = shapeBounds(corners);
+    if (bounds.width() < 1 || bounds.height() < 1 || bounds.width() > 30'000 || bounds.height() > 30'000 || bounds.width() * bounds.height() > 100'000'000)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    const LayerTransform placed{.origin = bounds.topLeft(), .size = bounds.size(), .sampling = transform.sampling};
+    // A uniform 1 × 1 mask already covers any shape.
+    if (isMask && image.width() == 1 && image.height() == 1)
+        return {image, placed};
+    const double factor = limit ? std::min(1.0, *limit / std::max(bounds.width(), bounds.height())) : 1;
+    const int width = std::max(1, int(std::ceil(bounds.width() * factor))), height = std::max(1, int(std::ceil(bounds.height() * factor)));
+    const Target target = imageCorners(corners, transform.flipX, transform.flipY);
+    const auto place = [&](QPointF point) { return QPointF((point.x() - bounds.left()) * factor, (point.y() - bounds.top()) * factor); };
+    QImage surface = BrushRaster::context(width, height, isMask);
+    QPainter painter(&surface);
+    painter.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
+    if (!isConvex(corners)) {
+        warpFolded(image, target, place, painter);
+        return {surface, placed};
+    }
+    const QRectF source(0, 0, image.width(), image.height());
+    QTransform map;
+    if (!QTransform::quadToQuad(QPolygonF({source.topLeft(), source.topRight(), source.bottomRight(), source.bottomLeft()}),
+                                QPolygonF({place(target.topLeft), place(target.topRight), place(target.bottomRight), place(target.bottomLeft)}), map))
+        throw ExportError(ExportError::Kind::render);
+    painter.setTransform(map);
+    painter.drawImage(source, image);
+    return {surface, placed};
+}
+
+DistortWarp::Trimmed DistortWarp::warpTrimmed(const QImage &image, const LayerTransform &transform, const Corners &corners)
+{
+    const Warped warped = warp(image, transform, corners, false);
+    const QRect full = warped.image.rect();
+    size_t edges[4] = {0, 0, 0, 0};
+    brush_alpha_bounds(warped.image.constBits(), size_t(full.width()), size_t(full.height()), size_t(warped.image.bytesPerLine()), edges);
+    const QRect crop(int(edges[0]), int(edges[1]), int(edges[2] - edges[0]), int(edges[3] - edges[1]));
+    // Nothing visible, or nothing to trim: the warp itself.
+    if (crop.width() < 1 || crop.height() < 1 || crop == full)
+        return {warped.image, warped.transform, full};
+    LayerTransform placed = warped.transform;
+    placed.origin += crop.topLeft();
+    placed.size = crop.size();
+    return {warped.image.copy(crop), placed, crop};
+}
+
+Corners DistortWarp::carried(const LayerTransform &placement, const LayerTransform &transform, const Corners &corners)
+{
+    QTransform toDocument;
+    toDocument.translate(transform.center().x(), transform.center().y());
+    toDocument.rotateRadians(transform.radians());
+    toDocument.scale(transform.size.width(), transform.size.height());
+    toDocument.translate(-0.5, -0.5);
+    const QTransform toUnit = toDocument.inverted();
+    const QTransform map = homography(corners);
+    const Corners own = DistortWarp::corners(placement);
+    Corners result;
+    for (size_t index = 0; index < 4; ++index)
+        result[index] = project(map, toUnit.map(own[index]));
+    return result;
+}
+
+DistortWarp::Warped DistortWarp::warpMask(const QImage &image, const LayerTransform &transform, const Corners &corners, double background,
+                                          std::optional<double> limit)
+{
+    const Warped warped = warp(image, transform, corners, true, limit);
+    if (background <= 0 || warped.image.cacheKey() == image.cacheKey())
+        return warped;
+    const int width = warped.image.width(), height = warped.image.height();
+    QImage surface = BrushRaster::context(width, height, true);
+    surface.fill(qRound(background * 255));
+    QPainter painter(&surface);
+    painter.setRenderHint(QPainter::Antialiasing, true);
+    const double sx = width / warped.transform.size.width(), sy = height / warped.transform.size.height();
+    QPainterPath shape;
+    for (size_t index = 0; index < 4; ++index) {
+        const QPointF point((corners[index].x() - warped.transform.origin.x()) * sx, (corners[index].y() - warped.transform.origin.y()) * sy);
+        if (index == 0)
+            shape.moveTo(point);
+        else
+            shape.lineTo(point);
+    }
+    shape.closeSubpath();
+    painter.setClipPath(shape);
+    BrushRaster::draw(warped.image, QRectF(0, 0, width, height), painter);
+    return {surface, warped.transform};
+}
+
+// Ctrl-drag on a handle: the corners move freely; Apply resamples.
+void EditorSession::beginDistort()
+{
+    if (!m_transformEdit || m_transformEdit->corners || !m_transformEdit->draft.isValid())
+        return;
+    m_transformEdit->corners = DistortWarp::corners(m_transformEdit->draft);
+    m_transformEdit->persistent = true;
+    notify();
+}
+
+// A twisted or collapsed shape is ignored.
+void EditorSession::previewCorners(const Corners &corners)
+{
+    if (!m_transformEdit || !m_transformEdit->corners || !DistortWarp::isUsable(corners))
+        return;
+    m_transformEdit->corners = corners;
+    notify();
+}
+
+// A group's layers take the box's perspective each.
+std::optional<DistortTarget> EditorSession::distortTarget(const ImageLayer &layer, const TransformEdit &edit, const Corners &shape) const
+{
+    if (!edit.group)
+        return edit.layerID == layer.id ? std::optional(DistortTarget{edit.draft, shape}) : std::nullopt;
+    const auto original = edit.group->originals.constFind(layer.id);
+    if (original == edit.group->originals.constEnd())
+        return std::nullopt;
+    const LayerTransform transform = original->following(edit.group->box, edit.draft);
+    const Corners corners = DistortWarp::carried(transform, edit.draft, shape);
+    return DistortWarp::isUsable(corners) ? std::optional(DistortTarget{transform, corners}) : std::nullopt;
+}
+
+// `image` is the layer with effects round it, mask applied.
+std::optional<DistortWarp::Warped> EditorSession::distortedEffects(const ImageLayer &layer, const QImage &image, double inset) const
+{
+    if (!m_transformEdit || m_transformEdit->mask || !m_transformEdit->corners)
+        return std::nullopt;
+    const std::optional<DistortTarget> target = distortTarget(layer, *m_transformEdit, *m_transformEdit->corners);
+    if (!target)
+        return std::nullopt;
+    return distortedEffects(layer, image, inset, *target);
+}
+
+std::optional<DistortWarp::Warped> EditorSession::distortedEffects(const ImageLayer &layer, const QImage &image, double inset,
+                                                                   const DistortTarget &target) const
+{
+    // The grown box takes the same perspective as the layer's.
+    const LayerTransform grown = LayerEffectsRenderer::placed(target.transform, image, inset);
+    const Corners carried = DistortWarp::carried(grown, target.transform, target.corners);
+    if (m_distortEffectsCache.contains(layer.id)) {
+        const DistortEffectsCache &cache = m_distortEffectsCache.at(layer.id);
+        if (cache.corners == carried && cache.image.cacheKey() == image.cacheKey())
+            return cache.result;
+    }
+    std::optional<DistortWarp::Warped> result;
+    // Swift's try?: effects that cannot warp are left off.
+    try {
+        result = DistortWarp::warp(image, grown, carried, false, 2048);
+    } catch (const ProjectError &) {
+    } catch (const ExportError &) {
+    }
+    m_distortEffectsCache.insert_or_assign(layer.id, DistortEffectsCache{carried, image, result});
+    return result;
+}
+
+// The layer warped into the pending distortion, at preview size.
+std::optional<DistortPreview> EditorSession::distortPreview(const ImageLayer &layer) const
+{
+    if (!m_transformEdit || m_transformEdit->mask || !m_transformEdit->corners || !layer.asset)
+        return std::nullopt;
+    const std::optional<DistortTarget> target = distortTarget(layer, *m_transformEdit, *m_transformEdit->corners);
+    if (!target)
+        return std::nullopt;
+    const std::optional<QImage> mask = layer.mask ? layer.mask->enabledImage() : std::nullopt;
+    const std::optional<ImageIdentity> maskID = mask ? std::optional(layer.mask->asset.identity()) : std::nullopt;
+    const auto cached = m_distortPreviewCache.find(layer.id);
+    if (cached != m_distortPreviewCache.end() && cached->second.corners == target->corners && cached->second.draft == target->transform
+        && cached->second.image == layer.asset->identity() && cached->second.mask == maskID)
+        return cached->second.result;
+    std::optional<DistortPreview> result;
+    try {
+        const DistortWarp::Warped warped = DistortWarp::warp(layer.asset->image(), target->transform, target->corners, false, 2048);
+        std::optional<QImage> warpedMask;
+        const std::optional<LayerMask> &owned = layer.mask;
+        if (owned && !owned->placement && owned->isLinked) {
+            // Swift's try?: a mask that cannot warp draws none.
+            try {
+                if (mask)
+                    warpedMask = DistortWarp::warp(*mask, target->transform, target->corners, true, 2048).image;
+            } catch (const ProjectError &) {
+            } catch (const ExportError &) {
+            }
+        } else if (owned) {
+            std::optional<QImage> carriedMask;
+            if (owned->isLinked && owned->placement) {
+                // A linked mask placed apart takes the same perspective.
+                const LayerTransform placement = owned->placement->following(layer.transform, target->transform);
+                const Corners carried = DistortWarp::carried(placement, target->transform, target->corners);
+                try {
+                    if (DistortWarp::isConvex(carried)) {
+                        const DistortWarp::Warped moved = DistortWarp::warpMask(owned->asset.image(), placement, carried, LayerMask::background(owned->asset.thumbnail), 2048);
+                        carriedMask = LayerMask(ImportedImage(moved.image, owned->asset.thumbnail, owned->asset.name), owned->isEnabled)
+                                          .clipImage(moved.transform, warped.transform, warped.image.width(), warped.image.height(), 2048);
+                    }
+                } catch (const ProjectError &) {
+                } catch (const ExportError &) {
+                }
+            }
+            // Otherwise the mask stays where it is on the document.
+            warpedMask = carriedMask ? carriedMask : owned->clipImage(owned->placement.value_or(layer.transform), warped.transform, warped.image.width(), warped.image.height(), 2048);
+        }
+        result = DistortPreview{warped.image, warpedMask, warped.transform};
+    } catch (const ProjectError &) {
+    } catch (const ExportError &) {
+    }
+    m_distortPreviewCache.insert_or_assign(layer.id, DistortPreviewCache{target->corners, target->transform, layer.asset->identity(), maskID, result});
+    return result;
+}
+
+// Apply: each layer resampled into its shape, one step.
+void EditorSession::commitDistort(const TransformEdit &edit, const Corners &shape)
+{
+    m_distortPreviewCache.clear();
+    std::vector<QUuid> ids;
+    if (edit.group) {
+        for (auto original = edit.group->originals.constBegin(); original != edit.group->originals.constEnd(); ++original)
+            ids.push_back(original.key());
+    } else {
+        ids.push_back(edit.layerID);
+    }
+    beginEdit(edit.group ? QStringLiteral("Distort Layers") : QStringLiteral("Distort"));
+    for (const QUuid &id : ids) {
+        const int index = indexOf(m_document->layers, id);
+        if (index < 0)
+            continue;
+        const std::optional<DistortTarget> target = distortTarget(m_document->layers[index], edit, shape);
+        if (!target)
+            continue;
+        // Warped effects stay on until the new pixels' effects land.
+        const std::optional<EffectsPreviewCache::Result> effects = effectsPreviews.rendered(id);
+        const std::optional<DistortWarp::Warped> warpedEffects =
+            effects ? distortedEffects(m_document->layers[index], effects->image, effects->inset, *target) : std::nullopt;
+        try {
+            distort(index, target->transform, target->corners);
+        } catch (const ProjectError &error) {
+            setBrushError(QString::fromUtf8(error.what()));
+        } catch (const ExportError &error) {
+            setBrushError(QString::fromUtf8(error.what()));
+        }
+        // Placed where warped: the commit also crops the layer.
+        if (warpedEffects)
+            effectsPreviews.seed(id, warpedEffects->image, warpedEffects->transform);
+    }
+    endEdit();
+    m_distortEffectsCache.clear();
+}
+
+// The layer at `index` resampled onto `corners`.
+void EditorSession::distort(int index, const LayerTransform &transform, const Corners &corners)
+{
+    const ImageLayer layer = m_document->layers[index];
+    if (!layer.asset)
+        return;
+    const DistortWarp::Trimmed warped = DistortWarp::warpTrimmed(layer.asset->image(), transform, corners);
+    const ImportedImage asset(warped.image, PixelAdjust::thumbnail(warped.image), layer.name);
+    std::optional<LayerMask> mask = layer.mask;
+    if (layer.mask && !layer.mask->placement && layer.mask->isLinked) {
+        const DistortWarp::Warped warpedMask = DistortWarp::warp(layer.mask->asset.image(), transform, corners, true);
+        // A uniform mask passes through; others crop with the pixels.
+        const bool uniform = warpedMask.image.cacheKey() == layer.mask->asset.image().cacheKey();
+        mask = layer.mask->replacing(uniform ? layer.mask->asset : LayerMask::assetFrom(warpedMask.image.copy(warped.crop)));
+    } else if (layer.mask && layer.mask->isLinked && layer.mask->placement
+               && DistortWarp::isConvex(DistortWarp::carried(layer.mask->placement->following(layer.transform, transform), transform, corners))) {
+        // A linked mask placed apart takes the same perspective.
+        const LayerTransform placement = layer.mask->placement->following(layer.transform, transform);
+        const DistortWarp::Warped moved = DistortWarp::warpMask(layer.mask->asset.image(), placement, DistortWarp::carried(placement, transform, corners),
+                                                                LayerMask::background(layer.mask->asset.thumbnail));
+        const bool uniform = moved.image.cacheKey() == layer.mask->asset.image().cacheKey();
+        mask = LayerMask(uniform ? layer.mask->asset : LayerMask::assetFrom(moved.image), layer.mask->isEnabled, moved.transform, true);
+    } else if (layer.mask) {
+        // An unlinked mask keeps its place on the document.
+        mask->placement = layer.mask->placement.value_or(layer.transform);
+    }
+    m_document->layers[index].asset = asset;
+    m_document->layers[index].transform = warped.transform;
+    m_document->layers[index].mask = mask;
+}
+
+// The selection's outline carried through the distortion.
+std::optional<QPainterPath> DistortWarp::mapPath(const QPainterPath &path, const QTransform &pixelToDocument, QSizeF pixelSize,
+                                                 const LayerTransform &transform, const Corners &corners)
+{
+    if (!isConvex(corners) || pixelSize.width() <= 0 || pixelSize.height() <= 0)
+        return std::nullopt;
+    const QTransform toPixels = pixelToDocument.inverted();
+    const QTransform map = homography(corners);
+    const auto carry = [&](QPointF point) {
+        const QPointF pixel = toPixels.map(point);
+        double u = pixel.x() / pixelSize.width(), v = pixel.y() / pixelSize.height();
+        if (transform.flipX)
+            u = 1 - u;
+        if (transform.flipY)
+            v = 1 - v;
+        return project(map, QPointF(u, v));
+    };
+    QPainterPath result;
+    result.setFillRule(path.fillRule());
+    for (int i = 0; i < path.elementCount(); ++i) {
+        const QPainterPath::Element element = path.elementAt(i);
+        if (element.isMoveTo()) {
+            result.moveTo(carry(element));
+        } else if (element.isLineTo()) {
+            result.lineTo(carry(element));
+        } else {
+            // A curve is its first control, then two data elements.
+            result.cubicTo(carry(element), carry(path.elementAt(i + 1)), carry(path.elementAt(i + 2)));
+            i += 2;
+        }
+    }
+    return result;
+}

src/Document/Distort.h

@@ -0,0 +1,72 @@
+#pragma once
+#include "Document/LayerTransform.h"
+#include "IO/ImageImporter.h"
+#include <QImage>
+#include <QPainterPath>
+#include <QRect>
+#include <optional>
+
+// Free distortion: the layer's four corners move on their own.
+namespace DistortWarp {
+struct Warped {
+    QImage image;
+    LayerTransform transform;
+};
+struct Trimmed {
+    QImage image;
+    LayerTransform transform;
+    // In the warp's pixels, for cropping a mask to match.
+    QRect crop;
+};
+
+// Clockwise from the top left, in document pixels.
+Corners corners(const LayerTransform &transform);
+// Four finite corners with some area to both halves.
+bool isUsable(const Corners &corners);
+// A shape a perspective can take, wound either way.
+bool isConvex(const Corners &corners);
+// The perspective taking the unit square onto `corners`.
+QTransform homography(const Corners &corners);
+// `image` under `transform`, resampled so its corners land on `corners`.
+Warped warp(const QImage &image, const LayerTransform &transform, const Corners &corners, bool isMask,
+            std::optional<double> limit = std::nullopt);
+// A full warp cropped to its visible pixels.
+Trimmed warpTrimmed(const QImage &image, const LayerTransform &transform, const Corners &corners);
+// Where `placement`'s corners land under `transform`'s perspective.
+Corners carried(const LayerTransform &placement, const LayerTransform &transform, const Corners &corners);
+// A path through the distortion; nil when the shape folds.
+std::optional<QPainterPath> mapPath(const QPainterPath &path, const QTransform &pixelToDocument, QSizeF pixelSize,
+                                    const LayerTransform &transform, const Corners &corners);
+// A mask warped like `warp`, its background outside the shape.
+Warped warpMask(const QImage &image, const LayerTransform &transform, const Corners &corners, double background,
+                std::optional<double> limit = std::nullopt);
+}
+
+// Where a distortion takes a layer: transform and corners.
+struct DistortTarget {
+    LayerTransform transform;
+    Corners corners;
+};
+
+// The layer warped for the canvas, its mask alongside.
+struct DistortPreview {
+    QImage image;
+    std::optional<QImage> mask;
+    LayerTransform transform;
+};
+
+// Effects last warped for a distortion, kept while corners rest.
+struct DistortEffectsCache {
+    Corners corners;
+    QImage image;
+    std::optional<DistortWarp::Warped> result;
+};
+
+// The canvas's last warped preview, kept while nothing changed.
+struct DistortPreviewCache {
+    Corners corners;
+    LayerTransform draft;
+    ImageIdentity image;
+    std::optional<ImageIdentity> mask;
+    std::optional<DistortPreview> result;
+};

src/Document/DocumentHistory.cpp

@@ -0,0 +1,131 @@
+#include "Document/DocumentHistory.h"
+#include <algorithm>
+#include <set>
+#include <utility>
+
+DocumentHistory::DocumentHistory(int entryLimit, qint64 retainedByteLimit)
+    : entryLimit(std::max(0, entryLimit)), retainedByteLimit(std::max<qint64>(0, retainedByteLimit))
+{
+}
+
+void DocumentHistory::reset()
+{
+    m_past.clear();
+    m_future.clear();
+    m_pending.reset();
+    m_depth = 0;
+    m_revision = QUuid::createUuid();
+    m_savedRevision = m_revision;
+}
+
+// A closed step changes these three; pending state is spent.
+void DocumentHistory::adopt(DocumentHistory &&staged) noexcept
+{
+    m_past = std::move(staged.m_past);
+    m_future = std::move(staged.m_future);
+    m_revision = staged.m_revision;
+}
+
+void DocumentHistory::begin(const QString &name, const std::optional<CanvasDocument> &document,
+                            std::optional<QUuid> selection)
+{
+    if (m_depth == 0) {
+        m_pending = Snapshot{document, selection, m_revision};
+        m_pendingName = name;
+    }
+    m_depth += 1;
+}
+
+void DocumentHistory::end(const std::optional<CanvasDocument> &document, std::optional<QUuid> selection)
+{
+    if (m_depth == 0)
+        return;
+    m_depth -= 1;
+    if (m_depth != 0 || !m_pending)
+        return;
+    const Snapshot before = *std::exchange(m_pending, std::nullopt);
+    // Selecting, navigating and no-op edits keep redo history.
+    if (before.document == document)
+        return;
+    m_revision = QUuid::createUuid();
+    m_past.push_back({m_pendingName, before, Snapshot{document, selection, m_revision}});
+    m_future.clear();
+    trim(document);
+}
+
+std::optional<DocumentHistory::Snapshot> DocumentHistory::undo()
+{
+    if (!canUndo())
+        return std::nullopt;
+    Entry entry = std::move(m_past.back());
+    m_past.pop_back();
+    m_future.push_back(entry);
+    m_revision = entry.before.revision;
+    trim(entry.before.document);
+    return entry.before;
+}
+
+std::optional<DocumentHistory::Snapshot> DocumentHistory::redo()
+{
+    if (!canRedo())
+        return std::nullopt;
+    Entry entry = std::move(m_future.back());
+    m_future.pop_back();
+    m_past.push_back(entry);
+    m_revision = entry.after.revision;
+    trim(entry.after.document);
+    return entry.after;
+}
+
+qint64 DocumentHistory::retainedBytes(const std::optional<CanvasDocument> &current) const
+{
+    const auto assets = [](const ImageLayer &layer) {
+        std::vector<const ImportedImage *> result;
+        if (layer.asset)
+            result.push_back(&*layer.asset);
+        if (layer.mask)
+            result.push_back(&layer.mask->asset);
+        return result;
+    };
+    std::set<ImageIdentity> seen;
+    if (current) {
+        for (const ImageLayer &layer : current->layers) {
+            for (const ImportedImage *asset : assets(layer)) {
+                seen.insert(asset->identity());
+                seen.insert({nullptr, asset->thumbnail.cacheKey()});
+            }
+        }
+    }
+    qint64 bytes = 0;
+    const auto count = [&](const Snapshot &snapshot) {
+        if (!snapshot.document)
+            return;
+        for (const ImageLayer &layer : snapshot.document->layers) {
+            for (const ImportedImage *asset : assets(layer)) {
+                if (seen.insert(asset->identity()).second)
+                    bytes += asset->byteCount();
+                if (seen.insert({nullptr, asset->thumbnail.cacheKey()}).second)
+                    bytes += asset->thumbnail.sizeInBytes();
+            }
+        }
+    };
+    for (const std::vector<Entry> *entries : {&m_past, &m_future}) {
+        for (const Entry &entry : *entries) {
+            count(entry.before);
+            count(entry.after);
+        }
+    }
+    return bytes;
+}
+
+void DocumentHistory::trim(const std::optional<CanvasDocument> &current)
+{
+    while (int(m_past.size() + m_future.size()) > entryLimit || retainedBytes(current) > retainedByteLimit) {
+        if (!m_past.empty())
+            m_past.erase(m_past.begin());
+        else if (!m_future.empty())
+            m_future.erase(m_future.begin());
+        else
+            break;
+    }
+}

src/Document/DocumentHistory.h

@@ -0,0 +1,55 @@
+#pragma once
+#include "Document/EditorSession+Model.h"
+#include <QString>
+#include <QUuid>
+#include <optional>
+#include <vector>
+
+// Snapshots share image data; layer edits copy no pixels.
+class DocumentHistory {
+public:
+    struct Snapshot {
+        std::optional<CanvasDocument> document;
+        std::optional<QUuid> activeLayerID;
+        QUuid revision;
+    };
+
+    const int entryLimit;
+    const qint64 retainedByteLimit;
+
+    explicit DocumentHistory(int entryLimit = 100, qint64 retainedByteLimit = 256LL * 1024 * 1024);
+
+    bool canUndo() const { return m_depth == 0 && !m_past.empty(); }
+    bool canRedo() const { return m_depth == 0 && !m_future.empty(); }
+    QString undoName() const { return m_past.empty() ? QString() : m_past.back().name; }
+    QString redoName() const { return m_future.empty() ? QString() : m_future.back().name; }
+    bool isModified() const { return m_revision != m_savedRevision; }
+    int undoCount() const { return int(m_past.size()); }
+    void markSaved() { m_savedRevision = m_revision; }
+    void reset();
+    // Takes the steps a staged copy of this history recorded.
+    void adopt(DocumentHistory &&staged) noexcept;
+
+    void begin(const QString &name, const std::optional<CanvasDocument> &document, std::optional<QUuid> selection);
+    void end(const std::optional<CanvasDocument> &document, std::optional<QUuid> selection);
+    std::optional<Snapshot> undo();
+    std::optional<Snapshot> redo();
+    qint64 retainedBytes(const std::optional<CanvasDocument> &current) const;
+
+private:
+    struct Entry {
+        QString name;
+        Snapshot before;
+        Snapshot after;
+    };
+
+    void trim(const std::optional<CanvasDocument> &current);
+
+    std::vector<Entry> m_past;
+    std::vector<Entry> m_future;
+    QUuid m_revision = QUuid::createUuid();
+    QUuid m_savedRevision = m_revision;
+    std::optional<Snapshot> m_pending;
+    QString m_pendingName = QStringLiteral("Edit");
+    int m_depth = 0;
+};

src/Document/EditorSession+Brush.cpp

@@ -0,0 +1,407 @@
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include "Logging.h"
+#include "Rendering/RasterSnapshot.h"
+#include <QtConcurrent>
+#include <chrono>
+#include <cmath>
+
+// Swift's EditorSession+Brush: the stroke from press to commit.
+namespace {
+bool sameAsset(const std::optional<ImportedImage> &a, const std::optional<ImportedImage> &b)
+{
+    return a.has_value() == b.has_value() && (!a || a->identity() == b->identity());
+}
+
+bool sameMask(const std::optional<LayerMask> &a, const std::optional<LayerMask> &b)
+{
+    return a.has_value() == b.has_value() && (!a || a->asset.identity() == b->asset.identity());
+}
+
+double uptime()
+{
+    return std::chrono::duration<double>(std::chrono::steady_clock::now().time_since_epoch()).count();
+}
+}
+
+bool EditorSession::canPaint() const
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    const std::optional<DocumentSelection> current = selection();
+    // A folder has no pixels: only its mask paints.
+    return canEditLayers() && m_selectedLayerIDs.size() == 1 && layer && (!layer->isGroup || m_isMaskSelected) && !(current && current->isEmpty())
+        && m_document->effectiveVisibleIDs().contains(layer->id) && (!m_isMaskSelected || (layer->mask && layer->mask->isEnabled))
+        && (m_isMaskSelected || !layer->adjustment);
+}
+
+// A tiled raster edit of pixels or mask, within budget.
+std::unique_ptr<BrushStroke> EditorSession::makeRasterEdit(const ImageLayer &layer, const BrushSettings &settings) const
+{
+    if (!m_document)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    auto stroke = std::make_unique<BrushStroke>(layer, m_isMaskSelected, settings, m_document->size());
+    qint64 used = 0;
+    for (const ImageLayer &other : m_document->layers) {
+        if (other.id == layer.id)
+            continue;
+        const std::optional<ImportedImage> &image = m_isMaskSelected ? (other.mask ? std::optional(other.mask->asset) : std::nullopt) : other.asset;
+        if (image)
+            used += qint64(image->size().width()) * image->size().height();
+    }
+    stroke->pixelLimit = 100'000'000 - used;
+    const std::optional<DocumentSelection> current = selection();
+    if (current)
+        stroke->selectionClip = current->clip(m_document->size());
+    if (!m_isMaskSelected && layer.mask) {
+        qint64 maskPixels = 0;
+        for (const ImageLayer &other : m_document->layers) {
+            if (other.id != layer.id && other.mask)
+                maskPixels += qint64(other.mask->asset.size().width()) * other.mask->asset.size().height();
+        }
+        stroke->pixelLimit = std::min<qint64>(stroke->pixelLimit, 100'000'000 - maskPixels);
+    }
+    return stroke;
+}
+
+void EditorSession::beginBrush(QPointF point)
+{
+    if (m_tool == NavigationTool::blur && m_blurMode != BlurToolMode::blur) {
+        beginWarp(point);
+        return;
+    }
+    // Spot Healing and Clone Stamp rework pixels, never a mask.
+    const bool paintsMasks = m_tool == NavigationTool::brush || m_tool == NavigationTool::blur;
+    if ((!paintsMasks && (!isBrushTool(m_tool) || m_isMaskSelected)) || !canPaint())
+        return;
+    const ImageLayer layer = activeLayer().value();
+    std::optional<BrushStroke::Clone> clone;
+    if (m_tool == NavigationTool::cloneStamp) {
+        const std::optional<QSizeF> offset = cloneStrokeOffset(point);
+        if (!offset) {
+            setBrushError(QStringLiteral("Alt-click where Clone Stamp should copy from first."));
+            return;
+        }
+        const std::optional<QImage> image = cloneSample(m_document.value());
+        if (!image)
+            return;
+        m_cloneOffset = offset;
+        clone = BrushStroke::Clone{*image, *offset};
+    }
+    // Blur paints a softened copy of the layer, in place.
+    if (m_tool == NavigationTool::blur) {
+        const std::optional<QImage> image = blurSample(m_document.value(), m_isMaskSelected);
+        if (!image)
+            return;
+        clone = BrushStroke::Clone{*image, QSizeF(0, 0)};
+    }
+    finishOpacityEdit();
+    try {
+        BrushSettings settings = m_brushSettings;
+        settings.healing = m_tool == NavigationTool::spotHealing;
+        settings.erasing = m_tool == NavigationTool::brush && m_brushMode == BrushToolMode::erase && !m_isMaskSelected;
+        settings.healingMode = m_spotHealingMode;
+        if (m_isMaskSelected) {
+            settings.red = m_maskPaintWhite ? 1 : 0;
+            settings.green = settings.red;
+            settings.blue = settings.red;
+        }
+        m_brushStroke = makeRasterEdit(layer, settings);
+        m_brushStroke->clone = clone;
+        m_brushStroke->isBlur = m_tool == NavigationTool::blur;
+        m_brushStroke->append(point);
+        m_lastBrushPoint = LastBrushPoint{point, layer.id, m_isMaskSelected};
+        ++m_brushRevision;
+        resumeFileRequests();
+        notify();
+    } catch (const ProjectError &error) {
+        cancelBrush();
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        cancelBrush();
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+}
+
+void EditorSession::continueBrush(QPointF point)
+{
+    if (m_warpStroke) {
+        m_warpStroke->append(point);
+        m_lastBrushPoint->point = point;
+        ++m_brushRevision;
+        notify();
+        return;
+    }
+    if (!m_brushStroke)
+        return;
+    try {
+        m_brushStroke->append(point);
+        m_lastBrushPoint->point = point;
+        ++m_brushRevision;
+        notify();
+    } catch (const ProjectError &error) {
+        cancelBrush();
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        cancelBrush();
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+}
+
+// Where a Shift-click's line starts, on the same target.
+std::optional<QPointF> EditorSession::shiftLineStart() const
+{
+    if (!m_lastBrushPoint || m_lastBrushPoint->layerID != m_activeLayerID || m_lastBrushPoint->mask != m_isMaskSelected)
+        return std::nullopt;
+    return m_lastBrushPoint->point;
+}
+
+void EditorSession::cancelBrush()
+{
+    m_warpStroke.reset();
+    m_brushStroke.reset();
+    ++m_brushRevision;
+    resumeFileRequests();
+    notify();
+}
+
+// Mouse-up's own call, before the next event arrives.
+bool EditorSession::finishBrushImmediately()
+{
+    if (m_warpStroke) {
+        if (m_isProjectBusy)
+            return false;
+        finishWarp();
+        return true;
+    }
+    if (!m_brushStroke)
+        return true;
+    if (m_isProjectBusy)
+        return false;
+    try {
+        m_brushStroke->flush();
+        if (m_brushStroke->settings.healing)
+            m_brushStroke->heal();
+        if (!m_brushStroke->patches().empty())
+            commitPaintSnapshot(*m_brushStroke);
+    } catch (const ProjectError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+    cancelBrush();
+    return true;
+}
+
+void EditorSession::finishBrush()
+{
+    finishBrushImmediately();
+}
+
+// Immutable tiles land at once, undo entry included.
+void EditorSession::commitPaintSnapshot(const BrushStroke &stroke)
+{
+    const PaintSnapshot result = stroke.paintSnapshot();
+    const int index = m_document ? indexOf(m_document->layers, stroke.layer.id) : -1;
+    if (!result.transform.isValid() || index < 0)
+        return;
+    const ImageLayer current = m_document->layers[index];
+    if (!sameAsset(current.asset, stroke.layer.asset) || current.transform != stroke.layer.transform)
+        return;
+    std::optional<LayerMask> mask = current.mask;
+    if (!stroke.isMask && mask && !mask->placement && result.bounds != stroke.sourceRect) {
+        // The mask grows with the layer, revealing past it.
+        const std::shared_ptr<const RasterSnapshot> raster = RasterSnapshot::replacing(mask->asset, stroke.sourceRect, {}, result.bounds, true);
+        mask = mask->replacing(ImportedImage(raster, raster->thumbnail(), mask->asset.name));
+    }
+    beginEdit(stroke.editName.value_or(stroke.isMask               ? QStringLiteral("Paint Mask")
+                                       : stroke.settings.erasing ? QStringLiteral("Erase")
+                                       : stroke.isBlur           ? QStringLiteral("Blur")
+                                       : stroke.clone            ? QStringLiteral("Clone Stamp")
+                                       : stroke.settings.healing ? QStringLiteral("Spot Healing")
+                                                                 : QStringLiteral("Brush Stroke")));
+    ImageLayer &layer = m_document->layers[index];
+    if (stroke.isMask) {
+        layer.mask = current.mask ? current.mask->replacing(result.asset) : LayerMask(result.asset);
+    } else {
+        layer.asset = result.asset;
+        layer.transform = result.transform;
+        layer.isGroup = false;
+        layer.mask = mask;
+        // Swift rebuilds the layer; effects stay, shape and text go.
+        layer.shape = std::nullopt;
+        layer.text = std::nullopt;
+    }
+    endEdit();
+}
+
+// A raster edit assembled off the UI thread; one step.
+void EditorSession::commitRasterEdit(std::shared_ptr<const BrushStroke> stroke, const QString &name, std::function<void()> alsoApply, std::function<void()> done)
+{
+    // The caller resumes from the event loop, like await.
+    const auto finish = [this, done] {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    };
+    if (!stroke->committedTransform().isValid()) {
+        setBrushError(QString::fromUtf8(ProjectError(ProjectError::Kind::tooLarge).what()));
+        finish();
+        return;
+    }
+    setIsProjectBusy(true);
+    const BrushCommit::Input input = stroke->commitInput();
+    m_committingRaster = CommittingRaster{std::move(stroke), name, std::move(alsoApply), std::move(done)};
+    m_rasterCommit.setFuture(QtConcurrent::run([input]() -> RasterMade {
+        try {
+            return RasterMade{BrushCommit::render(input), std::nullopt};
+        } catch (const ExportError &error) {
+            return RasterMade{std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::finishRasterCommit()
+{
+    const RasterMade made = m_rasterCommit.result();
+    const CommittingRaster pending = std::move(m_committingRaster.value());
+    m_committingRaster = std::nullopt;
+    const auto finish = [&] {
+        setIsProjectBusy(false);
+        ++m_brushRevision;
+        notify();
+        if (pending.done)
+            QMetaObject::invokeMethod(this, pending.done, Qt::QueuedConnection);
+    };
+    if (made.failure) {
+        setBrushError(made.failure);
+        finish();
+        return;
+    }
+    const BrushStroke &stroke = *pending.stroke;
+    const BrushCommit::Output &result = *made.output;
+    try {
+        const LayerTransform transform = stroke.transform(result.pixelBounds.translated(stroke.committedBounds().topLeft()));
+        if (!transform.isValid())
+            throw ProjectError(ProjectError::Kind::tooLarge);
+        std::optional<LayerMask> mask = stroke.layer.mask;
+        if (!stroke.isMask && mask && !mask->placement)
+            mask = mask->replacing(BrushCommit::expandMask(mask->asset, stroke.commitInput(), result.pixelBounds));
+        // Never over content that changed underneath it.
+        const int index = m_document ? indexOf(m_document->layers, stroke.layer.id) : -1;
+        if (index < 0) {
+            finish();
+            return;
+        }
+        const ImageLayer current = m_document->layers[index];
+        if (!sameAsset(current.asset, stroke.layer.asset) || current.transform != stroke.layer.transform || !sameMask(current.mask, stroke.layer.mask)) {
+            finish();
+            return;
+        }
+        beginEdit(pending.name);
+        ImageLayer &layer = m_document->layers[index];
+        if (stroke.isMask) {
+            layer.mask = current.mask ? current.mask->replacing(result.asset) : LayerMask(result.asset);
+        } else {
+            layer.asset = result.asset;
+            layer.transform = transform;
+            layer.isGroup = false;
+            // Swift rebuilds the layer; effects stay, shape and text go.
+            layer.shape = std::nullopt;
+            layer.text = std::nullopt;
+            if (mask && current.mask)
+                mask->isEnabled = current.mask->isEnabled;
+            layer.mask = mask;
+        }
+        if (pending.alsoApply)
+            pending.alsoApply();
+        endEdit();
+    } catch (const ProjectError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+    finish();
+}
+
+bool EditorSession::usesOpacityKeys() const
+{
+    return isBrushTool(m_tool) || m_tool == NavigationTool::gradient || m_tool == NavigationTool::move;
+}
+
+void EditorSession::typeOpacityDigit(int digit, std::optional<double> time)
+{
+    if (!usesOpacityKeys() || m_brushStroke || m_isProjectBusy || digit < 0 || digit > 9)
+        return;
+    const double now = time.value_or(uptime());
+    int percent = digit == 0 ? 100 : digit * 10;
+    if (m_pendingOpacityDigit && now - m_pendingOpacityDigit->time < 0.6) {
+        percent = std::max(1, m_pendingOpacityDigit->digit * 10 + digit);
+        m_pendingOpacityDigit = std::nullopt;
+    } else {
+        m_pendingOpacityDigit = PendingOpacityDigit{digit, now};
+    }
+    const double value = percent / 100.0;
+    if (isBrushTool(m_tool)) {
+        m_brushSettings.opacity = value;
+        notify();
+    } else if (m_tool == NavigationTool::gradient) {
+        setGradientSettings(GradientSettings{m_gradientSettings.shape, m_gradientSettings.style, m_gradientSettings.reversed, value});
+    } else {
+        setSelectedLayersOpacity(value);
+    }
+}
+
+// Shift-[ and Shift-]: hardness in Photoshop's quarters.
+void EditorSession::changeBrushHardness(bool increase)
+{
+    if (m_brushStroke)
+        return;
+    const double quarter = m_brushSettings.hardness * 4;
+    const double step = increase ? std::floor(quarter + 0.001) + 1 : std::ceil(quarter - 0.001) - 1;
+    m_brushSettings.hardness = std::min(4.0, std::max(0.0, step)) / 4;
+    notify();
+}
+
+void EditorSession::changeBrushSize(bool increase)
+{
+    if (m_brushStroke)
+        return;
+    // A fifth, at least a pixel: 2 would stay 2.
+    const double current = m_brushSettings.diameter;
+    const double stepped = increase ? std::max(current + 1, std::round(current * 1.2)) : std::min(current - 1, std::round(current / 1.2));
+    m_brushSettings.diameter = std::min(2000.0, std::max(1.0, stepped));
+    notify();
+}
+
+void EditorSession::setBrushSettings(const BrushSettings &settings)
+{
+    m_brushSettings = settings;
+    refreshGradient();
+    notify();
+}
+
+void EditorSession::setBrushMode(BrushToolMode mode)
+{
+    m_brushMode = mode;
+    notify();
+}
+
+void EditorSession::setBlurMode(BlurToolMode mode)
+{
+    m_blurMode = mode;
+    notify();
+}
+
+void EditorSession::setSpotHealingMode(SpotHealingMode mode)
+{
+    m_spotHealingMode = mode;
+    notify();
+}
+
+void EditorSession::setMaskPaintWhite(bool white)
+{
+    m_maskPaintWhite = white;
+    refreshGradient();
+    notify();
+}

src/Document/EditorSession+Brush.h

@@ -0,0 +1,24 @@
+// Swift's EditorSession+Brush extension: a stroke from press to commit.
+public:
+    bool canPaint() const;
+    std::unique_ptr<BrushStroke> makeRasterEdit(const ImageLayer &layer, const BrushSettings &settings = BrushSettings()) const;
+    void beginBrush(QPointF point);
+    void continueBrush(QPointF point);
+    // Where a Shift-click's line starts; nil on another target.
+    std::optional<QPointF> shiftLineStart() const;
+    void cancelBrush();
+    // Mouse-up's own call; false while busy.
+    bool finishBrushImmediately();
+    void finishBrush();
+    void commitPaintSnapshot(const BrushStroke &stroke);
+    // Off the UI thread; `done` follows the step or refusal.
+    void commitRasterEdit(std::shared_ptr<const BrushStroke> stroke, const QString &name, std::function<void()> alsoApply = {},
+                          std::function<void()> done = {});
+    bool usesOpacityKeys() const;
+    // Digits set 10%…100%; two quick ones, an exact value.
+    void typeOpacityDigit(int digit, std::optional<double> time = std::nullopt);
+    void changeBrushHardness(bool increase);
+    void changeBrushSize(bool increase);
+
+private:
+    void finishRasterCommit();

src/Document/EditorSession+Imports.cpp

@@ -0,0 +1,124 @@
+#include "Document/EditorSession.h"
+#include "Logging.h"
+#include <QtConcurrent>
+
+bool EditorSession::canStartProjectOperation() const
+{
+    return !m_textDraft && !m_isProjectBusy && !m_isImporting && !m_brushStroke && !m_warpStroke && !m_showsNewDocument && !m_showsImporter
+        && !m_renamingLayerID && !m_importError && !m_levels && !m_adjustmentEditingID;
+}
+
+void EditorSession::waitForFileRequest(std::function<void()> ready)
+{
+    if (canStartProjectOperation())
+        ready();
+    else
+        m_fileRequestWaiters.push_back(std::move(ready));
+}
+
+void EditorSession::resumeFileRequests()
+{
+    if (!canStartProjectOperation())
+        return;
+    // Each waiter looks again when its turn comes.
+    for (const std::function<void()> &waiter : std::exchange(m_fileRequestWaiters, {}))
+        QMetaObject::invokeMethod(this, [this, waiter] { waitForFileRequest(waiter); }, Qt::QueuedConnection);
+}
+
+void EditorSession::waitForProjectAccess(std::function<void()> ready)
+{
+    if (!m_isProjectBusy)
+        ready();
+    else
+        m_projectWaiters.push_back(std::move(ready));
+}
+
+void EditorSession::updateBusyIndicator()
+{
+    // Quick operations never dim the controls.
+    if (m_isProjectBusy) {
+        if (!m_busyTimer.isActive() && !m_showsBusy)
+            m_busyTimer.start();
+    } else {
+        m_busyTimer.stop();
+        m_showsBusy = false;
+    }
+}
+
+void EditorSession::importImages(const QList<QUrl> &urls, std::optional<QPointF> point, std::function<void()> done)
+{
+    if (urls.isEmpty()) {
+        if (done)
+            done();
+        return;
+    }
+    if (m_brushStroke)
+        finishBrush();
+    cancelCrop();
+    commitTransform();
+    waitForProjectAccess([this, urls, point, done] {
+        m_pendingImports.push_back({urls, point, done});
+        if (!m_isImporting) {
+            setIsImporting(true);
+            drainImports();
+        }
+    });
+}
+
+void EditorSession::drainImports()
+{
+    if (m_pendingImports.empty()) {
+        setIsImporting(false);
+        if (!m_importFailures.isEmpty())
+            setImportError(std::exchange(m_importFailures, {}).join("\n\n"));
+        return;
+    }
+    beginEdit(QStringLiteral("Import Images"));
+    // Without a canvas the first image makes it.
+    m_importPoint = m_document ? m_pendingImports.front().point : std::nullopt;
+    m_importIndex = 0;
+    decodeNext();
+}
+
+void EditorSession::decodeNext()
+{
+    const ImportRequest &request = m_pendingImports.front();
+    if (m_importIndex == request.urls.size()) {
+        endEdit();
+        qCInfo(lcApp) << "imported a request of" << request.urls.size() << "files";
+        // The caller goes on once the queue's state is settled.
+        if (request.done)
+            QMetaObject::invokeMethod(this, request.done, Qt::QueuedConnection);
+        m_pendingImports.pop_front();
+        drainImports();
+        return;
+    }
+    qint64 usedPixels = 0;
+    if (m_document) {
+        for (const ImageLayer &layer : m_document->layers) {
+            if (layer.asset)
+                usedPixels += qint64(layer.asset->size().width()) * layer.asset->size().height();
+        }
+    }
+    const QUrl url = request.urls[m_importIndex];
+    m_decoder.setFuture(QtConcurrent::run([url, remaining = 100'000'000 - usedPixels]() -> Decoded {
+        try {
+            if (!url.isLocalFile())
+                throw ImageImportError(ImageImportError::Kind::unsupported);
+            return {ImageImporter::decode(url.toLocalFile(), remaining), QString()};
+        } catch (const ImageImportError &error) {
+            return {std::nullopt, url.fileName() + ": " + error.what()};
+        }
+    }));
+}
+
+void EditorSession::finishDecode()
+{
+    const Decoded decoded = m_decoder.result();
+    if (decoded.asset)
+        insert(*decoded.asset, m_importPoint);
+    else
+        m_importFailures << decoded.failure;
+    m_importIndex += 1;
+    decodeNext();
+}

src/Document/EditorSession+Jobs.h

@@ -0,0 +1,136 @@
+#pragma once
+#include "Document/BrushStroke.h"
+#include "Document/Filters.h"
+#include "Document/HueSaturation.h"
+#include "Document/Selection.h"
+#include "IO/ImageImporter.h"
+#include "IO/ProjectStore.h"
+#include <QImage>
+#include <QPainterPath>
+#include <QSet>
+#include <QUrl>
+#include <functional>
+#include <map>
+#include <optional>
+
+// The private structs of EditorSession, its private base of types.
+struct SessionJobs {
+    // A brush family's tip while another family is in use.
+    struct ParkedTip {
+        double diameter;
+        double hardness;
+        double opacity;
+    };
+    struct ImportRequest {
+        QList<QUrl> urls;
+        std::optional<QPointF> point;
+        std::function<void()> done;
+    };
+    struct Decoded {
+        std::optional<ImportedImage> asset;
+        QString failure;
+    };
+    struct Baked {
+        std::map<QUuid, ImportedImage> images;
+        std::optional<QString> failure;
+    };
+    // What an invert waits for and writes back.
+    struct Inverting {
+        QUuid layerID;
+        bool mask;
+        std::optional<ImageIdentity> image;
+        std::optional<ImageIdentity> maskImage;
+        std::function<void()> done;
+    };
+    struct Inverted {
+        std::optional<QImage> image;
+        std::optional<QString> failure;
+    };
+    // What a wand click waits for and applies.
+    struct Wanding {
+        SelectionMode mode;
+        QUuid documentID;
+        std::function<void()> done;
+    };
+    struct Wanded {
+        std::optional<QPainterPath> path;
+        std::optional<QString> failure;
+    };
+    // The last stroke's end, for a Shift-click's line.
+    struct LastBrushPoint {
+        QPointF point;
+        QUuid layerID;
+        bool mask;
+    };
+    struct PendingOpacityDigit {
+        int digit;
+        double time;
+    };
+    // A raster edit's result on the way to the layer.
+    struct RasterMade {
+        std::optional<BrushCommit::Output> output;
+        std::optional<QString> failure;
+    };
+    struct CommittingRaster {
+        std::shared_ptr<const BrushStroke> stroke;
+        QString name;
+        std::function<void()> alsoApply;
+        std::function<void()> done;
+    };
+    // A filter's preview, and its result for the layer.
+    struct Filtered {
+        std::optional<QImage> image;
+        std::optional<QString> failure;
+        // The job's settings, which the preview was made with.
+        FilterSettings settings;
+    };
+    struct FilterMade {
+        std::optional<ImportedImage> asset;
+        std::optional<LayerTransform> transform;
+        std::optional<QString> failure;
+    };
+    // The crop's resized project, or why it failed.
+    struct Cropped {
+        std::optional<ProjectSnapshot> snapshot;
+        std::optional<QString> failure;
+    };
+    // Levels' result for the layer.
+    struct Leveled {
+        std::optional<ImportedImage> asset;
+        std::optional<QString> failure;
+    };
+    // A commit's caller, waiting for the preview or the result.
+    struct Committing {
+        QUuid editID;
+        std::function<void()> done;
+    };
+    // Hue/Saturation's pixels, or why they failed.
+    struct Adjusted {
+        std::optional<AdjustedPixels> pixels;
+        std::optional<QString> failure;
+    };
+    // The layers beneath an adjustment, drawn, or why not.
+    struct AdjustmentInput {
+        std::optional<ImportedImage> asset;
+        std::optional<QString> failure;
+    };
+    // A Hue/Saturation commit keeps its edit and its caller.
+    struct CommittingHue {
+        HueSaturationEdit edit;
+        std::function<void()> done;
+    };
+    // A merge worked out once: what goes, what it becomes.
+    struct MergePlan {
+        std::vector<QUuid> ids;
+        QSet<QUuid> removed;
+        QString name;
+        std::optional<QUuid> parent;
+        QUuid anchor;
+        QString action;
+    };
+    struct TransformDuplicate {
+        std::vector<QUuid> copies;
+        QSet<QUuid> source;
+        QUuid primary;
+    };
+};

src/Document/EditorSession+Layers.cpp

@@ -0,0 +1,181 @@
+#include "Document/EditorSession.h"
+#include <algorithm>
+#include <map>
+
+void EditorSession::addBlankLayer()
+{
+    if (!canEditLayers())
+        return;
+    const std::vector<ImageLayer> &existing = m_document->layers;
+    int number = 1;
+    const auto taken = [&](const QString &name) {
+        return std::any_of(existing.begin(), existing.end(), [&](const ImageLayer &layer) { return layer.name == name; });
+    };
+    while (taken(QStringLiteral("Layer %1").arg(number)))
+        ++number;
+    ImageLayer layer(QStringLiteral("Layer %1").arg(number), m_document->size());
+    const std::optional<ImageLayer> active = activeLayer();
+    const bool intoFolder = active && active->isGroup;
+    layer.parentID = intoFolder ? m_activeLayerID : active ? active->parentID : std::nullopt;
+    if (layer.parentID)
+        m_collapsedGroupIDs.remove(*layer.parentID);
+    const int activeIndex = indexOf(existing, m_activeLayerID);
+    int insertion = activeIndex >= 0 ? activeIndex + 1 : int(existing.size());
+    // In a folder the layer goes above its topmost contents.
+    if (intoFolder) {
+        std::map<QUuid, std::optional<QUuid>> parents;
+        for (const ImageLayer &each : existing)
+            parents[each.id] = each.parentID;
+        const auto isInside = [&](QUuid id) {
+            std::optional<QUuid> parent = parents[id];
+            for (int steps = 0; parent && steps < 64; ++steps) {
+                if (*parent == *m_activeLayerID)
+                    return true;
+                parent = parents.count(*parent) ? parents[*parent] : std::nullopt;
+            }
+            return false;
+        };
+        for (int index = int(existing.size()) - 1; index >= 0; --index) {
+            if (isInside(existing[index].id)) {
+                insertion = std::max(insertion, index + 1);
+                break;
+            }
+        }
+    }
+    beginEdit(QStringLiteral("New Blank Layer"));
+    m_document->layers.insert(m_document->layers.begin() + insertion, layer);
+    setActiveLayerID(layer.id);
+    endEdit();
+}
+
+void EditorSession::deleteLayer(QUuid id)
+{
+    if (!canEditLayers() || indexOf(m_document->layers, id) < 0)
+        return;
+    if (!deleteWithLiveMaskChoice({id}))
+        finishDeletingLayer(id, {});
+}
+
+void EditorSession::deleteActiveLayer()
+{
+    if (m_activeLayerID)
+        deleteLayer(*m_activeLayerID);
+}
+
+void EditorSession::deleteSelectedLayers()
+{
+    if (!canEditLayers())
+        return;
+    // Captured first: deleting moves the active layer.
+    std::vector<QUuid> ids;
+    for (const ImageLayer &layer : m_document->layers) {
+        if (m_selectedLayerIDs.contains(layer.id))
+            ids.push_back(layer.id);
+    }
+    if (ids.size() <= 1)
+        deleteActiveLayer();
+    else if (!deleteWithLiveMaskChoice(ids))
+        finishDeletingLayers(ids, {});
+}
+
+void EditorSession::renameLayer(QUuid id, const QString &name)
+{
+    const QString trimmed = name.trimmed();
+    const int index = m_document ? indexOf(m_document->layers, id) : -1;
+    if (m_isProjectBusy || m_isImporting || trimmed.isEmpty() || index < 0)
+        return;
+    beginEdit(QStringLiteral("Rename Layer"));
+    m_document->layers[index].name = trimmed;
+    endEdit();
+}
+
+void EditorSession::toggleLayerVisibility(QUuid id)
+{
+    const int index = canEditLayers() ? indexOf(m_document->layers, id) : -1;
+    if (index < 0)
+        return;
+    beginEdit(m_document->layers[index].isVisible ? QStringLiteral("Hide Layer") : QStringLiteral("Show Layer"));
+    m_document->layers[index].isVisible = !m_document->layers[index].isVisible;
+    endEdit();
+}
+
+std::optional<bool> EditorSession::beginVisibilitySwipe(QUuid id)
+{
+    const int index = canEditLayers() ? indexOf(m_document->layers, id) : -1;
+    if (index < 0)
+        return std::nullopt;
+    const bool visible = !m_document->layers[index].isVisible;
+    beginEdit(visible ? QStringLiteral("Show Layer") : QStringLiteral("Hide Layer"));
+    setVisibilityInSwipe(id, visible);
+    return visible;
+}
+
+void EditorSession::setVisibilityInSwipe(QUuid id, bool visible)
+{
+    const int index = m_document ? indexOf(m_document->layers, id) : -1;
+    if (index < 0 || m_document->layers[index].isVisible == visible)
+        return;
+    m_document->layers[index].isVisible = visible;
+    notify();
+}
+
+void EditorSession::endVisibilitySwipe()
+{
+    endEdit();
+}
+
+void EditorSession::reorderLayers(const std::set<int> &offsets, int destination)
+{
+    if (!canEditLayers())
+        return;
+    std::vector<ImageLayer> listed(m_document->layers.rbegin(), m_document->layers.rend());
+    const int count = int(listed.size());
+    const bool inRange = std::all_of(offsets.begin(), offsets.end(), [&](int offset) { return offset >= 0 && offset < count; });
+    if (!inRange || destination < 0 || destination > count)
+        return;
+    // SwiftUI's move: the picked rows land before `destination`.
+    std::vector<ImageLayer> moved, kept;
+    int before = 0;
+    for (int index = 0; index < count; ++index) {
+        const bool picked = offsets.count(index) > 0;
+        (picked ? moved : kept).push_back(listed[index]);
+        before += picked && index < destination;
+    }
+    kept.insert(kept.begin() + (destination - before), moved.begin(), moved.end());
+    beginEdit(QStringLiteral("Reorder Layers"));
+    m_document->layers.assign(kept.rbegin(), kept.rend());
+    endEdit();
+}
+
+bool EditorSession::canMoveActiveLayer(int offset) const
+{
+    const std::optional<ImageLayer> active = canEditLayers() ? activeLayer() : std::nullopt;
+    if (!active)
+        return false;
+    int position = -1, siblings = 0;
+    for (const ImageLayer &layer : m_document->layers) {
+        if (layer.parentID != active->parentID)
+            continue;
+        if (layer.id == active->id)
+            position = siblings;
+        ++siblings;
+    }
+    return position >= 0 && position + offset >= 0 && position + offset < siblings;
+}
+
+void EditorSession::moveActiveLayer(int offset)
+{
+    if (!canMoveActiveLayer(offset))
+        return;
+    const ImageLayer active = *activeLayer();
+    std::vector<int> siblings;
+    for (int index = 0; index < int(m_document->layers.size()); ++index) {
+        if (m_document->layers[index].parentID == active.parentID)
+            siblings.push_back(index);
+    }
+    const int from = indexOf(m_document->layers, active.id);
+    const auto position = std::find(siblings.begin(), siblings.end(), from) - siblings.begin();
+    beginEdit(QStringLiteral("Reorder Layers"));
+    std::swap(m_document->layers[from], m_document->layers[siblings[position + offset]]);
+    endEdit();
+}

src/Document/EditorSession+Model.cpp

@@ -0,0 +1,126 @@
+#include "Document/EditorSession+Model.h"
+#include "IO/ProjectStore.h"
+#include <algorithm>
+
+ImageLayer::ImageLayer(ImportedImage imported, QPointF origin)
+    : id(QUuid::createUuid()), asset(std::move(imported)),
+      transform{.origin = origin, .size = QSizeF(asset->size())}, name(asset->name)
+{
+}
+
+int indexOf(const std::vector<ImageLayer> &layers, std::optional<QUuid> id)
+{
+    const auto found = std::find_if(layers.begin(), layers.end(), [&](const ImageLayer &layer) { return id && layer.id == *id; });
+    return found == layers.end() ? -1 : int(found - layers.begin());
+}
+
+ImageLayer::ImageLayer(QString name, QSizeF blankSize)
+    : id(QUuid::createUuid()), transform{.origin = QPointF(0, 0), .size = blankSize}, name(std::move(name))
+{
+}
+
+ImageLayer::ImageLayer(const ProjectLayerRecord &record, std::optional<ImportedImage> asset, std::optional<LayerMask> mask)
+    : id(record.id), asset(std::move(asset)), transform(record.transform), name(record.name), isVisible(record.isVisible),
+      parentID(record.parentID), isGroup(record.isGroup == true), opacity(record.opacity.value_or(1)),
+      blendMode(record.blendMode.value_or(LayerBlendMode::normal)), maskSourceID(record.maskSourceID), mask(std::move(mask)),
+      adjustment(record.adjustment), effects(record.effects)
+{
+}
+
+bool operator==(const ImageLayer &lhs, const ImageLayer &rhs)
+{
+    const bool sameImage = lhs.asset.has_value() == rhs.asset.has_value()
+        && (!lhs.asset || lhs.asset->identity() == rhs.asset->identity());
+    return lhs.id == rhs.id && lhs.name == rhs.name && lhs.isVisible == rhs.isVisible
+        && lhs.transform == rhs.transform && sameImage && lhs.parentID == rhs.parentID
+        && lhs.isGroup == rhs.isGroup && lhs.opacity == rhs.opacity && lhs.blendMode == rhs.blendMode
+        && lhs.mask == rhs.mask && lhs.maskSourceID == rhs.maskSourceID && lhs.adjustment == rhs.adjustment && lhs.shape == rhs.shape
+        && lhs.text == rhs.text && lhs.effects == rhs.effects;
+}
+
+CanvasDocument::CanvasDocument(int width, int height)
+    : id(QUuid::createUuid()), width(width), height(height)
+{
+}
+
+CanvasDocument::CanvasDocument(const ProjectSnapshot &snapshot)
+    : id(snapshot.manifest.documentID), width(int(snapshot.manifest.width)), height(int(snapshot.manifest.height)),
+      resolution(snapshot.manifest.resolution.value_or(72))
+{
+    for (const ProjectLayerRecord &record : snapshot.manifest.layers) {
+        const auto image = snapshot.images.find(record.id);
+        layers.push_back(ImageLayer(record, image == snapshot.images.end() ? std::nullopt : std::optional(image->second),
+                                    snapshot.mask(record)));
+    }
+}
+
+namespace {
+// Swift's CharacterSet.whitespaces: category Zs and tab.
+bool isWhitespace(QChar character)
+{
+    return character == u'\t' || character.category() == QChar::Separator_Space;
+}
+
+// Swift's Int(String): optional sign, then ASCII digits only.
+std::optional<int> wholeNumber(QStringView text)
+{
+    qsizetype index = text.startsWith(u'+') || text.startsWith(u'-') ? 1 : 0;
+    if (index == text.size())
+        return std::nullopt;
+    for (; index < text.size(); ++index) {
+        if (text[index] < u'0' || text[index] > u'9')
+            return std::nullopt;
+    }
+    bool fits = false;
+    const int number = text.toInt(&fits);
+    return fits ? std::optional<int>(number) : std::nullopt;
+}
+}
+
+std::optional<int> CanvasDocument::validDimension(const QString &value)
+{
+    qsizetype first = 0, last = value.size();
+    while (first < last && isWhitespace(value[first]))
+        ++first;
+    while (last > first && isWhitespace(value[last - 1]))
+        --last;
+    const std::optional<int> number = wholeNumber(QStringView(value).mid(first, last - first));
+    if (!number || *number < 1 || *number > 30'000)
+        return std::nullopt;
+    return number;
+}
+
+QString label(NavigationTool tool)
+{
+    switch (tool) {
+    case NavigationTool::move: return QStringLiteral("Move / Transform (V)");
+    case NavigationTool::marquee: return QStringLiteral("Marquee (M)");
+    case NavigationTool::lasso: return QStringLiteral("Lasso (L)");
+    case NavigationTool::wand: return QStringLiteral("Magic Wand (W)");
+    case NavigationTool::crop: return QStringLiteral("Crop (C)");
+    case NavigationTool::brush: return QStringLiteral("Brush (B) · Eraser (E)");
+    case NavigationTool::spotHealing: return QStringLiteral("Spot Healing Brush (J)");
+    case NavigationTool::cloneStamp: return QStringLiteral("Clone Stamp (S) · Alt-click sets the source");
+    case NavigationTool::blur: return QStringLiteral("Smear (R)");
+    case NavigationTool::gradient: return QStringLiteral("Gradient (G)");
+    case NavigationTool::shape: return QStringLiteral("Shape (U) · Shift-U switches Rectangle/Ellipse");
+    case NavigationTool::type: return QStringLiteral("Type (T)");
+    case NavigationTool::eyedropper: return QStringLiteral("Eyedropper (I)");
+    case NavigationTool::hand: return QStringLiteral("Hand (H)");
+    case NavigationTool::zoom: return QStringLiteral("Zoom (Z)");
+    // No tool sits in no rail: nothing to name.
+    case NavigationTool::idle: break;
+    }
+    throw std::logic_error("no tool has no label");
+}
+
+bool isBrushTool(NavigationTool tool)
+{
+    return tool == NavigationTool::brush || tool == NavigationTool::spotHealing
+        || tool == NavigationTool::cloneStamp || tool == NavigationTool::blur;
+}
+
+bool isSelectionTool(NavigationTool tool)
+{
+    return tool == NavigationTool::marquee || tool == NavigationTool::lasso || tool == NavigationTool::wand;
+}

src/Document/EditorSession+Model.h

@@ -0,0 +1,85 @@
+#pragma once
+#include "Document/LayerAdjustment.h"
+#include "Document/LayerAppearance.h"
+#include "Document/LayerGroups.h"
+#include "Document/LayerMask.h"
+#include "Document/LayerTransform.h"
+#include "Document/Selection.h"
+#include "Document/ShapeTool.h"
+#include "Document/TypeTool.h"
+#include "IO/ImageImporter.h"
+#include <QString>
+#include <QUuid>
+#include <optional>
+#include <vector>
+
+struct ImageLayer {
+    QUuid id;
+    std::optional<ImportedImage> asset;
+    LayerTransform transform;
+    QString name;
+    bool isVisible = true;
+    std::optional<QUuid> parentID;
+    bool isGroup = false;
+    double opacity = 1;
+    LayerBlendMode blendMode = LayerBlendMode::normal;
+    std::optional<QUuid> maskSourceID;
+    std::optional<LayerMask> mask;
+    std::optional<LayerAdjustment> adjustment;
+    // Set on layers the Shape tool made; see `liveShape`.
+    std::optional<LayerShape> shape;
+    // A stroke, shadows or overlay drawn round the pixels.
+    std::optional<LayerEffects> effects;
+    std::optional<LayerText> text;
+
+    ImageLayer(ImportedImage imported, QPointF origin);
+    ImageLayer(QString name, QSizeF blankSize);
+    // A layer as a project records it.
+    ImageLayer(const ProjectLayerRecord &record, std::optional<ImportedImage> asset, std::optional<LayerMask> mask);
+
+    QPointF origin() const { return transform.origin; }
+    QSizeF size() const { return transform.size; }
+    ProjectLayerRecord hierarchyRecord() const;
+    LayerTransform maskTransform() const;
+    // The shape this layer still is: none once repainted otherwise.
+    std::optional<LayerShape> liveShape() const;
+    // The text this layer still is: none once repainted otherwise.
+    std::optional<LayerText> liveText() const;
+    friend bool operator==(const ImageLayer &lhs, const ImageLayer &rhs);
+};
+
+// The layer's place in the list, or -1.
+int indexOf(const std::vector<ImageLayer> &layers, std::optional<QUuid> id);
+
+struct ProjectSnapshot;
+
+struct CanvasDocument {
+    QUuid id;
+    int width;
+    int height;
+    double resolution = 72;
+    std::vector<ImageLayer> layers; // Bottom to top.
+    // Part of the document so undo covers it; never saved.
+    std::optional<DocumentSelection> selection;
+
+    CanvasDocument(int width, int height);
+    // The canvas a project holds, each layer as recorded.
+    explicit CanvasDocument(const ProjectSnapshot &snapshot);
+
+    QSizeF size() const { return QSizeF(width, height); }
+    static std::optional<int> validDimension(const QString &value);
+    std::vector<LayerHierarchy::Entry> hierarchyEntries() const;
+    QSet<QUuid> effectiveVisibleIDs() const;
+    std::vector<ImageLayer> renderLayers() const;
+    friend bool operator==(const CanvasDocument &lhs, const CanvasDocument &rhs) = default;
+};
+
+enum class NavigationTool {
+    move, marquee, lasso, wand, crop, brush, spotHealing, cloneStamp, blur,
+    gradient, shape, type, eyedropper, hand, zoom, idle
+};
+
+// The rail's tooltip: the tool's name and its key.
+QString label(NavigationTool tool);
+bool isBrushTool(NavigationTool tool);
+bool isSelectionTool(NavigationTool tool);

src/Document/EditorSession+Projects.cpp

@@ -0,0 +1,71 @@
+#include "Document/EditorSession.h"
+#include "Logging.h"
+
+std::optional<ProjectSnapshot> EditorSession::projectSnapshot() const
+{
+    if (!m_document)
+        return std::nullopt;
+    ProjectSnapshot snapshot{.manifest = {.resolution = m_document->resolution, .documentID = m_document->id, .width = m_document->width,
+                                          .height = m_document->height, .activeLayerID = m_activeLayerID, .layers = {}},
+                             .images = {}};
+    for (const ImageLayer &layer : m_document->layers) {
+        if (layer.asset)
+            snapshot.images.insert({layer.id, *layer.asset});
+        if (layer.mask)
+            snapshot.masks.insert({layer.id, layer.mask->asset});
+        snapshot.manifest.layers.push_back(layer.hierarchyRecord());
+    }
+    return snapshot;
+}
+
+void EditorSession::installProject(const ProjectSnapshot &snapshot, const QString &path)
+{
+    m_collapsedGroupIDs.clear();
+    m_isMaskSelected = false;
+    m_cropRect.reset();
+    m_transformEdit = std::nullopt;
+    m_document = CanvasDocument(snapshot);
+    // Install loads shapes and text; Swift's resize rebuild does not.
+    for (size_t index = 0; index < m_document->layers.size(); ++index) {
+        ImageLayer &layer = m_document->layers[index];
+        layer.shape = LayerShape::loaded(snapshot.manifest.layers[index].shape, layer.asset);
+        layer.text = LayerText::loaded(snapshot.manifest.layers[index].text, layer.asset);
+    }
+    setActiveLayerID(snapshot.manifest.activeLayerID);
+    m_projectPath = path;
+    m_renamingLayerID = std::nullopt;
+    history.reset();
+    viewport.fit(m_document->size());
+    qCInfo(lcApp).noquote() << "installed a project of" << int(m_document->layers.size()) << "layers from" << path;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::setProjectPath(std::optional<QString> path)
+{
+    m_projectPath = std::move(path);
+    notify();
+}
+
+void EditorSession::clearProject()
+{
+    m_collapsedGroupIDs.clear();
+    m_isMaskSelected = false;
+    m_cropRect.reset();
+    m_transformEdit = std::nullopt;
+    m_document = std::nullopt;
+    setActiveLayerID(std::nullopt);
+    m_renamingLayerID = std::nullopt;
+    m_projectPath = std::nullopt;
+    history.reset();
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::createNewProject(int width, int height)
+{
+    if (m_isProjectBusy || m_isImporting || width < 1 || width > 30'000 || height < 1 || height > 30'000)
+        return;
+    clearProject();
+    createDocument(width, height, true);
+}

src/Document/EditorSession+Projects.h

@@ -0,0 +1,7 @@
+// Swift's EditorSession+Projects extension: snapshots in and out.
+public:
+    std::optional<ProjectSnapshot> projectSnapshot() const;
+    // Only a package that loaded and validated whole is installed.
+    void installProject(const ProjectSnapshot &snapshot, const QString &path);
+    void clearProject();
+    void createNewProject(int width, int height);

src/Document/EditorSession+Transform.cpp

@@ -0,0 +1,296 @@
+#include "Document/Distort.h"
+#include "Document/EditorSession.h"
+#include <algorithm>
+
+bool EditorSession::canTransform() const
+{
+    if (!canEditLayers())
+        return false;
+    if (transformsAsGroup())
+        return !groupTransformMembers().empty();
+    const std::optional<ImageLayer> active = activeLayer();
+    return active && active->asset && m_document->effectiveVisibleIDs().contains(active->id);
+}
+
+bool EditorSession::transformsAsGroup() const
+{
+    const std::optional<ImageLayer> active = activeLayer();
+    return m_selectedLayerIDs.size() > 1 || (m_selectedLayerIDs.size() == 1 && active && active->isGroup);
+}
+
+std::vector<ImageLayer> EditorSession::groupTransformMembers() const
+{
+    if (!transformsAsGroup() || !m_document)
+        return {};
+    QHash<QUuid, std::optional<QUuid>> parents;
+    for (const ImageLayer &layer : m_document->layers)
+        parents.insert(layer.id, layer.parentID);
+    const QSet<QUuid> visible = m_document->effectiveVisibleIDs();
+    std::vector<ImageLayer> members;
+    for (const ImageLayer &layer : m_document->layers) {
+        if (!layer.asset || !visible.contains(layer.id))
+            continue;
+        // Selected itself, or inside a selected folder.
+        std::optional<QUuid> current = layer.id;
+        for (int step = 0; step < 64 && current; ++step) {
+            if (m_selectedLayerIDs.contains(*current)) {
+                members.push_back(layer);
+                break;
+            }
+            current = parents.value(*current);
+        }
+    }
+    return members;
+}
+
+std::optional<LayerTransform> EditorSession::groupTransformBox() const
+{
+    const std::vector<ImageLayer> members = groupTransformMembers();
+    if (members.empty())
+        return std::nullopt;
+    QPointF low = DistortWarp::corners(members.front().transform)[0], high = low;
+    for (const ImageLayer &member : members) {
+        for (const QPointF corner : DistortWarp::corners(member.transform)) {
+            low = {std::min(low.x(), corner.x()), std::min(low.y(), corner.y())};
+            high = {std::max(high.x(), corner.x()), std::max(high.y(), corner.y())};
+        }
+    }
+    // Rounding can leave a unit layer's box under one pixel.
+    return LayerTransform{.origin = low, .size = {std::max(1.0, high.x() - low.x()), std::max(1.0, high.y() - low.y())}};
+}
+
+void EditorSession::beginTransform(bool persistent)
+{
+    cancelCrop();
+    const std::optional<ImageLayer> layer = activeLayer();
+    if (!canTransform() || !layer)
+        return;
+    m_tool = NavigationTool::move;
+    if (transformsAsGroup()) {
+        const std::optional<LayerTransform> box = groupTransformBox();
+        if (!box)
+            return;
+        QHash<QUuid, LayerTransform> originals;
+        for (const ImageLayer &member : groupTransformMembers())
+            originals.insert(member.id, member.transform);
+        m_transformEdit = TransformEdit{layer->id, *box, TransformGroup{*box, originals}, false, persistent, std::nullopt, nullptr};
+    } else {
+        // Linked, layer and mask move together as the layer.
+        const bool maskAlone = transformTargetsMask();
+        m_transformEdit = TransformEdit{layer->id, maskAlone ? layer->maskTransform() : layer->transform, std::nullopt, maskAlone, persistent, std::nullopt, nullptr};
+    }
+    notify();
+}
+
+void EditorSession::setTransformAutoSelect(bool picks)
+{
+    m_transformAutoSelect = picks;
+    notify();
+}
+
+void EditorSession::setShowsTransformControls(bool shows)
+{
+    m_showsTransformControls = shows;
+    notify();
+}
+
+void EditorSession::setLocksTransformRatio(bool locks)
+{
+    m_locksTransformRatio = locks;
+    notify();
+}
+
+// A folder has no pixels to copy: it just moves.
+void EditorSession::beginDuplicateTransform()
+{
+    if (m_transformDuplicate || !m_activeLayerID)
+        return;
+    const QUuid primary = *m_activeLayerID;
+    commitTransform();
+    if (!canTransform())
+        return;
+    const QSet<QUuid> selection = m_selectedLayerIDs;
+    // Bottom to top: the copies keep their order.
+    std::vector<QUuid> targets;
+    for (const ImageLayer &layer : m_document->layers) {
+        if (selection.contains(layer.id) && !layer.isGroup)
+            targets.push_back(layer.id);
+    }
+    if (targets.empty())
+        return;
+    beginEdit(targets.size() > 1 ? QStringLiteral("Duplicate Layers") : QStringLiteral("Duplicate Layer"));
+    std::vector<QUuid> copies;
+    for (const QUuid &id : targets) {
+        selectLayer(id);
+        duplicateActiveLayer();
+        copies.push_back(m_activeLayerID.value());
+    }
+    m_transformDuplicate = TransformDuplicate{copies, selection, primary};
+    selectLayers(QSet<QUuid>(copies.begin(), copies.end()), copies.back());
+    beginTransform(false);
+}
+
+// The duplicate's step closes with its transform.
+void EditorSession::endDuplicateTransform()
+{
+    if (!m_transformDuplicate)
+        return;
+    m_transformDuplicate = std::nullopt;
+    endEdit();
+}
+
+void EditorSession::previewTransform(const LayerTransform &value)
+{
+    if (!value.isValid() || !m_transformEdit)
+        return;
+    m_transformEdit->draft = value;
+    notify();
+}
+
+void EditorSession::commitTransform()
+{
+    snapGuides = {};
+    // Whatever ends a transform ends these two as well.
+    if (m_blendPreview) {
+        m_blendPreview = std::nullopt;
+        notify();
+    }
+    finishOpacityEdit();
+    if (!m_transformEdit)
+        return;
+    if (m_transformEdit->floating) {
+        // Unchanged: restored exactly, so soft edges keep no seam.
+        const TransformEdit edit = *std::exchange(m_transformEdit, std::nullopt);
+        if (edit.draft == edit.floating->original && !edit.corners)
+            cancelFloatingTransform(*edit.floating);
+        else
+            mergeFloatingTransform(edit, *edit.floating);
+        return;
+    }
+    if (m_transformEdit->mask) {
+        commitMaskTransform();
+        return;
+    }
+    if (m_transformEdit->corners) {
+        const TransformEdit edit = *m_transformEdit;
+        m_transformEdit = std::nullopt;
+        commitDistort(edit, *edit.corners);
+        endDuplicateTransform();
+        return;
+    }
+    // One transaction: an edit that moves nothing records nothing.
+    beginEdit(m_transformEdit->group ? QStringLiteral("Transform Layers") : QStringLiteral("Transform Layer"));
+    for (int index = 0; index < int(m_document->layers.size()); ++index) {
+        ImageLayer &layer = m_document->layers[size_t(index)];
+        // A layer the box squeezes under a pixel stays put.
+        const std::optional<LayerTransform> moved = pendingTransform(layer);
+        if (!moved || !moved->isValid())
+            continue;
+        // A linked mask goes along; an unlinked one stays.
+        if (layer.mask)
+            layer.mask->placement = layer.mask->placementMovingLayer(layer.transform, *moved);
+        layer.transform = *moved;
+        redrawShape(index);
+    }
+    m_transformEdit = std::nullopt;
+    endEdit();
+    endDuplicateTransform();
+}
+
+void EditorSession::cancelTransform()
+{
+    snapGuides = {};
+    if (!m_transformEdit)
+        return;
+    const std::shared_ptr<const FloatingTransform> floating = std::exchange(m_transformEdit, std::nullopt)->floating;
+    if (m_transformDuplicate) {
+        // The copies go and the old selection returns.
+        const TransformDuplicate duplicate = *m_transformDuplicate;
+        std::erase_if(m_document->layers, [&](const ImageLayer &layer) {
+            return std::find(duplicate.copies.begin(), duplicate.copies.end(), layer.id) != duplicate.copies.end();
+        });
+        selectLayers(duplicate.source, duplicate.primary);
+        endDuplicateTransform();
+    }
+    if (floating)
+        cancelFloatingTransform(*floating);
+    notify();
+}
+
+std::optional<QSizeF> EditorSession::transformPixelSize() const
+{
+    if (m_transformEdit && m_transformEdit->group)
+        return m_transformEdit->group->box.size;
+    if (!m_transformEdit && transformsAsGroup()) {
+        const std::optional<LayerTransform> box = groupTransformBox();
+        return box ? std::optional(box->size) : std::nullopt;
+    }
+    // A mask has no pixel size to scale against.
+    if (transformTargetsMask())
+        return std::nullopt;
+    if (m_transformEdit && m_transformEdit->floating)
+        return m_transformEdit->floating->pixelSize;
+    const std::optional<ImageLayer> active = activeLayer();
+    return active && active->asset ? std::optional(QSizeF(active->asset->size())) : std::nullopt;
+}
+
+bool EditorSession::transformTargetsMask() const
+{
+    if (m_transformEdit)
+        return m_transformEdit->mask;
+    const std::optional<ImageLayer> active = activeLayer();
+    return m_isMaskSelected && active && active->mask && !active->mask->isLinked;
+}
+
+LayerTransform EditorSession::displayedTransform(const ImageLayer &layer) const
+{
+    if (const std::optional<LayerTransform> pending = pendingTransform(layer))
+        return *pending;
+    // Content-Aware Fill past the edge previews on the grown layer.
+    if (m_filterEdit && m_filterEdit->grownTransform && m_filterEdit->previewImage(layer.id))
+        return *m_filterEdit->grownTransform;
+    return layer.transform;
+}
+
+LayerTransform EditorSession::editedTransform(const ImageLayer &layer) const
+{
+    if (m_transformEdit && m_transformEdit->layerID == layer.id)
+        return m_transformEdit->draft;
+    // Before an edit the handles sit on the group's box.
+    if (!m_transformEdit && layer.id == m_activeLayerID && transformsAsGroup())
+        return groupTransformBox().value_or(layer.transform);
+    return layer.id == m_activeLayerID && transformTargetsMask() ? layer.maskTransform() : layer.transform;
+}
+
+std::optional<LayerTransform> EditorSession::pendingTransform(const ImageLayer &layer) const
+{
+    // An edit of the mask alone moves no layer.
+    if (!m_transformEdit || m_transformEdit->mask)
+        return std::nullopt;
+    if (m_transformEdit->group) {
+        const TransformGroup &group = *m_transformEdit->group;
+        const auto original = group.originals.constFind(layer.id);
+        return original == group.originals.constEnd() ? std::nullopt : std::optional(original->following(group.box, m_transformEdit->draft));
+    }
+    return m_transformEdit->layerID == layer.id ? std::optional(m_transformEdit->draft) : std::nullopt;
+}
+
+void EditorSession::nudgeLayer(double dx, double dy)
+{
+    const bool alreadyEditing = m_transformEdit.has_value();
+    if (!alreadyEditing)
+        beginTransform(false);
+    if (!m_transformEdit)
+        return;
+    LayerTransform value = m_transformEdit->draft;
+    value.origin += QPointF(dx, dy);
+    previewTransform(value);
+    if (const std::optional<Corners> corners = m_transformEdit->corners) {
+        Corners moved = *corners;
+        for (QPointF &corner : moved)
+            corner += QPointF(dx, dy);
+        previewCorners(moved);
+    }
+    if (!alreadyEditing)
+        commitTransform();
+}

src/Document/EditorSession.cpp

@@ -0,0 +1,347 @@
+#include "Document/EditorSession.h"
+#include "Logging.h"
+#include <cmath>
+
+EditorSession::EditorSession(QObject *parent) : QObject(parent), m_busyTimer(this)
+{
+    // A coarse timer may fire a twentieth early.
+    m_busyTimer.setTimerType(Qt::PreciseTimer);
+    m_busyTimer.setSingleShot(true);
+    m_busyTimer.setInterval(busyIndicatorDelay);
+    connect(&m_busyTimer, &QTimer::timeout, this, [this] {
+        m_showsBusy = true;
+        notify();
+    });
+    connect(&m_decoder, &QFutureWatcher<Decoded>::finished, this, &EditorSession::finishDecode);
+    connect(&m_baker, &QFutureWatcher<Baked>::finished, this, &EditorSession::finishBake);
+    connect(&m_inverter, &QFutureWatcher<Inverted>::finished, this, &EditorSession::finishInvert);
+    connect(&m_wand, &QFutureWatcher<Wanded>::finished, this, &EditorSession::finishWand);
+    connect(&m_filterPreview, &QFutureWatcher<Filtered>::finished, this, &EditorSession::finishFilterPreview);
+    connect(&m_filterCommit, &QFutureWatcher<FilterMade>::finished, this, &EditorSession::finishFilterCommit);
+    connect(&m_cropCommit, &QFutureWatcher<Cropped>::finished, this, &EditorSession::finishCropCommit);
+    connect(&m_levelsHistogram, &QFutureWatcher<std::optional<LevelsHistogram>>::finished, this, &EditorSession::finishLevelsHistogram);
+    connect(&m_levelsPreview, &QFutureWatcher<std::optional<QImage>>::finished, this, &EditorSession::finishLevelsPreview);
+    connect(&m_levelsCommit, &QFutureWatcher<Leveled>::finished, this, &EditorSession::finishLevelsCommit);
+    connect(&m_hueSaturationTask, &QFutureWatcher<Adjusted>::finished, this, &EditorSession::finishHueSaturationPreview);
+    connect(&m_hueSaturationCommit, &QFutureWatcher<Adjusted>::finished, this, &EditorSession::finishHueSaturationCommit);
+    connect(&m_rasterCommit, &QFutureWatcher<RasterMade>::finished, this, &EditorSession::finishRasterCommit);
+}
+
+void EditorSession::setActiveLayerID(std::optional<QUuid> id)
+{
+    // Another layer: its pixels are the target again.
+    if (id != m_activeLayerID)
+        m_isMaskSelected = false;
+    m_activeLayerID = id;
+    m_selectedLayerIDs = id ? QSet<QUuid>{*id} : QSet<QUuid>();
+    notify();
+}
+
+std::optional<ImageLayer> EditorSession::activeLayer() const
+{
+    if (!m_document || !m_activeLayerID)
+        return std::nullopt;
+    const auto found = std::find_if(m_document->layers.begin(), m_document->layers.end(),
+                                    [&](const ImageLayer &layer) { return layer.id == *m_activeLayerID; });
+    return found == m_document->layers.end() ? std::nullopt : std::optional(*found);
+}
+
+void EditorSession::setIsProjectBusy(bool busy)
+{
+    m_isProjectBusy = busy;
+    // Freed waiters each look again when their turn comes.
+    if (!busy) {
+        for (const std::function<void()> &waiter : std::exchange(m_projectWaiters, {}))
+            QMetaObject::invokeMethod(this, [this, waiter] { waitForProjectAccess(waiter); }, Qt::QueuedConnection);
+    }
+    resumeFileRequests();
+    updateBusyIndicator();
+    notify();
+}
+
+void EditorSession::setShowsNewDocument(bool shows)
+{
+    m_showsNewDocument = shows;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::setShowsImporter(bool shows)
+{
+    m_showsImporter = shows;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::setIsImporting(bool importing)
+{
+    m_isImporting = importing;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::setImportError(std::optional<QString> error)
+{
+    m_importError = std::move(error);
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::setBrushError(std::optional<QString> error)
+{
+    m_brushError = std::move(error);
+    notify();
+}
+
+void EditorSession::setRenamingLayerID(std::optional<QUuid> id)
+{
+    m_renamingLayerID = id;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::setAdjustmentEditingID(std::optional<QUuid> id)
+{
+    m_adjustmentEditingID = id;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::selectLayer(std::optional<QUuid> id)
+{
+    dropEffectSelection();
+    if (id != m_activeLayerID && !finishText())
+        return;
+    if (m_brushStroke || m_warpStroke || m_levels)
+        return;
+    if (id != m_activeLayerID) {
+        commitTransform();
+        resolveGradient();
+    }
+    setActiveLayerID(id);
+}
+
+void EditorSession::selectTool(NavigationTool value)
+{
+    if (m_tool != value && !finishText())
+        return;
+    if (m_isProjectBusy || m_brushStroke || m_warpStroke || m_levels)
+        return;
+    if (m_tool != value) {
+        commitTransform();
+        cancelCrop();
+        resolveGradient();
+        cancelLasso();
+        cancelShape();
+    }
+    // Each family's tip parks on leaving; sessions begin in 0.
+    const int from = tipFamily(m_tool), to = tipFamily(value);
+    if (from != to) {
+        const ParkedTip parked = m_parkedBrushTips.at(to);
+        m_parkedBrushTips[from] = ParkedTip{m_brushSettings.diameter, m_brushSettings.hardness, m_brushSettings.opacity};
+        m_brushSettings.diameter = parked.diameter;
+        m_brushSettings.hardness = parked.hardness;
+        m_brushSettings.opacity = parked.opacity;
+    }
+    m_tool = value;
+    // The Crop tool opens on the whole canvas, freely.
+    if (value == NavigationTool::crop && !m_cropRect && m_document) {
+        m_cropRatioChoice = QStringLiteral("Free");
+        m_cropRect = QRectF(QPointF(0, 0), m_document->size());
+    }
+    notify();
+}
+
+int EditorSession::tipFamily(NavigationTool tool)
+{
+    return tool == NavigationTool::cloneStamp ? 1 : tool == NavigationTool::blur ? 2 : 0;
+}
+
+// Tab: the tool's next mode; others arrive with their tools.
+void EditorSession::cycleToolMode()
+{
+    if (m_isProjectBusy || m_brushStroke || m_warpStroke)
+        return;
+    if (m_tool == NavigationTool::marquee)
+        toggleMarqueeKind();
+    else if (m_tool == NavigationTool::lasso)
+        toggleLassoKind();
+    else if (m_tool == NavigationTool::shape)
+        toggleShapeKind();
+    else if (m_tool == NavigationTool::brush)
+        setBrushMode(m_brushMode == BrushToolMode::paint ? BrushToolMode::erase : BrushToolMode::paint);
+    else if (m_tool == NavigationTool::gradient)
+        setGradientSettings(GradientSettings{allGradientShapes[(size_t(m_gradientSettings.shape) + 1) % allGradientShapes.size()], m_gradientSettings.style,
+                                             m_gradientSettings.reversed, m_gradientSettings.opacity});
+    else if (m_tool == NavigationTool::blur)
+        setBlurMode(allBlurToolModes[(size_t(m_blurMode) + 1) % allBlurToolModes.size()]);
+    else if (m_tool == NavigationTool::spotHealing)
+        setSpotHealingMode(allSpotHealingModes[(size_t(m_spotHealingMode) + 1) % allSpotHealingModes.size()]);
+    else if (m_tool == NavigationTool::cloneStamp)
+        setCloneSettings(CloneSettings{m_cloneSettings.aligned, !m_cloneSettings.sampleAllLayers});
+}
+
+bool EditorSession::canUseHistory() const
+{
+    return !m_textDraft && !m_isProjectBusy && !m_isImporting && !m_brushStroke && !m_warpStroke && !m_showsNewDocument && !m_showsImporter
+        && !m_renamingLayerID && !m_importError && !m_transformEdit && !m_levels;
+}
+
+void EditorSession::undo()
+{
+    // As Photoshop, the first Undo discards a pending gradient.
+    if (m_gradientEdit) {
+        cancelGradient();
+        return;
+    }
+    if (!canUndo())
+        return;
+    if (const std::optional<DocumentHistory::Snapshot> snapshot = history.undo())
+        restore(*snapshot);
+}
+
+void EditorSession::redo()
+{
+    if (!canRedo())
+        return;
+    if (const std::optional<DocumentHistory::Snapshot> snapshot = history.redo())
+        restore(*snapshot);
+}
+
+void EditorSession::restore(const DocumentHistory::Snapshot &snapshot)
+{
+    cancelCrop();
+    cancelGradient();
+    const auto canvas = [](const std::optional<CanvasDocument> &document) { return document ? std::optional(document->id) : std::nullopt; };
+    const bool changedCanvas = canvas(m_document) != canvas(snapshot.document);
+    // The mask stays the target while layer and mask remain.
+    const bool keepsMaskTarget = m_isMaskSelected && m_activeLayerID == snapshot.activeLayerID;
+    m_document = snapshot.document;
+    setActiveLayerID(snapshot.activeLayerID);
+    const std::optional<ImageLayer> active = activeLayer();
+    m_isMaskSelected = keepsMaskTarget && active && active->mask;
+    if (changedCanvas && m_document)
+        viewport.fit(m_document->size());
+    qCInfo(lcApp) << "restored a history snapshot of" << (m_document ? int(m_document->layers.size()) : 0) << "layers";
+    notify();
+}
+
+void EditorSession::beginEdit(const QString &name)
+{
+    history.begin(name, m_document, m_activeLayerID);
+    // An open transaction closes undo and redo: observers must know.
+    notify();
+}
+
+void EditorSession::endEdit()
+{
+    history.end(m_document, m_activeLayerID);
+    notify();
+}
+
+bool EditorSession::canEditLayers() const
+{
+    return !m_textDraft && m_document && !m_brushStroke && !m_warpStroke && !m_isProjectBusy && !m_isImporting && !m_showsNewDocument && !m_showsImporter
+        && !m_renamingLayerID && !m_transformEdit && !m_cropRect && !m_gradientEdit && !m_filterEdit && !m_pixelMove && !m_levels
+        && !m_hueSaturation && !m_adjustmentEditingID;
+}
+
+void EditorSession::insert(const ImportedImage &asset, std::optional<QPointF> centeredAt)
+{
+    beginEdit(QStringLiteral("Import Image"));
+    if (!m_document) {
+        m_document = CanvasDocument(asset.size().width(), asset.size().height());
+        viewport.fit(m_document->size());
+    }
+    const QPointF center = centeredAt.value_or(QPointF(m_document->width / 2.0, m_document->height / 2.0));
+    ImageLayer layer(asset, QPointF(std::floor(center.x() - asset.size().width() / 2.0), std::floor(center.y() - asset.size().height() / 2.0)));
+    // Beside the active layer, or inside the active folder.
+    const std::optional<ImageLayer> active = activeLayer();
+    layer.parentID = active && active->isGroup ? m_activeLayerID : active ? active->parentID : std::nullopt;
+    if (layer.parentID)
+        m_collapsedGroupIDs.remove(*layer.parentID);
+    m_document->layers.push_back(layer);
+    setActiveLayerID(layer.id);
+    endEdit();
+}
+
+void EditorSession::createDocument(int width, int height, bool emptyLayer)
+{
+    if (m_isProjectBusy || m_isImporting || width < 1 || width > 30'000 || height < 1 || height > 30'000)
+        return;
+    commitTransform();
+    beginEdit(QStringLiteral("New Canvas"));
+    CanvasDocument document(width, height);
+    // File > New starts with a selected blank "Layer 1".
+    std::optional<QUuid> selected;
+    if (emptyLayer) {
+        document.layers.push_back(ImageLayer(QStringLiteral("Layer 1"), document.size()));
+        selected = document.layers.back().id;
+    }
+    m_document = document;
+    setActiveLayerID(selected);
+    m_renamingLayerID = std::nullopt;
+    viewport.fit(m_document->size());
+    m_showsNewDocument = false;
+    // A closed sheet or an ended rename frees file requests.
+    resumeFileRequests();
+    qCInfo(lcApp) << "new canvas" << width << "x" << height;
+    endEdit();
+}
+
+void EditorSession::fit()
+{
+    if (!m_document)
+        return;
+    const CanvasViewport before = viewport;
+    viewport.fit(m_document->size());
+    if (!(viewport == before))
+        notify();
+}
+
+void EditorSession::zoom(double value, std::optional<QPointF> anchor)
+{
+    if (!m_document)
+        return;
+    // The viewport refuses a zoom that is no number.
+    const CanvasViewport before = viewport;
+    viewport.setZoom(value, anchor.value_or(viewport.center()), m_document->size());
+    if (!(viewport == before))
+        notify();
+}
+
+void EditorSession::setShowsPixelGrid(bool shows)
+{
+    m_showsPixelGrid = shows;
+    notify();
+}
+
+void EditorSession::setCropRect(std::optional<QRectF> rect)
+{
+    if (std::exchange(m_cropRect, rect) != rect)
+        notify();
+}
+
+void EditorSession::setCropRatioChoice(const QString &choice)
+{
+    if (std::exchange(m_cropRatioChoice, choice) != choice)
+        notify();
+}
+
+void EditorSession::setCropError(std::optional<QString> error)
+{
+    if (std::exchange(m_cropError, error) != error)
+        notify();
+}
+
+void EditorSession::setShowsSampleRing(bool shows)
+{
+    m_showsSampleRing = shows;
+    notify();
+}
+
+void EditorSession::requestCanvasFocus()
+{
+    ++m_canvasFocusRequest;
+    notify();
+}

src/Document/EditorSession.h

@@ -0,0 +1,437 @@
+#pragma once
+#include "Document/BrushStroke.h"
+#include "Document/CloneStamp.h"
+#include "Document/ColorPalette.h"
+#include "Document/Gradient.h"
+#include "Document/Distort.h"
+#include "Document/Filters.h"
+#include "Document/LevelsAutomatic.h"
+#include "Document/FloatingSelection.h"
+#include "Document/MagicWand.h"
+#include "Document/SelectionClipboard.h"
+#include "Document/SelectionEdits.h"
+#include "Document/SmudgeLiquify.h"
+#include "Document/DocumentHistory.h"
+#include "Document/EditorSession+Jobs.h"
+#include "Document/EditorSession+Model.h"
+#include "IO/ProjectStore.h"
+#include "Rendering/CanvasViewport.h"
+#include "Rendering/EffectsPreviewCache.h"
+#include <QFutureWatcher>
+#include <QObject>
+#include <QSet>
+#include <QTimer>
+#include <QUrl>
+#include <deque>
+#include <map>
+#include <functional>
+#include <set>
+
+class QPainter;
+
+// The one editor state; `changed` stands in for Swift's observation.
+class EditorSession : public QObject, private SessionJobs {
+    Q_OBJECT
+public:
+    explicit EditorSession(QObject *parent = nullptr);
+
+    // The canvas edits this in place, then calls notify().
+    CanvasViewport viewport;
+    DocumentHistory history;
+    // Swift's @ObservationIgnored tuple: the canvas draws it unannounced.
+    SnapGuides snapGuides;
+    // The canvas's effects previews, rendered off the UI thread.
+    EffectsPreviewCache effectsPreviews;
+
+    const std::optional<CanvasDocument> &document() const { return m_document; }
+    NavigationTool tool() const { return m_tool; }
+    const QSet<QUuid> &collapsedGroupIDs() const { return m_collapsedGroupIDs; }
+    const QSet<QUuid> &selectedLayerIDs() const { return m_selectedLayerIDs; }
+    std::optional<QUuid> activeLayerID() const { return m_activeLayerID; }
+    void setActiveLayerID(std::optional<QUuid> id);
+    std::optional<ImageLayer> activeLayer() const;
+
+    // Blocks overlapping edits at once.
+    bool isProjectBusy() const { return m_isProjectBusy; }
+    void setIsProjectBusy(bool busy);
+    // Busy this long, in milliseconds, before the controls dim.
+    static constexpr int busyIndicatorDelay = 250;
+    bool showsBusy() const { return m_showsBusy; }
+    bool showsNewDocument() const { return m_showsNewDocument; }
+    void setShowsNewDocument(bool shows);
+    bool showsImporter() const { return m_showsImporter; }
+    void setShowsImporter(bool shows);
+    bool isImporting() const { return m_isImporting; }
+    void setIsImporting(bool importing);
+    std::optional<QString> importError() const { return m_importError; }
+    void setImportError(std::optional<QString> error);
+    std::optional<QUuid> renamingLayerID() const { return m_renamingLayerID; }
+    void setRenamingLayerID(std::optional<QUuid> id);
+
+    // The first tab's first canvas sheet skips the clipboard.
+    bool skipsInitialClipboardCanvasSize = false;
+    // Where the project lives on disk, once opened or saved.
+    const std::optional<QString> &projectPath() const { return m_projectPath; }
+    void setProjectPath(std::optional<QString> path);
+
+    bool canStartProjectOperation() const;
+    // `ready` runs at once, or when the session allows.
+    void waitForFileRequest(std::function<void()> ready);
+    void waitForProjectAccess(std::function<void()> ready);
+
+    void selectLayer(std::optional<QUuid> id);
+    void selectTool(NavigationTool value);
+    // Tab: the current tool's next mode.
+    void cycleToolMode();
+
+    // Whether the mask thumbnail, not the layer's, is the target.
+    bool isMaskSelected() const { return m_isMaskSelected; }
+
+    // A blend mode tried on by hovering; nothing is saved.
+    struct BlendPreview {
+        QUuid layerID;
+        LayerBlendMode mode;
+        friend bool operator==(const BlendPreview &, const BlendPreview &) = default;
+    };
+    const std::optional<BlendPreview> &blendPreview() const { return m_blendPreview; }
+
+    // A pending transform; layers change when it is committed.
+    const std::optional<TransformEdit> &transformEdit() const { return m_transformEdit; }
+    bool canTransform() const;
+    // Several layers, or a folder's contents, move in one box.
+    bool transformsAsGroup() const;
+    std::vector<ImageLayer> groupTransformMembers() const;
+    std::optional<LayerTransform> groupTransformBox() const;
+    void beginTransform(bool persistent = true);
+    // Alt-drag: the selection's copies move, one step.
+    void beginDuplicateTransform();
+    // The Move tool's settings (TransformInspector, Ctrl+H).
+    bool transformAutoSelect() const { return m_transformAutoSelect; }
+    void setTransformAutoSelect(bool picks);
+    bool showsTransformControls() const { return m_showsTransformControls; }
+    void setShowsTransformControls(bool shows);
+    bool locksTransformRatio() const { return m_locksTransformRatio; }
+    void setLocksTransformRatio(bool locks);
+    void previewTransform(const LayerTransform &value);
+    void commitTransform();
+    void cancelTransform();
+    std::optional<QSizeF> transformPixelSize() const;
+    LayerTransform displayedTransform(const ImageLayer &layer) const;
+    LayerTransform editedTransform(const ImageLayer &layer) const;
+    std::optional<LayerTransform> pendingTransform(const ImageLayer &layer) const;
+    // An unlinked mask, selected, transforms without its layer.
+    bool transformTargetsMask() const;
+    void nudgeLayer(double dx, double dy);
+
+    const std::optional<LassoDraft> &lassoDraft() const { return m_lassoDraft; }
+    LassoKind lassoKind() const { return m_lassoKind; }
+    void setLassoKind(LassoKind kind);
+    LassoKind marqueeKind() const { return m_marqueeKind; }
+    void setMarqueeKind(LassoKind kind);
+    // The options bar's mode; Shift adds, Alt subtracts.
+    SelectionMode selectionModeChoice() const { return m_selectionModeChoice; }
+    void setSelectionModeChoice(SelectionMode mode);
+    std::optional<SelectionMode> heldSelectionMode() const { return m_heldSelectionMode; }
+    bool selectionAntialiased() const { return m_selectionAntialiased; }
+    void setSelectionAntialiased(bool antialiased);
+    // Pixels Expand and Contract grow or shrink by.
+    int selectionExpandAmount() const { return m_selectionExpandAmount; }
+    void setSelectionExpandAmount(int amount);
+    int selectionContractAmount() const { return m_selectionContractAmount; }
+    void setSelectionContractAmount(int amount);
+    const std::optional<DocumentSelection> &selectionMoveOrigin() const { return m_selectionMoveOrigin; }
+    const WandSettings &wandSettings() const { return m_wandSettings; }
+    void setWandSettings(const WandSettings &settings);
+    const BrushStroke *brushStroke() const { return m_brushStroke.get(); }
+    const BrushSettings &brushSettings() const { return m_brushSettings; }
+    void setBrushSettings(const BrushSettings &settings);
+    BrushToolMode brushMode() const { return m_brushMode; }
+    void setBrushMode(BrushToolMode mode);
+    BlurToolMode blurMode() const { return m_blurMode; }
+    void setBlurMode(BlurToolMode mode);
+    // Smudge or Liquify under way; the canvas shows its copy.
+    const WarpStroke *warpStroke() const { return m_warpStroke.get(); }
+    SpotHealingMode spotHealingMode() const { return m_spotHealingMode; }
+    void setSpotHealingMode(SpotHealingMode mode);
+    bool maskPaintWhite() const { return m_maskPaintWhite; }
+    void setMaskPaintWhite(bool white);
+    const PaletteColor &backgroundColor() const { return m_backgroundColor; }
+    void setBackgroundColor(const PaletteColor &color);
+    // The open colour picker; the session notifies its changes.
+    const std::optional<ColorPickerState> &colorPicker() const { return m_colorPicker; }
+    const GradientSettings &gradientSettings() const { return m_gradientSettings; }
+    void setGradientSettings(const GradientSettings &settings);
+    // A gradient drawn and not yet applied.
+    const std::optional<GradientEdit> &gradientEdit() const { return m_gradientEdit; }
+    // Clone Stamp's Alt-clicked source, in document pixels, and options.
+    const std::optional<QPointF> &cloneSource() const { return m_cloneSource; }
+    const CloneSettings &cloneSettings() const { return m_cloneSettings; }
+    void setCloneSettings(const CloneSettings &settings);
+    // Counts every change to a stroke, so the canvas redraws.
+    int brushRevision() const { return m_brushRevision; }
+    // Selected pixels on the move, from a Ctrl-drag or Ctrl-arrow.
+    const PixelMove *pixelMove() const { return m_pixelMove.get(); }
+
+    // The one filter open; its sheet shows while it lasts.
+    const std::optional<FilterEdit> &filterEdit() const { return m_filterEdit; }
+    // The last committed filter's settings, where the next one starts.
+    const FilterSettings &filterSettings() const { return m_filterSettings; }
+    const std::optional<LevelsEdit> &levels() const { return m_levels; }
+    const std::optional<HueSaturationEdit> &hueSaturation() const { return m_hueSaturation; }
+    // The newest preview, waiting for the running one.
+    const std::optional<HueSaturationJob> &hueSaturationPending() const { return m_hueSaturationPending; }
+    std::optional<HueSampleMode> hueSampleMode() const { return m_hueSampleMode; }
+    bool hueTargeting() const { return m_hueTargeting; }
+    // The adjustment layer the colour editors write to.
+    std::optional<QUuid> adjustmentEditingID() const { return m_adjustmentEditingID; }
+    void setAdjustmentEditingID(std::optional<QUuid> id);
+    // Its settings as its editor opened, for Cancel.
+    const std::optional<LayerAdjustment> &adjustmentOriginal() const { return m_adjustmentOriginal; }
+    // The effect whose panel is open, bound to its layer.
+    const std::optional<LayerEffectSelection> &effectsEditing() const { return m_effectsEditing; }
+    void setEffectsEditing(std::optional<LayerEffectSelection> selection);
+    // Its layer's effects as the panel opened, for Cancel.
+    const std::optional<LayerEffects> &effectsEditingOriginal() const { return m_effectsEditingOriginal; }
+    void setEffectsEditingOriginal(std::optional<LayerEffects> original);
+    // The effect chosen in the list; see selectedEffect.
+    const std::optional<LayerEffectSelection> &effectSelection() const { return m_effectSelection; }
+
+    bool isModified() const { return history.isModified(); }
+    bool canUseHistory() const;
+    // Undo's first press discards a pending gradient.
+    bool canUndo() const { return canUseHistory() && (history.canUndo() || m_gradientEdit); }
+    bool canRedo() const { return canUseHistory() && history.canRedo(); }
+    void undo();
+    void redo();
+    // A transaction that nests; a whole gesture is one entry.
+    void beginEdit(const QString &name);
+    void endEdit();
+
+    bool canEditLayers() const;
+    void addBlankLayer();
+    void deleteLayer(QUuid id);
+    void deleteActiveLayer();
+    // Every selected layer, folders with their contents: one step.
+    void deleteSelectedLayers();
+    void renameLayer(QUuid id, const QString &name);
+    void toggleLayerVisibility(QUuid id);
+    // Pressing an eye, then dragging over others: one undo step.
+    std::optional<bool> beginVisibilitySwipe(QUuid id);
+    void setVisibilityInSwipe(QUuid id, bool visible);
+    void endVisibilitySwipe();
+    // The list runs top down; the document stores bottom up.
+    void reorderLayers(const std::set<int> &offsets, int destination);
+    bool canMoveActiveLayer(int offset) const;
+    void moveActiveLayer(int offset);
+
+    // Files decode off the UI thread; `done` follows this request.
+    void importImages(const QList<QUrl> &urls, std::optional<QPointF> point = std::nullopt, std::function<void()> done = {});
+    void insert(const ImportedImage &asset, std::optional<QPointF> centeredAt = std::nullopt);
+    void createDocument(int width, int height, bool emptyLayer = false);
+    void fit();
+    void zoom(double value, std::optional<QPointF> anchor = std::nullopt);
+    // The View menu's grid, shown from 800%.
+    bool showsPixelGrid() const { return m_showsPixelGrid; }
+    void setShowsPixelGrid(bool shows);
+    // The Eyedropper's ring: the sampled colour over the original.
+    bool showsSampleRing() const { return m_showsSampleRing; }
+    void setShowsSampleRing(bool shows);
+    // The Crop tool's frame in document pixels, once drawn.
+    const std::optional<QRectF> &cropRect() const { return m_cropRect; }
+    void setCropRect(std::optional<QRectF> rect);
+    // Swift's ratio names: Free, Original, 1:1, 4:3 and 16:9.
+    const QString &cropRatioChoice() const { return m_cropRatioChoice; }
+    void setCropRatioChoice(const QString &choice);
+    const std::optional<QString> &cropError() const { return m_cropError; }
+    void setCropError(std::optional<QString> error);
+    // Counted requests; the canvas takes focus on each new one.
+    int canvasFocusRequest() const { return m_canvasFocusRequest; }
+    void requestCanvasFocus();
+
+    // What Paste puts back in place, until another copy.
+    const std::optional<PixelClipboard> &pixelClipboard() const { return m_pixelClipboard; }
+    // What went wrong in a tool; the window shows it.
+    const std::optional<QString> &brushError() const { return m_brushError; }
+    void setBrushError(std::optional<QString> error);
+
+    void notify() { emit changed(); }
+
+    // Swift's extensions, each declared in its twin's header.
+#include "Document/EditorSession+Projects.h"
+#include "IO/ImageResizer+Session.h"
+#include "Document/LayerMask+Session.h"
+#include "Document/LayerAppearance+Session.h"
+#include "Document/Crop+Session.h"
+#include "Document/Distort+Session.h"
+#include "Document/LayerMerge+Session.h"
+#include "Document/LayerFlip+Session.h"
+#include "Document/Selection+Session.h"
+#include "Document/MagicWand+Session.h"
+#include "Document/EditorSession+Brush.h"
+#include "Document/CloneStamp+Session.h"
+#include "Document/ColorPalette+Session.h"
+#include "Document/Gradient+Session.h"
+#include "Document/ShapeTool+Session.h"
+#include "Document/TypeTool+Session.h"
+#include "Document/SmudgeLiquify+Session.h"
+#include "Document/BlurTool+Session.h"
+#include "Document/SelectionEdits+Session.h"
+#include "Document/SelectionClipboard+Session.h"
+#include "Document/FloatingSelection+Session.h"
+#include "Document/Filters+Session.h"
+#include "Document/Levels+Session.h"
+#include "Document/LevelsAutomatic+Session.h"
+#include "Document/HueSaturation+Session.h"
+#include "Document/MaskTracing+Session.h"
+#include "Document/LayerGroups+Session.h"
+#include "Document/LiveLayerMask+Session.h"
+#include "Document/LayerAdjustment+Session.h"
+#include "Document/AdjustmentEditing+Session.h"
+#include "Document/LayerEffects+Session.h"
+signals:
+    void changed();
+
+private:
+    // Copies layers into this document in place, as Swift does.
+    friend class ProjectWorkspace;
+    void restore(const DocumentHistory::Snapshot &snapshot);
+    void resumeFileRequests();
+    void updateBusyIndicator();
+    void drainImports();
+    void decodeNext();
+    void finishDecode();
+    void endDuplicateTransform();
+    // Clone Stamp is family 1, the Smear 2, others 0.
+    static int tipFamily(NavigationTool tool);
+
+    std::optional<CanvasDocument> m_document;
+    NavigationTool m_tool = NavigationTool::move;
+    QSet<QUuid> m_collapsedGroupIDs;
+    QSet<QUuid> m_selectedLayerIDs;
+    // The canvas reads previews from a const session: mutable caches.
+    mutable std::map<QUuid, DistortPreviewCache> m_distortPreviewCache;
+    mutable std::map<QUuid, DistortEffectsCache> m_distortEffectsCache;
+    mutable std::optional<MaskDistortPreviewCache> m_maskDistortPreviewCache;
+
+    std::optional<TransformEdit> m_transformEdit;
+    std::optional<TransformDuplicate> m_transformDuplicate;
+    bool m_transformAutoSelect = false;
+    bool m_showsTransformControls = true;
+    bool m_locksTransformRatio = true;
+    std::optional<BlendPreview> m_blendPreview;
+    std::optional<QUuid> m_opacityEditLayerID;
+    bool m_isMaskSelected = false;
+    std::optional<QUuid> m_activeLayerID;
+    bool m_isProjectBusy = false;
+    bool m_showsNewDocument = false;
+    bool m_showsImporter = false;
+    bool m_isImporting = false;
+    std::optional<QString> m_importError;
+    std::optional<QUuid> m_renamingLayerID;
+    std::optional<QString> m_projectPath;
+    bool m_showsPixelGrid = true;
+    bool m_showsSampleRing = true;
+    std::optional<QRectF> m_cropRect;
+    QString m_cropRatioChoice = QStringLiteral("Free");
+    std::optional<QString> m_cropError;
+    QFutureWatcher<Cropped> m_cropCommit;
+    std::function<void()> m_committingCrop;
+    int m_canvasFocusRequest = 0;
+    bool m_showsBusy = false;
+    QTimer m_busyTimer;
+    std::vector<std::function<void()>> m_projectWaiters;
+    std::vector<std::function<void()>> m_fileRequestWaiters;
+    std::deque<ImportRequest> m_pendingImports;
+    // Where the request now decoding stands.
+    qsizetype m_importIndex = 0;
+    std::optional<QPointF> m_importPoint;
+    QStringList m_importFailures;
+    QFutureWatcher<Decoded> m_decoder;
+    std::optional<QString> m_brushError;
+    // The layers a bake under way will delete.
+    std::vector<QUuid> m_bakingIDs;
+    QFutureWatcher<Baked> m_baker;
+    std::optional<Inverting> m_inverting;
+    QFutureWatcher<Inverted> m_inverter;
+    WandSettings m_wandSettings;
+    std::optional<Wanding> m_wanding;
+    QFutureWatcher<Wanded> m_wand;
+    std::unique_ptr<BrushStroke> m_brushStroke;
+    std::unique_ptr<PixelMove> m_pixelMove;
+    BrushSettings m_brushSettings;
+    BrushToolMode m_brushMode = BrushToolMode::paint;
+    BlurToolMode m_blurMode = BlurToolMode::liquify;
+    std::unique_ptr<WarpStroke> m_warpStroke;
+    SpotHealingMode m_spotHealingMode = SpotHealingMode::contentAware;
+    bool m_maskPaintWhite = false;
+    PaletteColor m_backgroundColor = PaletteColor::white();
+    std::optional<ColorPickerState> m_colorPicker;
+    GradientSettings m_gradientSettings;
+    std::optional<GradientEdit> m_gradientEdit;
+    ShapeKind m_shapeKind = ShapeKind::rectangle;
+    double m_shapeCornerRadius = 0;
+    double m_shapeLineWidth = 4;
+    std::optional<ShapeDraft> m_shapeDraft;
+    mutable std::map<QUuid, ShapePreview> m_shapeTransformPreviewCache;
+    std::optional<TextDraft> m_textDraft;
+    LayerTextStyle m_textDefaults;
+    std::optional<QPointF> m_cloneSource;
+    CloneSettings m_cloneSettings;
+    // The offset aligned strokes keep, fixed as a stroke begins.
+    std::optional<QSizeF> m_cloneOffset;
+    // Clone Stamp and the Smear keep their own soft tips.
+    std::map<int, ParkedTip> m_parkedBrushTips{{1, {40, 0, 1}}, {2, {40, 0, 1}}};
+    std::optional<LastBrushPoint> m_lastBrushPoint;
+    int m_brushRevision = 0;
+    std::optional<PendingOpacityDigit> m_pendingOpacityDigit;
+    std::optional<CommittingRaster> m_committingRaster;
+    QFutureWatcher<RasterMade> m_rasterCommit;
+    std::optional<FilterEdit> m_filterEdit;
+    FilterSettings m_filterSettings;
+    QFutureWatcher<Filtered> m_filterPreview;
+    // Out until its finish runs: setFuture drops a pending signal.
+    bool m_filterRendering = false;
+    // Which edit's preview is being made; a cancelled edit's drops.
+    QUuid m_filterPreviewFor;
+    std::vector<Committing> m_filterCommitWaiting;
+    std::optional<Committing> m_committingFilter;
+    QFutureWatcher<FilterMade> m_filterCommit;
+    std::optional<LevelsEdit> m_levels;
+    // Which edit a running preview serves; others drop.
+    QUuid m_levelsPreviewFor;
+    QFutureWatcher<std::optional<QImage>> m_levelsPreview;
+    bool m_levelsRendering = false;
+    QFutureWatcher<std::optional<LevelsHistogram>> m_levelsHistogram;
+    std::optional<Committing> m_committingLevels;
+    QFutureWatcher<Leveled> m_levelsCommit;
+    std::optional<HueSaturationEdit> m_hueSaturation;
+    std::optional<HueSaturationJob> m_hueSaturationPending;
+    // Which edit a running preview serves; others drop.
+    QUuid m_hueSaturationPreviewFor;
+    QFutureWatcher<Adjusted> m_hueSaturationTask;
+    bool m_hueSaturationRendering = false;
+    std::optional<CommittingHue> m_committingHueSaturation;
+    QFutureWatcher<Adjusted> m_hueSaturationCommit;
+    std::optional<HueSampleMode> m_hueSampleMode;
+    bool m_hueTargeting = false;
+    std::optional<HueTargetDrag> m_hueTargetDrag;
+    std::optional<QUuid> m_adjustmentEditingID;
+    std::optional<LayerAdjustment> m_adjustmentOriginal;
+    std::optional<LayerEffectSelection> m_effectsEditing;
+    std::optional<LayerEffects> m_effectsEditingOriginal;
+    std::optional<LayerEffectSelection> m_effectSelection;
+    std::optional<PixelClipboard> m_pixelClipboard;
+    // Swift's pasteboard change count, counted from the first copy.
+    int m_clipboardChanges = 0;
+    bool m_watchingClipboard = false;
+    std::optional<LassoDraft> m_lassoDraft;
+    LassoKind m_lassoKind = LassoKind::freehand;
+    LassoKind m_marqueeKind = LassoKind::rectangle;
+    SelectionMode m_selectionModeChoice = SelectionMode::replace;
+    std::optional<SelectionMode> m_heldSelectionMode;
+    // The selection as a drag began; one step per drag.
+    std::optional<DocumentSelection> m_selectionMoveOrigin;
+    bool m_selectionAntialiased = true;
+    int m_selectionExpandAmount = 1;
+    int m_selectionContractAmount = 1;
+};

src/Document/Filters+Session.cpp

@@ -0,0 +1,335 @@
+#include "Document/BrushStroke.h"
+#include "Document/ContentFill.h"
+#include "Document/EditorSession.h"
+#include "Document/PixelAdjust.h"
+#include "Document/SubjectRemoval.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include "Logging.h"
+#include <QtConcurrent>
+
+// Swift's session extension in Filters.swift.
+
+// `canAdjustColors` already refuses an empty selection.
+bool EditorSession::canContentAwareFill() const
+{
+    return canAdjustColors() && selection().has_value() && !m_hueSaturation;
+}
+
+void EditorSession::beginFilter(FilterKind kind)
+{
+    if (kind == FilterKind::contentAwareFill && !canContentAwareFill())
+        return;
+    // Swift beeps; `canAdjustColors` reads the open filter.
+    if (m_hueSaturation || !canAdjustColors()) {
+        qCWarning(lcApp) << "a filter cannot open here";
+        return;
+    }
+    // A pending gradient lands first, then the filter begins.
+    if (m_gradientEdit) {
+        commitGradient([this, kind] { beginFilter(kind); });
+        return;
+    }
+    commitTransform();
+    cancelCrop();
+    cancelLasso();
+    const ImageLayer layer = activeLayer().value();
+    const QSizeF size = m_document->size();
+    const std::optional<DocumentSelection> current = selection();
+    FilterSettings settings = m_filterSettings;
+    // Gradient Map starts from the palette, as Photoshop's.
+    if (kind == FilterKind::gradientMap)
+        settings.gradientMap = GradientMapSettings{AdjustmentColor(foregroundColor()), AdjustmentColor(m_backgroundColor)};
+    try {
+        // Content-Aware Fill extends the layer over the selection's reach.
+        std::optional<QRectF> area;
+        if (kind == FilterKind::contentAwareFill && current) {
+            const QRectF box = current->path.boundingRect().intersected(QRectF(QPointF(0, 0), size));
+            if (!box.isEmpty())
+                area = box;
+        }
+        m_filterEdit.emplace(kind, layer, current ? std::optional(current->clip(size)) : std::nullopt, settings, area);
+        updateFilter(m_filterEdit->settings, true);
+    } catch (const ProjectError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+}
+
+void EditorSession::updateFilter(const FilterSettings &settings, bool preview)
+{
+    if (!m_filterEdit || m_filterEdit->committing)
+        return;
+    FilterEdit &edit = *m_filterEdit;
+    edit.settings = settings.normalized();
+    edit.preview = preview;
+    // A bigger blur needs more room round the layer.
+    if (FilterEdit::blurMargin(edit.kind, edit.settings) > edit.grownMargin) {
+        try {
+            edit.growForBlur();
+            edit.preparedPreview.reset();
+        } catch (const ProjectError &error) {
+            setBrushError(QString::fromUtf8(error.what()));
+        } catch (const ExportError &error) {
+            setBrushError(QString::fromUtf8(error.what()));
+        }
+    }
+    // An adjustment layer takes the settings; its pixels stay.
+    if (previewAdjustmentEditing(preview)) {
+        notify();
+        return;
+    }
+    // Automatic kinds render once.
+    if (isAutomatic(edit.kind) && edit.preparedPreview && edit.preparedSettings == edit.settings) {
+        ++m_brushRevision;
+        notify();
+        return;
+    }
+    if (!preview) {
+        // A job behind another edit's run is Swift's running task.
+        if (m_filterRendering && m_filterPreviewFor == edit.id)
+            edit.pending.reset();
+        edit.preparedPreview.reset();
+        ++m_brushRevision;
+        notify();
+        return;
+    }
+    edit.pending = edit.previewJob();
+    renderFilterPreview();
+}
+
+// Renders the newest job; one that arrives mid-render waits.
+void EditorSession::renderFilterPreview()
+{
+    if (!m_filterEdit || !m_filterEdit->pending)
+        return;
+    FilterEdit &edit = *m_filterEdit;
+    edit.preparing = true;
+    edit.previewError = std::nullopt;
+    // One worker at a time: a job waits its turn.
+    if (!m_filterRendering) {
+        const FilterJob job = *std::exchange(edit.pending, std::nullopt);
+        m_filterPreviewFor = edit.id;
+        m_filterRendering = true;
+        m_filterPreview.setFuture(QtConcurrent::run([job]() -> Filtered {
+            try {
+                return Filtered{PixelFilter::run(job), std::nullopt, job.settings};
+            } catch (const ContentFillError &error) {
+                return Filtered{std::nullopt, QString::fromUtf8(error.what()), job.settings};
+            } catch (const SubjectRemovalError &error) {
+                return Filtered{std::nullopt, QString::fromUtf8(error.what()), job.settings};
+            } catch (const ExportError &error) {
+                return Filtered{std::nullopt, QString::fromUtf8(error.what()), job.settings};
+            }
+        }));
+    }
+    notify();
+}
+
+void EditorSession::finishFilterPreview()
+{
+    m_filterRendering = false;
+    const Filtered result = m_filterPreview.result();
+    // An edit cancelled meanwhile takes nothing; a new one waited.
+    if (m_filterEdit && m_filterEdit->id == m_filterPreviewFor) {
+        FilterEdit &edit = *m_filterEdit;
+        edit.preparing = false;
+        edit.previewError = result.failure;
+        if (edit.preview || isAutomatic(edit.kind)) {
+            edit.preparedPreview = result.image;
+            edit.preparedSettings = result.settings;
+            ++m_brushRevision;
+        }
+        notify();
+    }
+    renderFilterPreview();
+    // Each waiting commit looks again; a gone edit's just returns.
+    for (const Committing &waiter : std::exchange(m_filterCommitWaiting, {})) {
+        if (m_filterEdit && m_filterEdit->id == waiter.editID)
+            commitFilter(waiter.done);
+        else
+            QMetaObject::invokeMethod(this, waiter.done, Qt::QueuedConnection);
+    }
+}
+
+void EditorSession::cancelFilter()
+{
+    // A Gradient Map colour being picked goes with the panel.
+    if (m_colorPicker && m_colorPicker->target.kind == ColorPickerTarget::Kind::gradientMap)
+        closeColorPicker(false);
+    if (finishAdjustmentEditing(false) || !m_filterEdit || m_filterEdit->committing)
+        return;
+    m_filterEdit.reset();
+    ++m_brushRevision;
+    notify();
+}
+
+void EditorSession::commitFilter(std::function<void()> done)
+{
+    // The caller resumes from the event loop, as after await.
+    const auto finish = [this, done] {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    };
+    if (m_colorPicker && m_colorPicker->target.kind == ColorPickerTarget::Kind::gradientMap)
+        closeColorPicker(true);
+    if (finishAdjustmentEditing(true) || !m_filterEdit || m_filterEdit->committing) {
+        finish();
+        return;
+    }
+    FilterEdit &edit = *m_filterEdit;
+    // An automatic filter commits what its preview made, once delivered.
+    if (isAutomatic(edit.kind) && edit.preparing) {
+        m_filterCommitWaiting.push_back(Committing{edit.id, done ? done : [] {}});
+        return;
+    }
+    if (isAutomatic(edit.kind) && (!edit.preparedPreview || edit.previewError)) {
+        finish();
+        return;
+    }
+    // Nothing to change closes as Cancel does, with no step.
+    if ((edit.kind == FilterKind::lensCorrection && edit.settings.distortion == 0)
+        || (edit.kind == FilterKind::exposure && edit.settings.exposure == ExposureSettings())
+        || (edit.kind == FilterKind::grain && edit.settings.grain.amount == 0)) {
+        cancelFilter();
+        finish();
+        return;
+    }
+    edit.committing = true;
+    // Swift cancels the running preview: its result and queue drop.
+    edit.pending.reset();
+    m_filterPreviewFor = QUuid();
+    m_filterSettings = edit.settings;
+    setIsProjectBusy(true);
+    m_committingFilter = Committing{edit.id, std::move(done)};
+    // A mask hides the background, which can come back.
+    if (edit.kind == FilterKind::removeBackground) {
+        commitBackgroundMask(edit);
+        return;
+    }
+    const std::optional<QImage> cached = isAutomatic(edit.kind) && edit.preparedSettings == edit.settings ? edit.preparedPreview : std::nullopt;
+    const bool spreads = edit.kind == FilterKind::gaussianBlur || edit.kind == FilterKind::motionBlur;
+    // A painted layer flattens in the worker, which catches failures.
+    m_filterCommit.setFuture(QtConcurrent::run([kind = edit.kind, original = edit.original, grownImage = edit.grownImage, settings = edit.settings,
+                                                selection = edit.selection, mapping = edit.mapping, seed = edit.seed, cached,
+                                                grown = edit.grownTransform, spreads]() -> FilterMade {
+        try {
+            QImage image = cached ? *cached : PixelFilter::run(FilterJob{kind, grownImage ? *grownImage : original.image(), settings, 1, selection, mapping, seed});
+            std::optional<LayerTransform> placed = grown;
+            // A blur is cut back to the pixels it left.
+            if (spreads && grown) {
+                const PixelFilter::Trimmed trimmed = PixelFilter::trimmed(image, *grown);
+                image = trimmed.image;
+                placed = trimmed.transform;
+            }
+            return FilterMade{ImportedImage(image, PixelAdjust::thumbnail(image), rawValue(kind)), placed, std::nullopt};
+        } catch (const ContentFillError &error) {
+            return FilterMade{std::nullopt, std::nullopt, QString::fromUtf8(error.what())};
+        } catch (const ExportError &error) {
+            return FilterMade{std::nullopt, std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::commitBackgroundMask(const FilterEdit &edit)
+{
+    // A mask in the layer's own grid is multiplied in.
+    const int index = indexOf(m_document->layers, edit.layerID);
+    std::optional<ImportedImage> existing;
+    if (index >= 0 && m_document->layers[index].mask) {
+        const LayerMask &owned = *m_document->layers[index].mask;
+        if (!owned.placement && owned.asset.size() == edit.original.size())
+            existing = owned.asset;
+    }
+    // Painted pixels and masks flatten in the worker.
+    m_filterCommit.setFuture(QtConcurrent::run([original = edit.original, existing, selection = edit.selection, mapping = edit.mapping,
+                                                settings = edit.settings]() -> FilterMade {
+        try {
+            const QImage source = original.image();
+            const std::optional<QImage> under = existing ? std::optional(existing->image()) : std::nullopt;
+            QImage mask = SubjectRemoval::subjectMask(source, under, settings);
+            // With a selection, only its part of the mask changes.
+            if (selection) {
+                QImage base = under ? *under : BrushRaster::context(source.width(), source.height(), true);
+                if (!under)
+                    base.fill(255);
+                mask = PixelAdjust::blend(mask, base, *selection, mapping, true);
+            }
+            return FilterMade{LayerMask::assetFrom(mask), std::nullopt, std::nullopt};
+        } catch (const SubjectRemovalError &error) {
+            return FilterMade{std::nullopt, std::nullopt, QString::fromUtf8(error.what())};
+        } catch (const ExportError &error) {
+            return FilterMade{std::nullopt, std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::finishFilterCommit()
+{
+    const FilterMade made = m_filterCommit.result();
+    const FilterEdit edit = std::exchange(m_filterEdit, std::nullopt).value();
+    const Committing pending = std::exchange(m_committingFilter, std::nullopt).value();
+    // The preview stayed up till now: the canvas never flashed.
+    const auto finish = [&] {
+        ++m_brushRevision;
+        setIsProjectBusy(false);
+        if (pending.done)
+            QMetaObject::invokeMethod(this, pending.done, Qt::QueuedConnection);
+    };
+    if (made.failure) {
+        setBrushError(made.failure);
+        finish();
+        return;
+    }
+    const int index = m_document ? indexOf(m_document->layers, edit.layerID) : -1;
+    if (index < 0) {
+        finish();
+        return;
+    }
+    const ImageLayer current = m_document->layers[index];
+    if (!current.asset || current.asset->identity() != edit.original.identity() || current.transform != edit.transform) {
+        finish();
+        return;
+    }
+    if (edit.kind == FilterKind::removeBackground) {
+        beginEdit(rawValue(edit.kind));
+        // Swift keeps the mask's switches; a placement would misplace it.
+        LayerMask mask = current.mask ? current.mask->replacing(*made.asset) : LayerMask(*made.asset);
+        mask.isEnabled = true;
+        mask.placement = std::nullopt;
+        m_document->layers[index].mask = mask;
+        m_isMaskSelected = true;
+        endEdit();
+        finish();
+        return;
+    }
+    try {
+        // The mask moves onto the new box, edge tone beyond.
+        std::optional<LayerMask> mask = current.mask;
+        if (edit.grownTransform && current.mask && !current.mask->placement
+            && (current.mask->asset.size().width() > 1 || current.mask->asset.size().height() > 1)) {
+            LayerMask enabled = *current.mask;
+            enabled.isEnabled = true;
+            const QSize size = made.asset->size();
+            const std::optional<QImage> carried = enabled.clipImage(current.transform, made.transform.value(), size.width(), size.height());
+            if (!carried)
+                throw ExportError(ExportError::Kind::render);
+            mask = current.mask->replacing(LayerMask::assetFrom(*carried));
+        }
+        beginEdit(rawValue(edit.kind));
+        ImageLayer layer = current;
+        layer.asset = made.asset;
+        layer.transform = made.transform.value_or(current.transform);
+        layer.mask = mask;
+        // Swift rebuilds the layer without shape, effects and text.
+        layer.shape = std::nullopt;
+        layer.effects = std::nullopt;
+        layer.text = std::nullopt;
+        m_document->layers[index] = layer;
+        endEdit();
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+    finish();
+}

src/Document/Filters+Session.h

@@ -0,0 +1,16 @@
+// Swift's Filters extension: one filter at a time.
+public:
+    bool canContentAwareFill() const;
+    void beginFilter(FilterKind kind);
+    // The settings normalized; Preview off drops the queue and preview.
+    void updateFilter(const FilterSettings &settings, bool preview);
+    void cancelFilter();
+    // Off the UI thread; `done` follows the step or refusal.
+    void commitFilter(std::function<void()> done = {});
+
+private:
+    void renderFilterPreview();
+    void finishFilterPreview();
+    void finishFilterCommit();
+    // Swift's commitBackgroundMask: the subject kept by a mask.
+    void commitBackgroundMask(const FilterEdit &edit);

src/Document/Filters.cpp

@@ -0,0 +1,242 @@
+#include "Document/Filters.h"
+#include "Document/BrushStroke.h"
+#include "Document/ContentFill.h"
+#include "Document/EditorSession+Model.h"
+#include "Document/PixelAdjust.h"
+#include "Document/SubjectRemoval.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include "Rendering/RasterSnapshot.h"
+#include <algorithm>
+#include <cmath>
+#include <numbers>
+
+extern "C" {
+#include "BrushPixels.h"
+#include "LensPixels.h"
+#include "NoisePixels.h"
+}
+
+QString rawValue(FilterKind kind)
+{
+    switch (kind) {
+    case FilterKind::gaussianBlur: return QStringLiteral("Gaussian Blur");
+    case FilterKind::motionBlur: return QStringLiteral("Motion Blur");
+    case FilterKind::addNoise: return QStringLiteral("Add Noise");
+    case FilterKind::lensCorrection: return QStringLiteral("Lens Correction");
+    case FilterKind::removeBackground: return QStringLiteral("Remove Background");
+    case FilterKind::contentAwareFill: return QStringLiteral("Content-Aware Fill");
+    case FilterKind::curves: return QStringLiteral("Curves");
+    case FilterKind::exposure: return QStringLiteral("Exposure");
+    case FilterKind::gradientMap: return QStringLiteral("Gradient Map");
+    case FilterKind::grain: return QStringLiteral("Grain");
+    }
+    throw std::logic_error("unknown filter kind");
+}
+
+bool isAutomatic(FilterKind kind)
+{
+    return kind == FilterKind::contentAwareFill || kind == FilterKind::removeBackground;
+}
+
+QString rawValue(BackgroundQuality quality)
+{
+    return quality == BackgroundQuality::basic ? QStringLiteral("Basic") : QStringLiteral("Advanced");
+}
+
+bool isImageAdjustment(FilterKind kind)
+{
+    return kind == FilterKind::curves || kind == FilterKind::exposure || kind == FilterKind::gradientMap || kind == FilterKind::grain;
+}
+
+FilterSettings FilterSettings::normalized() const
+{
+    using ImageAdjustmentPixels::clamp;
+    FilterSettings result = *this;
+    result.radius = clamp(radius, 0.1, 250, 1);
+    result.angle = clamp(angle, -90, 90, 0);
+    result.distance = clamp(distance, 1, 2000, 10);
+    result.amount = clamp(amount, 0.1, 400, 10);
+    result.distortion = clamp(distortion, -100, 100, 0);
+    result.refineEdges = clamp(refineEdges, 0, 40, 12);
+    result.matteContrast = clamp(matteContrast, 0, 100, 25);
+    result.shiftEdge = clamp(shiftEdge, -10, 10, 0);
+    result.exposure = exposure.normalized();
+    result.gradientMap = gradientMap.normalized();
+    result.grain = grain.normalized();
+    return result;
+}
+
+PixelFilter::Trimmed PixelFilter::trimmed(const QImage &image, const LayerTransform &placed)
+{
+    const QRectF full(image.rect());
+    // Bounds read premultiplied bytes; other formats get drawn.
+    QImage pixels = image;
+    if (pixels.format() != QImage::Format_RGBA8888_Premultiplied) {
+        pixels = BrushRaster::context(image.width(), image.height(), false);
+        QPainter painter(&pixels);
+        BrushRaster::draw(image, full, painter);
+    }
+    size_t edges[4] = {0, 0, 0, 0};
+    brush_alpha_bounds(pixels.constBits(), size_t(pixels.width()), size_t(pixels.height()), size_t(pixels.bytesPerLine()), edges);
+    const QRect crop(int(edges[0]), int(edges[1]), int(edges[2] - edges[0]), int(edges[3] - edges[1]));
+    if (crop.width() < 1 || crop.height() < 1 || QRectF(crop) == full)
+        return {image, placed};
+    LayerTransform result = placed;
+    result.size = {crop.width() * placed.size.width() / full.width(), crop.height() * placed.size.height() / full.height()};
+    const QPointF middle = BrushRaster::pixelToDocument(placed, image.width(), image.height()).map(QRectF(crop).center());
+    result.origin = {middle.x() - result.size.width() / 2, middle.y() - result.size.height() / 2};
+    // Swift's crop shares pixels; a copy may find no memory.
+    const QImage cropped = image.copy(crop);
+    if (cropped.isNull())
+        throw ExportError(ExportError::Kind::render);
+    return {cropped, result};
+}
+
+namespace {
+// The image drawn into a fresh premultiplied context.
+QImage drawn(const QImage &image)
+{
+    QImage context = BrushRaster::context(image.width(), image.height(), false);
+    QPainter painter(&context);
+    BrushRaster::draw(image, QRectF(0, 0, image.width(), image.height()), painter);
+    return context;
+}
+}
+
+QImage PixelFilter::run(const FilterJob &job)
+{
+    const FilterSettings settings = job.settings.normalized();
+    const int width = job.image.width(), height = job.image.height();
+    QImage image;
+    switch (job.kind) {
+    case FilterKind::curves: image = settings.curves.apply(job.image); break;
+    case FilterKind::exposure: image = settings.exposure.apply(job.image); break;
+    case FilterKind::gradientMap: image = settings.gradientMap.apply(job.image); break;
+    // Grain sits in layer pixels; the job's seed patterns it.
+    case FilterKind::grain: image = settings.grain.apply(job.image, QPointF(), 1 / job.scale, job.seed); break;
+    case FilterKind::removeBackground: image = SubjectRemoval::run(job.image, settings); break;
+    case FilterKind::contentAwareFill: image = ContentFill::run(job); break;
+    // Unclamped: a blur spreads into the room made for it.
+    case FilterKind::gaussianBlur: image = PixelAdjust::gaussianBlur(drawn(job.image), settings.radius * job.scale, false); break;
+    case FilterKind::motionBlur:
+        image = PixelAdjust::motionBlur(drawn(job.image), settings.distance * job.scale * motionRadiusPerPixel, settings.angle * std::numbers::pi / 180);
+        break;
+    case FilterKind::addNoise:
+        image = drawn(job.image);
+        noise_add(image.bits(), size_t(width), size_t(height), size_t(image.bytesPerLine()), float(settings.amount), settings.gaussian ? 1 : 0,
+                  settings.monochromatic ? 1 : 0, job.seed);
+        break;
+    case FilterKind::lensCorrection: {
+        // Relative to the image's size, so previews bend alike.
+        const QImage source = drawn(job.image);
+        image = BrushRaster::context(width, height, false);
+        lens_distort(source.constBits(), image.bits(), size_t(width), size_t(height), size_t(source.bytesPerLine()), settings.distortion / 100 * lensStrength);
+        break;
+    }
+    }
+    if (!job.selection)
+        return image;
+    return PixelAdjust::blend(image, job.image, *job.selection, job.mapping, false);
+}
+
+namespace {
+QRectF integral(const QRectF &rect)
+{
+    return QRectF(QPointF(std::floor(rect.left()), std::floor(rect.top())), QPointF(std::ceil(rect.right()), std::ceil(rect.bottom())));
+}
+}
+
+FilterEdit::FilterEdit(FilterKind kind, const ImageLayer &layer, std::optional<SelectionClip> selection, const FilterSettings &settings,
+                       std::optional<QRectF> area)
+    : kind(kind), layerID(layer.id), original(layer.asset ? *layer.asset : throw ProjectError(ProjectError::Kind::invalid)),
+      transform(layer.transform), selection(std::move(selection)), settings(settings.normalized())
+{
+    prepare(original, transform);
+    if (area)
+        grow(integral(mapping.inverted().mapRect(*area)));
+    growForBlur();
+}
+
+double FilterEdit::blurMargin(FilterKind kind, const FilterSettings &settings)
+{
+    switch (kind) {
+    case FilterKind::gaussianBlur: return settings.radius * 3 + 2;
+    case FilterKind::motionBlur: return settings.distance / 2 + 2;
+    default: return 0;
+    }
+}
+
+void FilterEdit::growForBlur()
+{
+    const double margin = blurMargin(kind, settings);
+    if (!(margin > grownMargin))
+        return;
+    const QSize size = original.size();
+    grow(QRectF(-margin, -margin, size.width() + 2 * margin, size.height() + 2 * margin));
+}
+
+// The layer's pixels in a grid covering `extent` (layer pixels).
+void FilterEdit::grow(const QRectF &extent)
+{
+    const QSize size = original.size();
+    const QRectF bounds(0, 0, size.width(), size.height());
+    const QRectF target = integral(bounds.united(extent));
+    if (target == bounds)
+        return;
+    if (target.width() > 30'000 || target.height() > 30'000 || target.width() * target.height() > 100'000'000)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    QImage grown = BrushRaster::context(int(target.width()), int(target.height()), false);
+    const QRectF inside = bounds.translated(-target.left(), -target.top());
+    QPainter painter(&grown);
+    if (original.raster)
+        original.raster->draw(inside, painter);
+    else
+        BrushRaster::draw(original.image(), inside, painter);
+    painter.end();
+    const QTransform toDocument = BrushRaster::pixelToDocument(transform, size.width(), size.height());
+    LayerTransform expanded = transform;
+    expanded.size = {target.width() * transform.size.width() / bounds.width(), target.height() * transform.size.height() / bounds.height()};
+    const QPointF middle = toDocument.map(target.center());
+    expanded.origin = {middle.x() - expanded.size.width() / 2, middle.y() - expanded.size.height() / 2};
+    grownImage = grown;
+    grownTransform = expanded;
+    grownMargin = std::min({bounds.left() - target.left(), bounds.top() - target.top(), target.right() - bounds.right(), target.bottom() - bounds.bottom()});
+    prepare(ImportedImage(grown, QImage(), QString()), expanded);
+}
+
+void FilterEdit::prepare(const ImportedImage &source, const LayerTransform &placed)
+{
+    const QSize size = source.size();
+    mapping = BrushRaster::pixelToDocument(placed, size.width(), size.height());
+    // Noise and grain preview whole: enlarged, grain looks coarse.
+    const bool whole = kind == FilterKind::addNoise || kind == FilterKind::grain || kind == FilterKind::contentAwareFill || kind == FilterKind::removeBackground;
+    const double factor = whole ? 1 : std::min(1.0, previewLimit / std::max(size.width(), size.height()));
+    if (!(factor < 1)) {
+        previewSource = source.image();
+        previewScale = 1;
+        previewMapping = mapping;
+        return;
+    }
+    const int width = std::max(1, int(size.width() * factor)), height = std::max(1, int(size.height() * factor));
+    previewSource = BrushRaster::context(width, height, false);
+    QPainter painter(&previewSource);
+    const QRectF rect(0, 0, width, height);
+    if (source.raster)
+        source.raster->draw(rect, painter);
+    else
+        BrushRaster::draw(source.image(), rect, painter);
+    painter.end();
+    previewScale = double(width) / size.width();
+    previewMapping = BrushRaster::pixelToDocument(placed, width, height);
+}
+
+std::optional<QImage> FilterEdit::previewImage(QUuid id) const
+{
+    return preview && id == layerID ? preparedPreview : std::nullopt;
+}
+
+FilterJob FilterEdit::previewJob() const
+{
+    return FilterJob{kind, previewSource, settings, previewScale, selection, previewMapping, seed};
+}

src/Document/Filters.h

@@ -0,0 +1,141 @@
+#pragma once
+#include "Document/Curves.h"
+#include "Document/ImageAdjustments.h"
+#include "Document/LayerTransform.h"
+#include "Document/Selection.h"
+#include "IO/ImageImporter.h"
+#include <QImage>
+#include <QRandomGenerator>
+#include <QTransform>
+#include <QUuid>
+#include <array>
+#include <cmath>
+
+struct ImageLayer;
+
+// The Filter and Image menus' filters.
+enum class FilterKind { gaussianBlur, motionBlur, addNoise, lensCorrection, removeBackground, contentAwareFill, curves, exposure, gradientMap, grain };
+// The menu's title, Swift's rawValue.
+QString rawValue(FilterKind kind);
+// Made without settings: its preview is its result.
+bool isAutomatic(FilterKind kind);
+// The Image menu's colour adjustments, not under Filter.
+bool isImageAdjustment(FilterKind kind);
+// Swift's allCases, the menus' order.
+inline constexpr std::array allFilterKinds{FilterKind::gaussianBlur,     FilterKind::motionBlur,       FilterKind::addNoise,
+                                           FilterKind::lensCorrection,   FilterKind::removeBackground, FilterKind::contentAwareFill,
+                                           FilterKind::curves,           FilterKind::exposure,         FilterKind::gradientMap,
+                                           FilterKind::grain};
+
+// Remove Background's mask: the model's alone, or refined.
+enum class BackgroundQuality { basic, advanced };
+QString rawValue(BackgroundQuality quality);
+inline constexpr std::array allBackgroundQualities{BackgroundQuality::basic, BackgroundQuality::advanced};
+
+// Every filter's settings; each filter reads only its own.
+struct FilterSettings {
+    // Gaussian Blur's radius in layer pixels: the blur's sigma.
+    double radius = 1;
+    // Motion Blur's angle, counterclockwise from horizontal, as Photoshop's.
+    double angle = 0;
+    double distance = 10;
+    // Add Noise's strength, Photoshop's percentage.
+    double amount = 10;
+    bool gaussian = false;
+    // The same noise on every channel: brightness alone.
+    bool monochromatic = false;
+    // Positive straightens barrel distortion, negative pincushion.
+    double distortion = 0;
+    CurvesSettings curves{};
+    ExposureSettings exposure{};
+    GradientMapSettings gradientMap{};
+    GrainSettings grain{};
+    BackgroundQuality backgroundQuality = BackgroundQuality::basic;
+    // How far, in pixels, the mask is pulled onto edges.
+    double refineEdges = 12;
+    // Pushes the mask's grays apart, 0 to 100, clearing haze.
+    double matteContrast = 25;
+    // Contracts (negative) or grows the mask's edge, in pixels.
+    double shiftEdge = 0;
+    FilterSettings normalized() const;
+    friend bool operator==(const FilterSettings &, const FilterSettings &) = default;
+};
+
+// What a filter runs on: grid, settings, selection, placement.
+struct FilterJob {
+    FilterKind kind;
+    QImage image;
+    FilterSettings settings;
+    // Pixels in `image` a layer pixel: smaller previews blur less.
+    double scale;
+    std::optional<SelectionClip> selection;
+    QTransform mapping;
+    // Add Noise's pattern: the same seed, the same grain.
+    quint32 seed = 0;
+};
+
+// Swift's PixelFilter.
+namespace PixelFilter {
+struct Trimmed {
+    QImage image;
+    LayerTransform transform;
+};
+
+// CIMotionBlur's radius a streak pixel: an even streak's spread.
+inline const double motionRadiusPerPixel = 1 / std::sqrt(12.0);
+// Remove Distortion at ±100 moves the corners this share.
+inline constexpr double lensStrength = 0.35;
+// The pixels that are there, placed where they were.
+Trimmed trimmed(const QImage &image, const LayerTransform &placed);
+// The filter over the job's image, through its selection.
+QImage run(const FilterJob &job);
+}
+
+// An open filter: the layer's pixels, preview copy, grown grid.
+class FilterEdit {
+public:
+    // Previews render from a copy no longer than this.
+    static constexpr double previewLimit = 2048;
+    // `area`: a document area the grid must cover (Content-Aware Fill).
+    FilterEdit(FilterKind kind, const ImageLayer &layer, std::optional<SelectionClip> selection, const FilterSettings &settings,
+               std::optional<QRectF> area);
+    // The room a blur needs round the layer, in pixels.
+    static double blurMargin(FilterKind kind, const FilterSettings &settings);
+    // Pads the grid for the blur; it only ever grows.
+    void growForBlur();
+    std::optional<QImage> previewImage(QUuid id) const;
+    FilterJob previewJob() const;
+
+    // Swift's object identity: a preview lands on its own edit.
+    const QUuid id = QUuid::createUuid();
+    const FilterKind kind;
+    const QUuid layerID;
+    const ImportedImage original;
+    const LayerTransform transform;
+    const std::optional<SelectionClip> selection;
+    QTransform mapping;
+    QImage previewSource;
+    double previewScale = 1;
+    QTransform previewMapping;
+    // The layer padded out, and the transform placing it.
+    std::optional<QImage> grownImage;
+    std::optional<LayerTransform> grownTransform;
+    // How far the padding reaches past every side.
+    double grownMargin = 0;
+    FilterSettings settings;
+    bool preview = true;
+    bool committing = false;
+    std::optional<QString> previewError;
+    bool preparing = false;
+    // Add Noise's grain, fixed while the panel is open.
+    const quint32 seed = QRandomGenerator::global()->generate();
+    std::optional<QImage> preparedPreview;
+    // The settings the prepared preview was made with.
+    std::optional<FilterSettings> preparedSettings;
+    std::optional<FilterJob> pending;
+
+private:
+    void grow(const QRectF &extent);
+    // Swift's prepared and prepare: mapping and preview copy.
+    void prepare(const ImportedImage &source, const LayerTransform &placed);
+};

src/Document/FloatingSelection+Session.h

@@ -0,0 +1,10 @@
+// Swift's FloatingSelection extension: Ctrl+T on selected pixels.
+public:
+    bool canTransformSelection() const;
+    void transformCommand();
+    void beginSelectionTransform(std::function<void()> done = {});
+    std::optional<QTransform> floatingSelectionTransform(const TransformEdit &edit) const;
+
+private:
+    void mergeFloatingTransform(const TransformEdit &edit, const FloatingTransform &floating);
+    void cancelFloatingTransform(const FloatingTransform &floating);

src/Document/FloatingSelection.cpp

@@ -0,0 +1,196 @@
+#include "Document/FloatingSelection.h"
+#include "Document/BrushStroke.h"
+#include "Document/Distort.h"
+#include "Document/EditorSession.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include "Logging.h"
+#include "Rendering/LayerRenderer.h"
+#include <cmath>
+
+// Swift's FloatingSelection extension: one "Transform Selection" step.
+namespace {
+QRectF integral(const QRectF &rect)
+{
+    return QRectF(QPointF(std::floor(rect.left()), std::floor(rect.top())), QPointF(std::ceil(rect.right()), std::ceil(rect.bottom())));
+}
+}
+
+bool EditorSession::canTransformSelection() const
+{
+    const std::optional<DocumentSelection> current = selection();
+    const std::optional<ImageLayer> layer = activeLayer();
+    // `canEditPixels` refuses an open edit and an empty selection.
+    return canEditPixels() && !m_isMaskSelected && current && layer && layer->asset;
+}
+
+// Ctrl+T: the selected pixels with a selection, else the layer.
+void EditorSession::transformCommand()
+{
+    if (canTransformSelection())
+        beginSelectionTransform();
+    else
+        beginTransform();
+}
+
+void EditorSession::beginSelectionTransform(std::function<void()> done)
+{
+    const auto finish = [this, done] {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    };
+    if (!canTransformSelection()) {
+        qCWarning(lcApp) << "no selected pixels to transform";
+        finish();
+        return;
+    }
+    const ImageLayer source = activeLayer().value();
+    std::optional<CopiedPixels> lifted;
+    try {
+        lifted = renderSelectedPixels(source, false);
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+        finish();
+        return;
+    }
+    if (!lifted) {
+        qCWarning(lcApp) << "no selected pixels to lift";
+        finish();
+        return;
+    }
+    const CanvasDocument before = m_document.value();
+    const std::optional<QUuid> beforeActive = m_activeLayerID;
+    // The outer step: closed by the merge or the restore.
+    beginEdit(QStringLiteral("Transform Selection"));
+    clearSelectedPixels([this, source, lifted = *lifted, before, beforeActive, finish] {
+        const int index = m_document ? indexOf(m_document->layers, source.id) : -1;
+        std::optional<QImage> thumbnail;
+        try {
+            thumbnail = PixelAdjust::thumbnail(lifted.image);
+        } catch (const ExportError &error) {
+            qCWarning(lcApp) << "the floating selection could not be made:" << error.what();
+        }
+        if (index < 0 || !thumbnail) {
+            m_document = before;
+            endEdit();
+            finish();
+            return;
+        }
+        ImageLayer floating(ImportedImage(lifted.image, *thumbnail, QStringLiteral("Floating Selection")), lifted.region.topLeft());
+        floating.name = QStringLiteral("Floating Selection");
+        floating.parentID = source.parentID;
+        floating.opacity = source.opacity;
+        floating.blendMode = source.blendMode;
+        m_document->layers.insert(m_document->layers.begin() + index + 1, floating);
+        setActiveLayerID(floating.id);
+        m_tool = NavigationTool::move;
+        const auto state = std::make_shared<const FloatingTransform>(FloatingTransform{source.id, before, beforeActive, floating.transform, lifted.region.size()});
+        m_transformEdit = TransformEdit{floating.id, floating.transform, std::nullopt, false, true, std::nullopt, state};
+        notify();
+        finish();
+    });
+}
+
+// Maps the original selection to where the pixels float now.
+std::optional<QTransform> EditorSession::floatingSelectionTransform(const TransformEdit &edit) const
+{
+    if (!edit.floating)
+        return std::nullopt;
+    const int width = int(edit.floating->pixelSize.width()), height = int(edit.floating->pixelSize.height());
+    return BrushRaster::pixelToDocument(edit.floating->original, width, height).inverted() * BrushRaster::pixelToDocument(edit.draft, width, height);
+}
+
+// The pixels back into their layer, the selection moved along.
+void EditorSession::mergeFloatingTransform(const TransformEdit &edit, const FloatingTransform &floating)
+{
+    try {
+        // The open edit holds both layers; checked reads suffice.
+        const QImage pixels = m_document->layers[indexOf(m_document->layers, edit.layerID)].asset.value().image();
+        const ImageLayer source = m_document->layers[indexOf(m_document->layers, floating.sourceID)];
+        // A distorted selection is warped into its shape first.
+        QImage placedImage = pixels;
+        LayerTransform placedTransform = edit.draft;
+        if (edit.corners) {
+            const DistortWarp::Trimmed warped = DistortWarp::warpTrimmed(pixels, edit.draft, *edit.corners);
+            placedImage = warped.image;
+            placedTransform = warped.transform;
+        }
+        const FloatingMerge::Merged merged = FloatingMerge::merge(placedImage, placedTransform, source);
+        const std::optional<DocumentSelection> current = selection();
+        std::optional<DocumentSelection> moved;
+        if (current && edit.corners) {
+            const QTransform placement = BrushRaster::pixelToDocument(floating.original, int(floating.pixelSize.width()), int(floating.pixelSize.height()));
+            if (const std::optional<QPainterPath> path = DistortWarp::mapPath(current->path, placement, floating.pixelSize, edit.draft, *edit.corners))
+                moved = DocumentSelection{*path, current->antialiased};
+        } else if (const std::optional<QTransform> shift = floatingSelectionTransform(edit); current && shift) {
+            moved = DocumentSelection{shift->map(current->path), current->antialiased};
+        }
+        std::erase_if(m_document->layers, [&](const ImageLayer &layer) { return layer.id == edit.layerID; });
+        ImageLayer &layer = m_document->layers[indexOf(m_document->layers, source.id)];
+        layer.asset = merged.asset;
+        layer.transform = merged.transform;
+        layer.isGroup = false;
+        layer.mask = merged.mask;
+        // Swift rebuilds the layer without shape, effects and text.
+        layer.shape = std::nullopt;
+        layer.effects = std::nullopt;
+        layer.text = std::nullopt;
+        m_document->selection = moved;
+        setActiveLayerID(source.id);
+    } catch (const ProjectError &error) {
+        m_document = floating.before;
+        setActiveLayerID(floating.beforeActive);
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        m_document = floating.before;
+        setActiveLayerID(floating.beforeActive);
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+    endEdit();
+}
+
+void EditorSession::cancelFloatingTransform(const FloatingTransform &floating)
+{
+    m_document = floating.before;
+    setActiveLayerID(floating.beforeActive);
+    endEdit();
+}
+
+FloatingMerge::Merged FloatingMerge::merge(const QImage &pixels, const LayerTransform &transform, const ImageLayer &source)
+{
+    if (!source.asset)
+        throw ProjectError(ProjectError::Kind::invalid);
+    const QImage sourceImage = source.asset->image();
+    const int width = sourceImage.width(), height = sourceImage.height();
+    const QTransform toDocument = BrushRaster::pixelToDocument(source.transform, width, height);
+    const QTransform toPixels = toDocument.inverted();
+    const QRectF floatingBounds = toPixels.mapRect(BrushRaster::pixelToDocument(transform, pixels.width(), pixels.height()).mapRect(QRectF(pixels.rect())));
+    const QRectF original(0, 0, width, height);
+    const QRectF extent = integral(original.united(floatingBounds));
+    if (extent.width() > 30'000 || extent.height() > 30'000 || extent.width() * extent.height() > 100'000'000)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    QImage context = BrushRaster::context(int(extent.width()), int(extent.height()), false);
+    const QRectF placed = original.translated(-extent.topLeft());
+    QPainter painter(&context);
+    BrushRaster::draw(sourceImage, placed, painter);
+    painter.setTransform(toPixels * QTransform::fromTranslate(-extent.left(), -extent.top()));
+    LayerRenderer::draw(pixels, transform, transform.center(), painter, {});
+    painter.end();
+    const ImportedImage asset(context, PixelAdjust::thumbnail(context), source.name);
+    LayerTransform merged = source.transform;
+    merged.size = {extent.width() * source.size().width() / width, extent.height() * source.size().height() / height};
+    const QPointF center = toDocument.map(extent.center());
+    merged.origin = {center.x() - merged.size.width() / 2, center.y() - merged.size.height() / 2};
+    std::optional<LayerMask> mask = source.mask;
+    if (source.mask && !source.mask->placement && extent != original) {
+        // The mask grows with the layer, revealing the new area.
+        QImage grown = BrushRaster::context(int(extent.width()), int(extent.height()), true);
+        grown.fill(Qt::white);
+        QPainter maskPainter(&grown);
+        BrushRaster::draw(source.mask->asset.image(), placed, maskPainter);
+        maskPainter.end();
+        mask = source.mask->replacing(LayerMask::assetFrom(grown));
+    }
+    return {asset, merged, mask};
+}

src/Document/FloatingSelection.h

@@ -0,0 +1,23 @@
+#pragma once
+#include "Document/EditorSession+Model.h"
+#include "Document/LayerTransform.h"
+#include <QImage>
+
+// Ctrl+T with a selection: the pixels float, then merge back.
+struct FloatingTransform {
+    QUuid sourceID;
+    CanvasDocument before;
+    std::optional<QUuid> beforeActive;
+    LayerTransform original;
+    QSizeF pixelSize;
+};
+
+// Floating pixels drawn back onto their layer, grown as needed.
+namespace FloatingMerge {
+struct Merged {
+    ImportedImage asset;
+    LayerTransform transform;
+    std::optional<LayerMask> mask;
+};
+Merged merge(const QImage &pixels, const LayerTransform &transform, const ImageLayer &source);
+}

src/Document/Gradient+Session.h

@@ -0,0 +1,14 @@
+// Swift's Gradient extension: a pending gradient, drawn until applied.
+public:
+    void beginGradient(QPointF point);
+    void moveGradient(std::optional<QPointF> start, std::optional<QPointF> end);
+    // Redraws the pending gradient from its ends, settings and palette.
+    void refreshGradient();
+    std::array<QColor, 2> gradientColors(bool mask) const;
+    // Ends a drag; a click without a line leaves nothing.
+    void endGradientDrag();
+    void cancelGradient();
+    // Off the UI thread; `done` follows the step or refusal.
+    void commitGradient(std::function<void()> done = {});
+    // Switching tools, layers or targets applies it, as Photoshop.
+    void resolveGradient();

src/Document/Gradient.cpp

@@ -0,0 +1,152 @@
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "IO/ProjectStore.h"
+#include <stdexcept>
+
+QString rawValue(GradientStyle style)
+{
+    switch (style) {
+    case GradientStyle::foregroundToBackground:
+        return QStringLiteral("Foreground to Background");
+    case GradientStyle::foregroundToTransparent:
+        return QStringLiteral("Foreground to Transparent");
+    }
+    throw std::logic_error("unknown gradient style");
+}
+
+QString rawValue(GradientShape shape)
+{
+    switch (shape) {
+    case GradientShape::linear:
+        return QStringLiteral("Linear");
+    case GradientShape::radial:
+        return QStringLiteral("Radial");
+    }
+    throw std::logic_error("unknown gradient shape");
+}
+
+void EditorSession::setGradientSettings(const GradientSettings &settings)
+{
+    m_gradientSettings = settings;
+    refreshGradient();
+    notify();
+}
+
+void EditorSession::beginGradient(QPointF point)
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    if (m_tool != NavigationTool::gradient || !(canPaint() || m_gradientEdit) || !layer)
+        return;
+    // A new line on this target replaces the pending one.
+    if (m_gradientEdit && m_gradientEdit->raster->layer.id == layer->id && m_gradientEdit->raster->isMask == m_isMaskSelected) {
+        m_gradientEdit->start = point;
+        m_gradientEdit->end = point;
+        refreshGradient();
+        return;
+    }
+    if (!canPaint())
+        return;
+    finishOpacityEdit();
+    try {
+        m_gradientEdit = GradientEdit{std::shared_ptr<BrushStroke>(makeRasterEdit(*layer)), point, point};
+        ++m_brushRevision;
+        notify();
+    } catch (const ProjectError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+}
+
+void EditorSession::moveGradient(std::optional<QPointF> start, std::optional<QPointF> end)
+{
+    if (!m_gradientEdit)
+        return;
+    if (start)
+        m_gradientEdit->start = *start;
+    if (end)
+        m_gradientEdit->end = *end;
+    refreshGradient();
+}
+
+void EditorSession::refreshGradient()
+{
+    if (!m_gradientEdit)
+        return;
+    if (m_gradientEdit->hasLine()) {
+        try {
+            m_gradientEdit->raster->fillGradient(m_gradientSettings.shape, m_gradientEdit->start, m_gradientEdit->end,
+                                                 gradientColors(m_gradientEdit->raster->isMask), m_gradientSettings.opacity);
+        } catch (const ProjectError &error) {
+            cancelGradient();
+            setBrushError(QString::fromUtf8(error.what()));
+            return;
+        } catch (const ExportError &error) {
+            cancelGradient();
+            setBrushError(QString::fromUtf8(error.what()));
+            return;
+        }
+    }
+    ++m_brushRevision;
+    notify();
+}
+
+std::array<QColor, 2> EditorSession::gradientColors(bool mask) const
+{
+    // A mask's colours are gray, the palette's red.
+    const auto color = [mask](const PaletteColor &value, double alpha) {
+        return mask ? QColor::fromRgbF(float(value.red), float(value.red), float(value.red), float(alpha)) : value.color(alpha);
+    };
+    const PaletteColor foreground = paletteColor(false);
+    std::array<QColor, 2> colors = m_gradientSettings.style == GradientStyle::foregroundToBackground
+                                       ? std::array{color(foreground, 1), color(paletteColor(true), 1)}
+                                       : std::array{color(foreground, 1), color(foreground, 0)};
+    if (m_gradientSettings.reversed)
+        std::swap(colors[0], colors[1]);
+    return colors;
+}
+
+void EditorSession::endGradientDrag()
+{
+    if (m_gradientEdit && !m_gradientEdit->hasLine())
+        cancelGradient();
+}
+
+void EditorSession::cancelGradient()
+{
+    if (!m_gradientEdit)
+        return;
+    m_gradientEdit = std::nullopt;
+    ++m_brushRevision;
+    notify();
+}
+
+void EditorSession::commitGradient(std::function<void()> done)
+{
+    if (!m_gradientEdit || m_isProjectBusy) {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+        return;
+    }
+    if (!m_gradientEdit->hasLine()) {
+        cancelGradient();
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+        return;
+    }
+    const std::shared_ptr<BrushStroke> raster = m_gradientEdit->raster;
+    commitRasterEdit(raster, raster->isMask ? QStringLiteral("Gradient Mask") : QStringLiteral("Gradient"), {}, [this, raster, done] {
+        // Swift's `gradientEdit === edit`: the same edit ends here.
+        if (m_gradientEdit && m_gradientEdit->raster == raster)
+            cancelGradient();
+        if (done)
+            done();
+    });
+}
+
+// Swift's `Task`: the commit starts once the caller returns.
+void EditorSession::resolveGradient()
+{
+    if (m_gradientEdit)
+        QMetaObject::invokeMethod(this, [this] { commitGradient(); }, Qt::QueuedConnection);
+}

src/Document/Gradient.h

@@ -0,0 +1,34 @@
+#pragma once
+#include <QPointF>
+#include <QString>
+#include <array>
+#include <cmath>
+#include <memory>
+
+class BrushStroke;
+
+enum class GradientStyle { foregroundToBackground, foregroundToTransparent };
+inline constexpr std::array allGradientStyles{GradientStyle::foregroundToBackground, GradientStyle::foregroundToTransparent};
+QString rawValue(GradientStyle style);
+
+// Linear runs start to end; radial centres on the start.
+enum class GradientShape { linear, radial };
+inline constexpr std::array allGradientShapes{GradientShape::linear, GradientShape::radial};
+QString rawValue(GradientShape shape);
+
+struct GradientSettings {
+    GradientShape shape = GradientShape::linear;
+    GradientStyle style = GradientStyle::foregroundToTransparent;
+    bool reversed = false;
+    double opacity = 1;
+    friend bool operator==(const GradientSettings &, const GradientSettings &) = default;
+};
+
+// A pending gradient on a layer or mask, document pixels.
+struct GradientEdit {
+    // Shared: the canvas draws it while a commit renders.
+    std::shared_ptr<BrushStroke> raster;
+    QPointF start;
+    QPointF end;
+    bool hasLine() const { return std::hypot(end.x() - start.x(), end.y() - start.y()) >= 0.5; }
+};

src/Document/GuidedMatte.cpp

@@ -0,0 +1,124 @@
+#include "Document/GuidedMatte.h"
+#include "IO/ImageExporter.h"
+#include <algorithm>
+#include <cmath>
+#include <new>
+
+namespace {
+// Floats; a failed allocation is a render error.
+std::vector<float> floats(size_t count)
+{
+    try {
+        return std::vector<float>(count);
+    } catch (const std::bad_alloc &) {
+        throw ExportError(ExportError::Kind::render);
+    }
+}
+
+QImage checked(const QImage &image)
+{
+    if (image.isNull())
+        throw ExportError(ExportError::Kind::render);
+    return image;
+}
+}
+
+std::vector<float> GuidedMatte::box(const std::vector<float> &source, int width, int height, int radius)
+{
+    const float span = float(radius * 2 + 1);
+    const size_t stride = size_t(width);
+    std::vector<float> pass = floats(stride * size_t(height));
+    for (int y = 0; y < height; ++y) {
+        const float *row = source.data() + size_t(y) * stride;
+        float sum = 0;
+        for (int x = -radius; x <= radius; ++x)
+            sum += row[std::clamp(x, 0, width - 1)];
+        float *out = pass.data() + size_t(y) * stride;
+        for (int x = 0; x < width; ++x) {
+            out[x] = sum / span;
+            sum -= row[std::clamp(x - radius, 0, width - 1)];
+            sum += row[std::clamp(x + radius + 1, 0, width - 1)];
+        }
+    }
+    // Each column sums as Swift's, a row at a time.
+    std::vector<float> result = floats(stride * size_t(height)), sums = floats(stride);
+    for (int y = -radius; y <= radius; ++y) {
+        const float *row = pass.data() + size_t(std::clamp(y, 0, height - 1)) * stride;
+        for (size_t x = 0; x < stride; ++x)
+            sums[x] += row[x];
+    }
+    for (int y = 0; y < height; ++y) {
+        float *out = result.data() + size_t(y) * stride;
+        const float *leaving = pass.data() + size_t(std::clamp(y - radius, 0, height - 1)) * stride;
+        const float *entering = pass.data() + size_t(std::clamp(y + radius + 1, 0, height - 1)) * stride;
+        for (size_t x = 0; x < stride; ++x) {
+            out[x] = sums[x] / span;
+            sums[x] -= leaving[x];
+            sums[x] += entering[x];
+        }
+    }
+    return result;
+}
+
+std::vector<float> GuidedMatte::filter(const std::vector<float> &mask, const std::vector<float> &guide, int width, int height, int radius, float epsilon)
+{
+    const size_t count = size_t(width) * size_t(height);
+    const std::vector<float> meanGuide = box(guide, width, height, radius), meanMask = box(mask, width, height, radius);
+    std::vector<float> slope = floats(count), offset = floats(count);
+    {
+        std::vector<float> squares = floats(count), products = floats(count);
+        for (size_t i = 0; i < count; ++i) {
+            squares[i] = guide[i] * guide[i];
+            products[i] = guide[i] * mask[i];
+        }
+        const std::vector<float> meanSquares = box(squares, width, height, radius), meanProducts = box(products, width, height, radius);
+        for (size_t i = 0; i < count; ++i) {
+            const float variance = meanSquares[i] - meanGuide[i] * meanGuide[i];
+            const float covariance = meanProducts[i] - meanGuide[i] * meanMask[i];
+            slope[i] = covariance / (variance + epsilon);
+            offset[i] = meanMask[i] - slope[i] * meanGuide[i];
+        }
+    }
+    const std::vector<float> meanSlope = box(slope, width, height, radius), meanOffset = box(offset, width, height, radius);
+    std::vector<float> result = floats(count);
+    for (size_t i = 0; i < count; ++i)
+        result[i] = std::min(1.0f, std::max(0.0f, meanSlope[i] * guide[i] + meanOffset[i]));
+    return result;
+}
+
+// Premultiplied first: Qt area-filters that, not gray, when shrinking.
+std::vector<float> GuidedMatte::levels(const QImage &image, int width, int height)
+{
+    const QImage source = checked(checked(image.convertToFormat(QImage::Format_RGBA8888_Premultiplied))
+                                      .scaled(width, height, Qt::IgnoreAspectRatio, Qt::SmoothTransformation)
+                                      .convertToFormat(QImage::Format_RGBA8888_Premultiplied));
+    std::vector<float> result = floats(size_t(width) * size_t(height));
+    for (int y = 0; y < height; ++y) {
+        const uchar *row = source.constScanLine(y);
+        for (int x = 0; x < width; ++x)
+            result[size_t(y) * size_t(width) + size_t(x)] = float(qGray(row[x * 4], row[x * 4 + 1], row[x * 4 + 2])) / 255;
+    }
+    return result;
+}
+
+QImage GuidedMatte::image(const std::vector<float> &levels, int width, int height)
+{
+    QImage result = checked(QImage(width, height, QImage::Format_Grayscale8));
+    for (int y = 0; y < height; ++y) {
+        uchar *row = result.scanLine(y);
+        for (int x = 0; x < width; ++x)
+            row[x] = uchar(std::min(255.0f, std::max(0.0f, levels[size_t(y) * size_t(width) + size_t(x)] * 255 + 0.5f)));
+    }
+    return result;
+}
+
+QImage GuidedMatte::refine(const QImage &mask, const QImage &guide, double radius, double limit)
+{
+    const QSize full = mask.size();
+    const double factor = std::min(1.0, limit / std::max(full.width(), full.height()));
+    const int width = std::max(1, int(std::round(full.width() * factor))), height = std::max(1, int(std::round(full.height() * factor)));
+    const int steps = std::max(1, int(std::round(radius * factor)));
+    const QImage small = image(filter(levels(mask, width, height), levels(guide, width, height), width, height, steps, 1e-4f), width, height);
+    // Drawn back up, filtered; Qt returns one already that size.
+    return checked(small.scaled(full, Qt::IgnoreAspectRatio, Qt::SmoothTransformation).convertToFormat(QImage::Format_Grayscale8));
+}

src/Document/GuidedMatte.h

@@ -0,0 +1,17 @@
+#pragma once
+#include <QImage>
+#include <vector>
+
+// Swift's GuidedMatte: a mask pulled onto its image's own edges.
+namespace GuidedMatte {
+// Mean over a (2r+1)² square, two running-sum passes.
+std::vector<float> box(const std::vector<float> &source, int width, int height, int radius);
+// `mask` refined by `guide`: both 0 to 1, one size.
+std::vector<float> filter(const std::vector<float> &mask, const std::vector<float> &guide, int width, int height, int radius, float epsilon);
+// `image`'s gray levels at this size, 0 to 1.
+std::vector<float> levels(const QImage &image, int width, int height);
+// Levels back to a gray image.
+QImage image(const std::vector<float> &levels, int width, int height);
+// Refined on a copy `limit` long at most, drawn back.
+QImage refine(const QImage &mask, const QImage &guide, double radius, double limit);
+}

src/Document/HueSaturation+Filter.cpp

@@ -0,0 +1,190 @@
+#include "Document/BrushStroke.h"
+#include "Rendering/PoolMap.h"
+#include "Document/HueSaturation.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include <QPainter>
+#include <QtConcurrent>
+#include <algorithm>
+#include <cmath>
+#include <numeric>
+
+namespace {
+struct HSL {
+    double hue;
+    double saturation;
+    double lightness;
+};
+
+HSL toHSL(double red, double green, double blue)
+{
+    const double high = std::max({red, green, blue}), low = std::min({red, green, blue});
+    const double lightness = (high + low) / 2, delta = high - low;
+    if (!(delta > 0))
+        return {0, 0, lightness};
+    const double saturation = delta / (1 - std::abs(2 * lightness - 1));
+    double hue = high == red ? (green - blue) / delta : high == green ? (blue - red) / delta + 2 : (red - green) / delta + 4;
+    hue *= 60;
+    if (hue < 0)
+        hue += 360;
+    return {hue, std::min(1.0, saturation), lightness};
+}
+
+HueSaturationFilter::Color toRGB(double hue, double saturation, double lightness)
+{
+    if (!(saturation > 0))
+        return {lightness, lightness, lightness};
+    const double chroma = (1 - std::abs(2 * lightness - 1)) * saturation, sector = hue / 60;
+    const double second = chroma * (1 - std::abs(std::fmod(sector, 2) - 1)), base = lightness - chroma / 2;
+    HueSaturationFilter::Color color{chroma, 0, second};
+    switch (int(sector)) {
+    case 0: color = {chroma, second, 0}; break;
+    case 1: color = {second, chroma, 0}; break;
+    case 2: color = {0, chroma, second}; break;
+    case 3: color = {0, second, chroma}; break;
+    case 4: color = {second, 0, chroma}; break;
+    }
+    const auto clamp = [base](double value) { return std::min(1.0, std::max(0.0, value + base)); };
+    return {clamp(color.red), clamp(color.green), clamp(color.blue)};
+}
+
+// Core Image's colour cube: unpremultiplied, trilinear, premultiplied back.
+void applyCube(uchar *bits, qsizetype stride, int width, const std::vector<float> &cube, const std::vector<int> &rows)
+{
+    constexpr int size = HueSaturationFilter::dimension, last = size - 1;
+    PoolMap::blocking(rows, [&](int y) {
+        uchar *pixel = bits + y * stride;
+        for (int x = 0; x < width; ++x, pixel += 4) {
+            const float alpha = pixel[3];
+            if (alpha == 0) {
+                std::fill_n(pixel, 3, uchar(0));
+                continue;
+            }
+            float place[3], fraction[3];
+            int low[3];
+            for (int channel = 0; channel < 3; ++channel) {
+                place[channel] = std::min(1.0f, pixel[channel] / alpha) * last;
+                low[channel] = std::min(int(place[channel]), last - 1);
+                fraction[channel] = place[channel] - float(low[channel]);
+            }
+            float out[3] = {0, 0, 0};
+            for (int corner = 0; corner < 8; ++corner) {
+                const int r = low[0] + (corner & 1), g = low[1] + (corner >> 1 & 1), b = low[2] + (corner >> 2 & 1);
+                const float share = ((corner & 1) ? fraction[0] : 1 - fraction[0]) * ((corner >> 1 & 1) ? fraction[1] : 1 - fraction[1])
+                    * ((corner >> 2 & 1) ? fraction[2] : 1 - fraction[2]);
+                const float *entry = &cube[size_t(((b * size + g) * size + r) * 4)];
+                for (int channel = 0; channel < 3; ++channel)
+                    out[channel] += share * entry[channel];
+            }
+            for (int channel = 0; channel < 3; ++channel)
+                pixel[channel] = uchar(std::lround(std::min(1.0f, std::max(0.0f, out[channel])) * alpha));
+        }
+    });
+}
+}
+
+std::vector<HueSaturationFilter::HueResponse> HueSaturationFilter::hueResponse(const HueSaturationSettings &settings)
+{
+    std::vector<HueResponse> result(361);
+    for (int degree = 0; degree <= 360; ++degree) {
+        HueResponse &response = result[size_t(degree)];
+        for (const auto &[colorRange, adjustment] : settings.adjustments) {
+            if (adjustment == RangeAdjustment())
+                continue;
+            const double weight = settings.weight(colorRange, degree);
+            if (!(weight > 0))
+                continue;
+            response.shift += adjustment.hue * weight;
+            response.saturation += adjustment.saturation * weight;
+            response.lightness += adjustment.lightness * weight;
+        }
+    }
+    return result;
+}
+
+std::vector<float> HueSaturationFilter::cube(const HueSaturationSettings &settings)
+{
+    const std::vector<HueResponse> response = hueResponse(settings);
+    std::vector<float> values(size_t(dimension * dimension * dimension * 4));
+    size_t index = 0;
+    const double step = dimension - 1;
+    for (int blue = 0; blue < dimension; ++blue) {
+        for (int green = 0; green < dimension; ++green) {
+            for (int red = 0; red < dimension; ++red) {
+                const Color color = adjust(red / step, green / step, blue / step, settings, response);
+                values[index] = float(color.red);
+                values[index + 1] = float(color.green);
+                values[index + 2] = float(color.blue);
+                values[index + 3] = 1;
+                index += 4;
+            }
+        }
+    }
+    return values;
+}
+
+HueSaturationFilter::Color HueSaturationFilter::adjust(double red, double green, double blue, const HueSaturationSettings &settings)
+{
+    return adjust(red, green, blue, settings, hueResponse(settings));
+}
+
+HueSaturationFilter::Color HueSaturationFilter::adjust(double red, double green, double blue, const HueSaturationSettings &settings,
+                                                       const std::vector<HueResponse> &response)
+{
+    auto [hue, saturation, lightness] = toHSL(red, green, blue);
+    double lightnessAmount = 0;
+    if (settings.colorize) {
+        hue = std::fmod(settings.hue(), 360);
+        saturation = std::min(1.0, std::max(0.0, settings.saturation() / 100));
+        lightnessAmount = settings.lightness() / 100;
+    } else {
+        // Every range adds in, weighted by its claim.
+        const HueResponse &sampled = response[size_t(std::min(response.size() - 1, size_t(std::max(0.0, std::round(hue)))))];
+        lightnessAmount = sampled.lightness / 100;
+        hue = std::fmod(hue + sampled.shift, 360);
+        if (hue < 0)
+            hue += 360;
+        // Multiplied, so neutral grays stay neutral.
+        saturation = std::min(1.0, std::max(0.0, saturation * (1 + sampled.saturation / 100)));
+    }
+    // Toward white above zero, black below, reaching either at 100.
+    const double amount = std::min(1.0, std::max(-1.0, lightnessAmount));
+    lightness = amount >= 0 ? lightness + (1 - lightness) * amount : lightness * (1 + amount);
+    return toRGB(hue, saturation, std::min(1.0, std::max(0.0, lightness)));
+}
+
+double HueSaturationFilter::shiftedHue(double hue, const HueSaturationSettings &settings)
+{
+    double shift = 0;
+    for (const auto &[colorRange, adjustment] : settings.adjustments) {
+        if (adjustment.hue != 0)
+            shift += adjustment.hue * settings.weight(colorRange, hue);
+    }
+    const double shifted = std::fmod(hue + shift, 360);
+    return shifted < 0 ? shifted + 360 : shifted;
+}
+
+AdjustedPixels HueSaturationFilter::run(const HueSaturationJob &job)
+{
+    const int width = job.image.width(), height = job.image.height();
+    QImage result = BrushRaster::context(width, height, false);
+    {
+        QPainter painter(&result);
+        BrushRaster::draw(job.image, QRectF(0, 0, width, height), painter);
+    }
+    std::vector<float> table;
+    std::vector<int> rows;
+    try {
+        table = cube(job.settings);
+        rows.resize(size_t(height));
+    } catch (const std::bad_alloc &) {
+        // Out of memory fails as a context would.
+        throw ExportError(ExportError::Kind::render);
+    }
+    std::iota(rows.begin(), rows.end(), 0);
+    applyCube(result.bits(), result.bytesPerLine(), width, table, rows);
+    // Swift blends over the image drawn again: the same pixels.
+    if (job.selection)
+        result = PixelAdjust::blend(result, job.image, *job.selection, job.pixelToDocument, false);
+    return {result, job.thumbnail ? std::optional(PixelAdjust::thumbnail(result)) : std::nullopt};
+}

src/Document/HueSaturation+Session.cpp

@@ -0,0 +1,276 @@
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include <QtConcurrent>
+
+namespace {
+// Swift's adjustedPixels: the render, or why it failed.
+SessionJobs::Adjusted adjusted(const HueSaturationJob &job)
+{
+    try {
+        return {HueSaturationFilter::run(job), std::nullopt};
+    } catch (const ExportError &error) {
+        return {std::nullopt, QString::fromUtf8(error.what())};
+    }
+}
+}
+
+// Swift's session extension in HueSaturation.swift.
+bool EditorSession::canAdjustColors() const
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    const std::optional<DocumentSelection> current = selection();
+    return !m_levels && !m_filterEdit && m_document && layer && !m_isProjectBusy && !m_isImporting && !m_brushStroke && !m_pixelMove && !m_renamingLayerID && !m_showsNewDocument
+        && !m_showsImporter && m_selectedLayerIDs.size() == 1 && !layer->isGroup && !m_isMaskSelected && layer->asset
+        && m_document->effectiveVisibleIDs().contains(layer->id) && !(current && current->isEmpty());
+}
+
+void EditorSession::beginHueSaturation()
+{
+    // Swift beeps.
+    if (m_hueSaturation || !canAdjustColors()) {
+        qCWarning(lcApp) << "Hue/Saturation cannot open here";
+        return;
+    }
+    // A pending gradient lands first, as beginLevels waits.
+    if (m_gradientEdit) {
+        commitGradient([this] { beginHueSaturation(); });
+        return;
+    }
+    commitTransform();
+    const ImageLayer layer = activeLayer().value();
+    const std::optional<DocumentSelection> current = selection();
+    try {
+        m_hueSaturation.emplace(layer.id, layer.asset.value(), current ? std::optional(current->clip(m_document->size())) : std::nullopt,
+                                layer.transform);
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+        return;
+    }
+    notify();
+}
+
+void EditorSession::updateHueSaturation(const HueSaturationSettings &settings, bool preview)
+{
+    if (!m_hueSaturation)
+        return;
+    HueSaturationEdit &edit = *m_hueSaturation;
+    edit.settings = settings;
+    edit.preview = preview;
+    // An adjustment layer takes the settings; its pixels stay.
+    if (previewAdjustmentEditing(preview)) {
+        notify();
+        return;
+    }
+    if (!preview || settings.isIdentity()) {
+        // Swift cancels the running preview: its result drops.
+        m_hueSaturationPreviewFor = QUuid();
+        m_hueSaturationPending.reset();
+        edit.setPreview(std::nullopt);
+        ++m_brushRevision;
+        notify();
+        return;
+    }
+    m_hueSaturationPending = HueSaturationJob{edit.previewSource, settings, edit.selection, edit.previewPixelToDocument, false};
+    renderPendingHuePreview();
+    notify();
+}
+
+// Renders the newest job; one that arrives mid-render waits.
+void EditorSession::renderPendingHuePreview()
+{
+    if (!m_hueSaturation || !m_hueSaturationPending || m_hueSaturationRendering)
+        return;
+    const HueSaturationJob job = *std::exchange(m_hueSaturationPending, std::nullopt);
+    m_hueSaturationPreviewFor = m_hueSaturation->id;
+    m_hueSaturationRendering = true;
+    m_hueSaturationTask.setFuture(QtConcurrent::run([job] { return adjusted(job); }));
+}
+
+void EditorSession::finishHueSaturationPreview()
+{
+    m_hueSaturationRendering = false;
+    const Adjusted made = m_hueSaturationTask.result();
+    // Swift's adjustedPixels shows a failure whichever edit asked.
+    if (made.failure)
+        setBrushError(made.failure);
+    if (m_hueSaturation && m_hueSaturation->id == std::exchange(m_hueSaturationPreviewFor, QUuid()) && made.pixels) {
+        m_hueSaturation->setPreview(made.pixels->image);
+        ++m_brushRevision;
+        notify();
+    }
+    renderPendingHuePreview();
+}
+
+void EditorSession::commitHueSaturation(std::function<void()> done)
+{
+    // The caller resumes from the event loop, as after await.
+    const auto finish = [this, done] {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    };
+    if (finishAdjustmentEditing(true) || !m_hueSaturation || m_committingHueSaturation) {
+        finish();
+        return;
+    }
+    m_hueSampleMode.reset();
+    m_hueTargeting = false;
+    m_hueTargetDrag.reset();
+    m_hueSaturationPending.reset();
+    m_hueSaturationPreviewFor = QUuid();
+    const HueSaturationEdit &edit = *m_hueSaturation;
+    if (edit.settings.isIdentity()) {
+        m_hueSaturation.reset();
+        ++m_brushRevision;
+        notify();
+        finish();
+        return;
+    }
+    // The preview stays on screen until the pixels land.
+    m_committingHueSaturation.emplace(CommittingHue{edit, std::move(done)});
+    setIsProjectBusy(true);
+    // A painted layer flattens there, where a failure is caught.
+    m_hueSaturationCommit.setFuture(QtConcurrent::run([original = edit.original, settings = edit.settings, selection = edit.selection,
+                                                       mapping = edit.pixelToDocument]() -> Adjusted {
+        try {
+            return adjusted(HueSaturationJob{original.image(), settings, selection, mapping, true});
+        } catch (const ExportError &error) {
+            return {std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::finishHueSaturationCommit()
+{
+    const Adjusted made = m_hueSaturationCommit.result();
+    const CommittingHue committing = std::exchange(m_committingHueSaturation, std::nullopt).value();
+    const HueSaturationEdit &edit = committing.edit;
+    const int index = m_document ? indexOf(m_document->layers, edit.layerID) : -1;
+    if (made.failure)
+        setBrushError(made.failure);
+    else if (index >= 0 && m_document->layers[index].asset && m_document->layers[index].asset->identity() == edit.original.identity()) {
+        beginEdit(QStringLiteral("Hue/Saturation"));
+        ImageLayer &layer = m_document->layers[index];
+        layer.asset = ImportedImage(made.pixels->image, made.pixels->thumbnail.value_or(made.pixels->image), layer.name);
+        // Swift rebuilds the layer without shape, effects and text.
+        layer.shape = std::nullopt;
+        layer.effects = std::nullopt;
+        layer.text = std::nullopt;
+        endEdit();
+    }
+    // Swift's defers: the project frees, then the edit goes.
+    setIsProjectBusy(false);
+    m_hueSaturation.reset();
+    ++m_brushRevision;
+    notify();
+    if (committing.done)
+        QMetaObject::invokeMethod(this, committing.done, Qt::QueuedConnection);
+}
+
+std::optional<double> EditorSession::sampledHue(QPointF point) const
+{
+    const std::optional<PaletteColor> color = sampleCompositeColor(point);
+    if (!color)
+        return std::nullopt;
+    const PickerHSB hsb(*color);
+    return hsb.saturation > 0.02 ? std::optional(hsb.hue) : std::nullopt;
+}
+
+void EditorSession::sampleHueRange(QPointF point)
+{
+    if (!m_hueSaturation || !m_hueSampleMode)
+        return;
+    HueSaturationSettings settings = m_hueSaturation->settings;
+    const std::optional<double> hue = settings.range != ColorRange::master && !settings.colorize ? sampledHue(point) : std::nullopt;
+    // Swift beeps: Master, Colorize, or a gray pixel.
+    if (!hue) {
+        qCWarning(lcApp) << "no hue to sample here";
+        return;
+    }
+    HueBand band = settings.band();
+    if (*m_hueSampleMode == HueSampleMode::replace)
+        band = band.centered(*hue);
+    else if (*m_hueSampleMode == HueSampleMode::add)
+        band.include(*hue);
+    else
+        band.exclude(*hue);
+    settings.setBand(band);
+    updateHueSaturation(settings, m_hueSaturation->preview);
+}
+
+bool EditorSession::beginHueTargeting(QPointF point)
+{
+    const std::optional<double> hue = m_hueSaturation && m_hueTargeting && !m_hueSaturation->settings.colorize ? sampledHue(point) : std::nullopt;
+    // Swift beeps.
+    if (!hue) {
+        qCWarning(lcApp) << "no colour to target here";
+        return false;
+    }
+    HueSaturationSettings settings = m_hueSaturation->settings;
+    // Swift's max(by:): the first of the strongest claims.
+    ColorRange range = colorRanges.front();
+    for (const ColorRange each : colorRanges) {
+        if (settings.weight(each, *hue) > settings.weight(range, *hue))
+            range = each;
+    }
+    settings.range = range;
+    const RangeAdjustment adjustment = settings.adjustments.contains(range) ? settings.adjustments.at(range) : RangeAdjustment();
+    m_hueTargetDrag = HueTargetDrag{range, adjustment.hue, adjustment.saturation};
+    updateHueSaturation(settings, m_hueSaturation->preview);
+    return true;
+}
+
+void EditorSession::dragHueTargeting(double viewDelta, bool adjustsHue)
+{
+    if (!m_hueSaturation || !m_hueTargetDrag)
+        return;
+    HueSaturationSettings settings = m_hueSaturation->settings;
+    const HueTargetDrag drag = *m_hueTargetDrag;
+    RangeAdjustment &adjustment = settings.adjustments[drag.range];
+    if (adjustsHue)
+        adjustment.hue = std::min(180.0, std::max(-180.0, drag.hue + viewDelta / 2));
+    else
+        adjustment.saturation = std::min(100.0, std::max(-100.0, drag.saturation + viewDelta / 2));
+    updateHueSaturation(settings, m_hueSaturation->preview);
+}
+
+void EditorSession::endHueTargeting()
+{
+    m_hueTargetDrag.reset();
+}
+
+void EditorSession::cancelHueSaturation()
+{
+    if (finishAdjustmentEditing(false))
+        return;
+    const bool armed = m_hueSampleMode || m_hueTargeting;
+    m_hueSampleMode.reset();
+    m_hueTargeting = false;
+    m_hueTargetDrag.reset();
+    if (!m_hueSaturation) {
+        if (armed)
+            notify();
+        return;
+    }
+    // A running preview drops: its edit is gone.
+    m_hueSaturationPending.reset();
+    m_hueSaturation.reset();
+    ++m_brushRevision;
+    notify();
+}
+
+void EditorSession::setHueSampleMode(std::optional<HueSampleMode> mode)
+{
+    if (m_hueSampleMode == mode)
+        return;
+    m_hueSampleMode = mode;
+    notify();
+}
+
+void EditorSession::setHueTargeting(bool targeting)
+{
+    if (m_hueTargeting == targeting)
+        return;
+    m_hueTargeting = targeting;
+    notify();
+}

src/Document/HueSaturation+Session.h

@@ -0,0 +1,26 @@
+// Swift's HueSaturation extension.
+public:
+    bool canAdjustColors() const;
+    void beginHueSaturation();
+    // Previews coalesce: the newest waits for the running one.
+    void updateHueSaturation(const HueSaturationSettings &settings, bool preview);
+    // Off the UI thread; `done` follows the step or refusal.
+    void commitHueSaturation(std::function<void()> done = {});
+    // The hue under a document point; near-grays have none.
+    std::optional<double> sampledHue(QPointF point) const;
+    // The eyedroppers: re-centre, widen or narrow the band.
+    void sampleHueRange(QPointF point);
+    // Targeted adjustment: the range owning the sampled colour.
+    bool beginHueTargeting(QPointF point);
+    // A unit every two view points, from the drag's start.
+    void dragHueTargeting(double viewDelta, bool adjustsHue);
+    void endHueTargeting();
+    void cancelHueSaturation();
+    // The sheet's writes, as Swift's sheet sets them.
+    void setHueSampleMode(std::optional<HueSampleMode> mode);
+    void setHueTargeting(bool targeting);
+
+private:
+    void renderPendingHuePreview();
+    void finishHueSaturationPreview();
+    void finishHueSaturationCommit();

src/Document/HueSaturation.cpp

@@ -0,0 +1,246 @@
+#include "Document/HueSaturation.h"
+#include "Document/BrushStroke.h"
+#include "Document/LayerTransform.h"
+#include "Rendering/RasterSnapshot.h"
+#include <QPainter>
+#include <cmath>
+#include <stdexcept>
+
+QString rawValue(ColorRange range)
+{
+    switch (range) {
+    case ColorRange::master: return QStringLiteral("Master");
+    case ColorRange::reds: return QStringLiteral("Reds");
+    case ColorRange::yellows: return QStringLiteral("Yellows");
+    case ColorRange::greens: return QStringLiteral("Greens");
+    case ColorRange::cyans: return QStringLiteral("Cyans");
+    case ColorRange::blues: return QStringLiteral("Blues");
+    case ColorRange::magentas: return QStringLiteral("Magentas");
+    }
+    throw std::logic_error("no such colour range");
+}
+
+HueBand defaultBand(ColorRange range)
+{
+    switch (range) {
+    case ColorRange::master: return {0, 0, 360, 360};
+    case ColorRange::reds: return {315, 345, 15, 45};
+    case ColorRange::yellows: return {15, 45, 75, 105};
+    case ColorRange::greens: return {75, 105, 135, 165};
+    case ColorRange::cyans: return {135, 165, 195, 225};
+    case ColorRange::blues: return {195, 225, 255, 285};
+    case ColorRange::magentas: return {255, 285, 315, 345};
+    }
+    throw std::logic_error("no such colour range");
+}
+
+namespace {
+// Swift's truncatingRemainder, brought into 0 to 360.
+double wrap(double value)
+{
+    const double remainder = std::fmod(value, 360);
+    return remainder < 0 ? remainder + 360 : remainder;
+}
+}
+
+double HueBand::forward(double from, double to)
+{
+    const double delta = std::fmod(to - from, 360);
+    return delta < 0 ? delta + 360 : delta;
+}
+
+double HueBand::weight(double hue) const
+{
+    const double span = forward(falloffStart, falloffEnd);
+    // Master covers everything.
+    if (!(span > 0))
+        return 1;
+    const double position = forward(falloffStart, hue);
+    if (!(position <= span))
+        return 0;
+    const double rampIn = forward(falloffStart, rangeStart), plateauEnd = forward(falloffStart, rangeEnd);
+    if (position < rampIn)
+        return rampIn > 0 ? position / rampIn : 1;
+    if (position <= plateauEnd)
+        return 1;
+    const double rampOut = span - plateauEnd;
+    return rampOut > 0 ? (span - position) / rampOut : 1;
+}
+
+std::array<double, 4> HueBand::handles() const
+{
+    return {falloffStart, rangeStart, rangeEnd, falloffEnd};
+}
+
+HueBand HueBand::centered(double hue) const
+{
+    const double core = forward(rangeStart, rangeEnd), leading = forward(falloffStart, rangeStart), trailing = forward(rangeEnd, falloffEnd);
+    const double start = wrap(hue - core / 2);
+    return {wrap(start - leading), start, wrap(start + core), wrap(start + core + trailing)};
+}
+
+void HueBand::include(double hue)
+{
+    if (!(weight(hue) < 1))
+        return;
+    const double shoulderIn = forward(falloffStart, rangeStart), shoulderOut = forward(rangeEnd, falloffEnd);
+    if (forward(hue, rangeStart) <= forward(rangeEnd, hue)) {
+        rangeStart = hue;
+        falloffStart = hue - shoulderIn;
+    } else {
+        rangeEnd = hue;
+        falloffEnd = hue + shoulderOut;
+    }
+    normalize();
+}
+
+void HueBand::exclude(double hue)
+{
+    if (!(weight(hue) > 0))
+        return;
+    const double shoulderIn = forward(falloffStart, rangeStart), shoulderOut = forward(rangeEnd, falloffEnd);
+    if (forward(falloffStart, hue) <= forward(hue, falloffEnd)) {
+        falloffStart = hue + 1;
+        rangeStart = hue + 1 + shoulderIn;
+    } else {
+        falloffEnd = hue - 1;
+        rangeEnd = hue - 1 - shoulderOut;
+    }
+    normalize();
+}
+
+void HueBand::normalize()
+{
+    falloffStart = wrap(falloffStart);
+    rangeStart = wrap(rangeStart);
+    rangeEnd = wrap(rangeEnd);
+    falloffEnd = wrap(falloffEnd);
+    // Under a full circle.
+    if (forward(falloffStart, falloffEnd) > 350)
+        falloffEnd = wrap(falloffStart + 350);
+}
+
+void HueBand::setHandle(int index, double degrees)
+{
+    HueBand updated = *this;
+    const double value = std::fmod(std::fmod(degrees, 360) + 360, 360);
+    (index == 0 ? updated.falloffStart : index == 1 ? updated.rangeStart : index == 2 ? updated.rangeEnd : updated.falloffEnd) = value;
+    const double span = forward(updated.falloffStart, updated.falloffEnd), toStart = forward(updated.falloffStart, updated.rangeStart),
+                 toEnd = forward(updated.falloffStart, updated.rangeEnd);
+    if (span > 1 && span <= 350 && toStart <= toEnd && toEnd <= span)
+        *this = updated;
+}
+
+QString rawValue(HueSampleMode mode)
+{
+    switch (mode) {
+    case HueSampleMode::replace: return QStringLiteral("Sample");
+    case HueSampleMode::add: return QStringLiteral("Add");
+    case HueSampleMode::remove: return QStringLiteral("Remove");
+    }
+    throw std::logic_error("no such sample mode");
+}
+
+QString help(HueSampleMode mode)
+{
+    switch (mode) {
+    case HueSampleMode::replace: return QStringLiteral("Click the image to center this range on that color");
+    case HueSampleMode::add: return QStringLiteral("Click the image to widen this range to include that color");
+    case HueSampleMode::remove: return QStringLiteral("Click the image to narrow this range to exclude that color");
+    }
+    throw std::logic_error("no such sample mode");
+}
+
+HueSaturationSettings::HueSaturationSettings(double hue, double saturation, double lightness, bool colorize, ColorRange range)
+    : range(range), colorize(colorize)
+{
+    for (const ColorRange each : allColorRanges)
+        bands.emplace(each, defaultBand(each));
+    adjustments[range] = RangeAdjustment{hue, saturation, lightness};
+}
+
+double HueSaturationSettings::hue() const
+{
+    return adjustments.contains(range) ? adjustments.at(range).hue : 0;
+}
+
+void HueSaturationSettings::setHue(double value)
+{
+    adjustments[range].hue = value;
+}
+
+double HueSaturationSettings::saturation() const
+{
+    return adjustments.contains(range) ? adjustments.at(range).saturation : 0;
+}
+
+void HueSaturationSettings::setSaturation(double value)
+{
+    adjustments[range].saturation = value;
+}
+
+double HueSaturationSettings::lightness() const
+{
+    return adjustments.contains(range) ? adjustments.at(range).lightness : 0;
+}
+
+void HueSaturationSettings::setLightness(double value)
+{
+    adjustments[range].lightness = value;
+}
+
+HueBand HueSaturationSettings::band() const
+{
+    return bands.contains(range) ? bands.at(range) : defaultBand(range);
+}
+
+void HueSaturationSettings::setBand(const HueBand &value)
+{
+    bands[range] = value;
+}
+
+HueSaturationSettings HueSaturationSettings::colorizeStart()
+{
+    return HueSaturationSettings(0, 25, 0, true);
+}
+
+bool HueSaturationSettings::isIdentity() const
+{
+    return !colorize && std::all_of(adjustments.begin(), adjustments.end(), [](const auto &entry) { return entry.second == RangeAdjustment(); });
+}
+
+double HueSaturationSettings::weight(ColorRange colorRange, double hue) const
+{
+    if (colorRange == ColorRange::master)
+        return 1;
+    const double inBand = (bands.contains(colorRange) ? bands.at(colorRange) : defaultBand(colorRange)).weight(hue);
+    return invertRange && colorRange == range ? 1 - inBand : inBand;
+}
+
+HueSaturationEdit::HueSaturationEdit(QUuid layerID, const ImportedImage &original, std::optional<SelectionClip> selection,
+                                     const LayerTransform &transform)
+    : layerID(layerID), original(original), selection(std::move(selection)),
+      pixelToDocument(BrushRaster::pixelToDocument(transform, original.size().width(), original.size().height()))
+{
+    const QSize size = original.size();
+    const double factor = std::min(1.0, double(previewLimit) / std::max(size.width(), size.height()));
+    if (factor < 1) {
+        const int width = std::max(1, int(size.width() * factor)), height = std::max(1, int(size.height() * factor));
+        previewSource = BrushRaster::context(width, height, false);
+        QPainter painter(&previewSource);
+        const QRectF rect(0, 0, width, height);
+        if (original.raster)
+            original.raster->draw(rect, painter);
+        else
+            BrushRaster::draw(original.image(), rect, painter);
+        previewPixelToDocument = BrushRaster::pixelToDocument(transform, width, height);
+    } else {
+        previewSource = original.image();
+        previewPixelToDocument = pixelToDocument;
+    }
+}
+
+std::optional<QImage> HueSaturationEdit::previewImage(QUuid layer) const
+{
+    return layer == layerID ? m_preparedPreview : std::nullopt;
+}

src/Document/HueSaturation.h

@@ -0,0 +1,160 @@
+#pragma once
+#include "Document/Selection.h"
+#include "IO/ImageImporter.h"
+#include <QImage>
+#include <QTransform>
+#include <QUuid>
+#include <array>
+#include <map>
+#include <optional>
+#include <vector>
+
+struct LayerTransform;
+
+// The six colour ranges and Master, as Photoshop's Cmd+U.
+enum class ColorRange { master, reds, yellows, greens, cyans, blues, magentas };
+inline constexpr std::array<ColorRange, 7> allColorRanges{ColorRange::master, ColorRange::reds,  ColorRange::yellows, ColorRange::greens,
+                                                          ColorRange::cyans,  ColorRange::blues, ColorRange::magentas};
+// Swift's colorRanges: every range but Master.
+inline constexpr std::array<ColorRange, 6> colorRanges{ColorRange::reds,  ColorRange::yellows, ColorRange::greens,
+                                                       ColorRange::cyans, ColorRange::blues,   ColorRange::magentas};
+QString rawValue(ColorRange range);
+
+// A hue band in degrees, wrapping at 360.
+struct HueBand {
+    double falloffStart;
+    double rangeStart;
+    double rangeEnd;
+    double falloffEnd;
+    // Degrees forward from one hue to another, 0 to 360.
+    static double forward(double from, double to);
+    // Full inside the range, ramping through each shoulder.
+    double weight(double hue) const;
+    std::array<double, 4> handles() const;
+    // Centred on a hue, keeping the core and shoulders.
+    HueBand centered(double hue) const;
+    // Widens to hold the hue, moving the nearer edge.
+    void include(double hue);
+    // Narrows to leave the hue out, shoulder included.
+    void exclude(double hue);
+    // Moves a handle unless the four would cross.
+    void setHandle(int index, double degrees);
+    friend bool operator==(const HueBand &, const HueBand &) = default;
+
+private:
+    void normalize();
+};
+
+// Photoshop's starting band for a range.
+HueBand defaultBand(ColorRange range);
+
+// Which eyedropper the panel arms.
+enum class HueSampleMode { replace, add, remove };
+inline constexpr std::array<HueSampleMode, 3> allHueSampleModes{HueSampleMode::replace, HueSampleMode::add, HueSampleMode::remove};
+QString rawValue(HueSampleMode mode);
+QString help(HueSampleMode mode);
+
+// A targeted-adjustment drag: its range and starting values.
+struct HueTargetDrag {
+    ColorRange range;
+    double hue;
+    double saturation;
+};
+
+struct RangeAdjustment {
+    double hue = 0;
+    double saturation = 0;
+    double lightness = 0;
+    friend bool operator==(const RangeAdjustment &, const RangeAdjustment &) = default;
+};
+
+// Each range keeps its own values; Master applies everywhere.
+struct HueSaturationSettings {
+    explicit HueSaturationSettings(double hue = 0, double saturation = 0, double lightness = 0, bool colorize = false,
+                                   ColorRange range = ColorRange::master);
+    ColorRange range;
+    bool colorize;
+    // Applies the range outside its band instead.
+    bool invertRange = false;
+    std::map<ColorRange, RangeAdjustment> adjustments;
+    std::map<ColorRange, HueBand> bands;
+    // Swift's computed properties: the selected range's values.
+    double hue() const;
+    void setHue(double value);
+    double saturation() const;
+    void setSaturation(double value);
+    double lightness() const;
+    void setLightness(double value);
+    HueBand band() const;
+    void setBand(const HueBand &value);
+    // Photoshop's start when Colorize is switched on.
+    static HueSaturationSettings colorizeStart();
+    bool isIdentity() const;
+    // Master everywhere; the others through their bands.
+    double weight(ColorRange colorRange, double hue) const;
+    friend bool operator==(const HueSaturationSettings &, const HueSaturationSettings &) = default;
+};
+
+struct HueSaturationJob {
+    QImage image;
+    HueSaturationSettings settings;
+    std::optional<SelectionClip> selection;
+    QTransform pixelToDocument;
+    // Previews skip the panel's thumbnail.
+    bool thumbnail = true;
+};
+
+struct AdjustedPixels {
+    QImage image;
+    std::optional<QImage> thumbnail;
+};
+
+// Swift's HueSaturationFilter: a colour cube built from the settings.
+namespace HueSaturationFilter {
+inline constexpr int dimension = 33;
+// Swift's tuple: how every range moves one hue.
+struct HueResponse {
+    double shift = 0;
+    double saturation = 0;
+    double lightness = 0;
+};
+struct Color {
+    double red;
+    double green;
+    double blue;
+};
+// Throws ExportError when out of memory.
+AdjustedPixels run(const HueSaturationJob &job);
+// Once a degree, 0 to 360.
+std::vector<HueResponse> hueResponse(const HueSaturationSettings &settings);
+// RGBA floats, red fastest, then green, then blue.
+std::vector<float> cube(const HueSaturationSettings &settings);
+Color adjust(double red, double green, double blue, const HueSaturationSettings &settings);
+Color adjust(double red, double green, double blue, const HueSaturationSettings &settings, const std::vector<HueResponse> &response);
+// The hue a spectrum swatch becomes.
+double shiftedHue(double hue, const HueSaturationSettings &settings);
+}
+
+// One open Hue/Saturation dialog over a layer's pixels.
+class HueSaturationEdit {
+public:
+    // Past 8000 pixels a side the preview copy is smaller.
+    static constexpr int previewLimit = 8000;
+    HueSaturationEdit(QUuid layerID, const ImportedImage &original, std::optional<SelectionClip> selection, const LayerTransform &transform);
+    std::optional<QImage> previewImage(QUuid layer) const;
+    void setPreview(std::optional<QImage> image) { m_preparedPreview = std::move(image); }
+
+    // Swift's object identity: results land on their own edit.
+    const QUuid id = QUuid::createUuid();
+    const QUuid layerID;
+    const ImportedImage original;
+    const std::optional<SelectionClip> selection;
+    const QTransform pixelToDocument;
+    QImage previewSource;
+    QTransform previewPixelToDocument;
+    HueSaturationSettings settings;
+    bool preview = true;
+
+private:
+    std::optional<QImage> m_preparedPreview;
+};

src/Document/ImageAdjustments.cpp

@@ -0,0 +1,133 @@
+#include "Document/ImageAdjustments.h"
+#include "Document/BrushStroke.h"
+#include "IO/ProjectStore.h"
+#include <QPainter>
+#include <array>
+#include <cmath>
+extern "C" {
+#include "AdjustPixels.h"
+#include "LevelsPixels.h"
+}
+
+QImage ImageAdjustmentPixels::run(const QImage &image, const std::function<void(uchar *, int, int, qsizetype)> &body)
+{
+    QImage context = BrushRaster::context(image.width(), image.height(), false);
+    {
+        QPainter painter(&context);
+        BrushRaster::draw(image, QRectF(0, 0, image.width(), image.height()), painter);
+    }
+    body(context.bits(), context.width(), context.height(), context.bytesPerLine());
+    return context;
+}
+
+double ImageAdjustmentPixels::clamp(double value, double low, double high, double fallback)
+{
+    return std::isfinite(value) ? std::min(high, std::max(low, value)) : fallback;
+}
+
+bool AdjustmentColor::isValid() const
+{
+    return red >= 0 && red <= 1 && green >= 0 && green <= 1 && blue >= 0 && blue <= 1;
+}
+
+AdjustmentColor AdjustmentColor::clamped() const
+{
+    using ImageAdjustmentPixels::clamp;
+    return {clamp(red, 0, 1, 0), clamp(green, 0, 1, 0), clamp(blue, 0, 1, 0)};
+}
+
+bool ExposureSettings::isValid() const
+{
+    return exposure >= exposureLow && exposure <= exposureHigh && offset >= offsetLow && offset <= offsetHigh && gamma >= gammaLow
+        && gamma <= gammaHigh;
+}
+
+ExposureSettings ExposureSettings::normalized() const
+{
+    using ImageAdjustmentPixels::clamp;
+    return {clamp(exposure, exposureLow, exposureHigh, 0), clamp(offset, offsetLow, offsetHigh, 0), clamp(gamma, gammaLow, gammaHigh, 1)};
+}
+
+std::array<float, 256> ExposureSettings::table() const
+{
+    const double scale = std::pow(2, exposure);
+    std::array<float, 256> result;
+    for (size_t index = 0; index < 256; ++index) {
+        const double encoded = double(index) / 255;
+        double linear = encoded <= 0.04045 ? encoded / 12.92 : std::pow((encoded + 0.055) / 1.055, 2.4);
+        linear = std::pow(std::max(0.0, linear * scale + offset), 1 / gamma);
+        const double output = linear <= 0.0031308 ? linear * 12.92 : 1.055 * std::pow(linear, 1 / 2.4) - 0.055;
+        result[index] = float(std::min(1.0, std::max(0.0, output)));
+    }
+    return result;
+}
+
+QImage ExposureSettings::apply(const QImage &image) const
+{
+    if (!isValid())
+        throw ProjectError(ProjectError::Kind::invalid);
+    const std::array<float, 256> curve = table();
+    std::array<float, 3 * 256> tables;
+    for (size_t channel = 0; channel < 3; ++channel)
+        std::copy(curve.begin(), curve.end(), tables.begin() + qsizetype(channel * 256));
+    return ImageAdjustmentPixels::run(image, [&](uchar *pixels, int width, int height, qsizetype) {
+        levels_apply(pixels, size_t(width) * size_t(height), tables.data());
+    });
+}
+
+bool GradientMapSettings::isValid() const
+{
+    return shadows.isValid() && highlights.isValid();
+}
+
+GradientMapSettings GradientMapSettings::normalized() const
+{
+    return {shadows.clamped(), highlights.clamped(), reversed};
+}
+
+GradientMapSettings::Ends GradientMapSettings::ends() const
+{
+    return reversed ? Ends{highlights, shadows} : Ends{shadows, highlights};
+}
+
+QImage GradientMapSettings::apply(const QImage &image) const
+{
+    if (!isValid())
+        throw ProjectError(ProjectError::Kind::invalid);
+    const auto [dark, light] = ends();
+    std::array<uint8_t, 3 * 256> table;
+    const auto byte = [](double value) { return uint8_t(std::min(255.0, std::max(0.0, std::round(value * 255)))); };
+    for (size_t index = 0; index < 256; ++index) {
+        const double t = double(index) / 255;
+        table[index * 3] = byte(dark.red + (light.red - dark.red) * t);
+        table[index * 3 + 1] = byte(dark.green + (light.green - dark.green) * t);
+        table[index * 3 + 2] = byte(dark.blue + (light.blue - dark.blue) * t);
+    }
+    return ImageAdjustmentPixels::run(image, [&](uchar *pixels, int width, int height, qsizetype stride) {
+        adjust_gradient_map(pixels, size_t(width), size_t(height), size_t(stride), table.data());
+    });
+}
+
+bool GrainSettings::isValid() const
+{
+    return amount >= amountLow && amount <= amountHigh && size >= sizeLow && size <= sizeHigh && roughness >= roughnessLow
+        && roughness <= roughnessHigh;
+}
+
+GrainSettings GrainSettings::normalized() const
+{
+    using ImageAdjustmentPixels::clamp;
+    return {clamp(amount, amountLow, amountHigh, 25), clamp(size, sizeLow, sizeHigh, 1.5), clamp(roughness, roughnessLow, roughnessHigh, 50), seed};
+}
+
+QImage GrainSettings::apply(const QImage &image, QPointF origin, double unitsPerPixel, std::optional<quint32> seed) const
+{
+    if (!isValid() || !std::isfinite(unitsPerPixel) || !(unitsPerPixel > 0))
+        throw ProjectError(ProjectError::Kind::invalid);
+    if (!(amount > 0))
+        return image;
+    const quint32 pattern = seed.value_or(this->seed);
+    return ImageAdjustmentPixels::run(image, [&](uchar *pixels, int width, int height, qsizetype stride) {
+        adjust_grain(pixels, size_t(width), size_t(height), size_t(stride), amount, size, roughness, pattern, origin.x(), origin.y(), unitsPerPixel);
+    });
+}

src/Document/ImageAdjustments.h

@@ -0,0 +1,76 @@
+#pragma once
+#include "Document/ColorPalette.h"
+#include <QImage>
+#include <QPointF>
+#include <array>
+#include <functional>
+#include <optional>
+
+// Swift's ImageAdjustmentPixels: a kernel over premultiplied RGBA.
+namespace ImageAdjustmentPixels {
+// The body gets pixels, width, height and bytes a row.
+QImage run(const QImage &image, const std::function<void(uchar *, int, int, qsizetype)> &body);
+// Within range; a value that is no number falls back.
+double clamp(double value, double low, double high, double fallback);
+}
+
+// A straight sRGB colour an adjustment stores, 0 to 1.
+struct AdjustmentColor {
+    AdjustmentColor(double red, double green, double blue) : red(red), green(green), blue(blue) {}
+    explicit AdjustmentColor(const PaletteColor &color) : AdjustmentColor(color.red, color.green, color.blue) {}
+    double red;
+    double green;
+    double blue;
+    bool isValid() const;
+    AdjustmentColor clamped() const;
+    friend bool operator==(const AdjustmentColor &, const AdjustmentColor &) = default;
+};
+
+// Photoshop's Exposure: stops and offset in linear light, then gamma.
+struct ExposureSettings {
+    static constexpr double exposureLow = -20, exposureHigh = 20;
+    static constexpr double offsetLow = -0.5, offsetHigh = 0.5;
+    static constexpr double gammaLow = 0.01, gammaHigh = 9.99;
+    double exposure = 0;
+    double offset = 0;
+    double gamma = 1;
+    bool isValid() const;
+    ExposureSettings normalized() const;
+    // Each input byte's output, 0 to 1, through linear light.
+    std::array<float, 256> table() const;
+    QImage apply(const QImage &image) const;
+    friend bool operator==(const ExposureSettings &, const ExposureSettings &) = default;
+};
+
+// Gradient Map: brightness picks a colour between the two ends.
+struct GradientMapSettings {
+    AdjustmentColor shadows{0, 0, 0};
+    AdjustmentColor highlights{1, 1, 1};
+    bool reversed = false;
+    bool isValid() const;
+    GradientMapSettings normalized() const;
+    // Swift's ends: the darkest tone's colour, then the lightest's.
+    struct Ends {
+        AdjustmentColor dark;
+        AdjustmentColor light;
+    };
+    Ends ends() const;
+    QImage apply(const QImage &image) const;
+    friend bool operator==(const GradientMapSettings &, const GradientMapSettings &) = default;
+};
+
+// Film grain, fixed in document space by its seed.
+struct GrainSettings {
+    static constexpr double amountLow = 0, amountHigh = 100;
+    static constexpr double sizeLow = 0.5, sizeHigh = 20;
+    static constexpr double roughnessLow = 0, roughnessHigh = 100;
+    double amount = 25;
+    double size = 1.5;
+    double roughness = 50;
+    quint32 seed = 0;
+    bool isValid() const;
+    GrainSettings normalized() const;
+    // Pixels placed in the document; a given seed wins.
+    QImage apply(const QImage &image, QPointF origin = QPointF(), double unitsPerPixel = 1, std::optional<quint32> seed = std::nullopt) const;
+    friend bool operator==(const GrainSettings &, const GrainSettings &) = default;
+};

src/Document/LayerAdjustment+Session.h

@@ -0,0 +1,6 @@
+// Swift's LayerAdjustment extension: adding and updating adjustments.
+public:
+    // Above the active layer, one step; then its editor opens.
+    void addAdjustment(AdjustmentKind kind);
+    // Valid settings alone, with no step of their own.
+    void updateAdjustment(QUuid id, const LayerAdjustment &value);

src/Document/LayerAdjustment.cpp

@@ -0,0 +1,157 @@
+#include "Document/LayerAdjustment.h"
+#include "Document/EditorSession.h"
+#include <QRandomGenerator>
+#include <cmath>
+
+QString rawValue(AdjustmentKind kind)
+{
+    switch (kind) {
+    case AdjustmentKind::hsv: return QStringLiteral("Hue/Saturation");
+    case AdjustmentKind::levels: return QStringLiteral("Levels");
+    case AdjustmentKind::curves: return QStringLiteral("Curves");
+    case AdjustmentKind::exposure: return QStringLiteral("Exposure");
+    case AdjustmentKind::gradientMap: return QStringLiteral("Gradient Map");
+    case AdjustmentKind::grain: return QStringLiteral("Grain");
+    }
+    throw std::logic_error("unknown adjustment kind");
+}
+
+std::optional<AdjustmentKind> adjustmentKind(const QString &text)
+{
+    for (const AdjustmentKind kind : allAdjustmentKinds) {
+        if (rawValue(kind) == text)
+            return kind;
+    }
+    return std::nullopt;
+}
+
+std::optional<FilterKind> filterKind(AdjustmentKind kind)
+{
+    switch (kind) {
+    case AdjustmentKind::curves: return FilterKind::curves;
+    case AdjustmentKind::exposure: return FilterKind::exposure;
+    case AdjustmentKind::gradientMap: return FilterKind::gradientMap;
+    case AdjustmentKind::grain: return FilterKind::grain;
+    case AdjustmentKind::hsv:
+    case AdjustmentKind::levels: return std::nullopt;
+    }
+    throw std::logic_error("unknown adjustment kind");
+}
+
+HueSaturationSettings LayerAdjustment::resolvedHSV() const
+{
+    return hsvSettings.value_or(HueSaturationSettings(hue, saturation, lightness, colorize));
+}
+
+ExposureSettings LayerAdjustment::exposure() const
+{
+    return exposureSettings.value_or(ExposureSettings());
+}
+
+void LayerAdjustment::setExposure(const ExposureSettings &value)
+{
+    exposureSettings = value;
+}
+
+GradientMapSettings LayerAdjustment::gradientMap() const
+{
+    return gradientMapSettings.value_or(GradientMapSettings());
+}
+
+void LayerAdjustment::setGradientMap(const GradientMapSettings &value)
+{
+    gradientMapSettings = value;
+}
+
+GrainSettings LayerAdjustment::grain() const
+{
+    return grainSettings.value_or(GrainSettings());
+}
+
+void LayerAdjustment::setGrain(const GrainSettings &value)
+{
+    grainSettings = value;
+}
+
+// Bounds refuse what is no number: no isFinite terms.
+bool LayerAdjustment::isValid() const
+{
+    const auto within = [](const RangeAdjustment &values) {
+        return std::abs(values.hue) <= 360 && std::abs(values.saturation) <= 100 && std::abs(values.lightness) <= 100;
+    };
+    if (!within(RangeAdjustment{hue, saturation, lightness}))
+        return false;
+    const HueSaturationSettings hsv = resolvedHSV();
+    for (const auto &[range, values] : hsv.adjustments) {
+        if (!within(values))
+            return false;
+    }
+    for (const auto &[range, band] : hsv.bands) {
+        for (const double handle : band.handles()) {
+            if (!std::isfinite(handle))
+                return false;
+        }
+    }
+    for (const LevelRange &range : levels.ranges) {
+        if (!(range == range.normalized()))
+            return false;
+    }
+    return curves.isValid() && exposure().isValid() && gradientMap().isValid() && grain().isValid();
+}
+
+QImage LayerAdjustment::apply(const QImage &image, std::optional<QRectF> region) const
+{
+    switch (kind) {
+    case AdjustmentKind::hsv:
+        return HueSaturationFilter::run(HueSaturationJob{image, resolvedHSV(), std::nullopt, QTransform(), false}).image;
+    case AdjustmentKind::levels: return LevelsFilter::run(LevelsJob{image, levels, std::nullopt, QTransform()});
+    case AdjustmentKind::curves: return curves.apply(image);
+    case AdjustmentKind::exposure: return exposure().apply(image);
+    case AdjustmentKind::gradientMap: return gradientMap().apply(image);
+    case AdjustmentKind::grain: {
+        // Grain sits in the document, however the image is cut.
+        const QRectF area = region.value_or(QRectF(0, 0, image.width(), image.height()));
+        return grain().apply(image, area.topLeft(), area.width() / std::max(1, image.width()));
+    }
+    }
+    throw std::logic_error("unknown adjustment kind");
+}
+
+void EditorSession::addAdjustment(AdjustmentKind kind)
+{
+    if (!canEditLayers() || m_document->layers.size() >= 10'000)
+        return;
+    ImageLayer layer(rawValue(kind), m_document->size());
+    LayerAdjustment adjustment{kind};
+    // A Gradient Map runs from foreground to background, as Photoshop's.
+    if (kind == AdjustmentKind::gradientMap)
+        adjustment.setGradientMap(GradientMapSettings{AdjustmentColor(foregroundColor()), AdjustmentColor(m_backgroundColor)});
+    // Each Grain layer rolls a pattern of its own.
+    if (kind == AdjustmentKind::grain) {
+        GrainSettings grain = adjustment.grain();
+        grain.seed = QRandomGenerator::global()->generate();
+        adjustment.setGrain(grain);
+    }
+    layer.adjustment = adjustment;
+    const std::optional<ImageLayer> active = activeLayer();
+    layer.parentID = active && active->isGroup ? m_activeLayerID : active ? active->parentID : std::nullopt;
+    const int activeIndex = indexOf(m_document->layers, m_activeLayerID);
+    const int index = activeIndex >= 0 ? activeIndex + 1 : int(m_document->layers.size());
+    beginEdit(QStringLiteral("New %1 Adjustment").arg(rawValue(kind)));
+    m_document->layers.insert(m_document->layers.begin() + index, layer);
+    if (layer.parentID)
+        m_collapsedGroupIDs.remove(*layer.parentID);
+    setActiveLayerID(layer.id);
+    endEdit();
+    setAdjustmentEditingID(layer.id);
+}
+
+void EditorSession::updateAdjustment(QUuid id, const LayerAdjustment &value)
+{
+    const int index = m_document ? indexOf(m_document->layers, id) : -1;
+    if (index < 0 || !value.isValid())
+        return;
+    m_document->layers[size_t(index)].adjustment = value;
+    ++m_brushRevision;
+    notify();
+}

src/Document/LayerAdjustment.h

@@ -0,0 +1,50 @@
+#pragma once
+#include "Document/Curves.h"
+#include "Document/Filters.h"
+#include "Document/HueSaturation.h"
+#include "Document/ImageAdjustments.h"
+#include "Document/Levels.h"
+#include <QRectF>
+#include <QString>
+#include <array>
+#include <optional>
+
+// An adjustment layer's kind, in the Layer menu's order.
+enum class AdjustmentKind { hsv, levels, curves, exposure, gradientMap, grain };
+inline constexpr std::array allAdjustmentKinds{AdjustmentKind::hsv,      AdjustmentKind::levels,      AdjustmentKind::curves,
+                                               AdjustmentKind::exposure, AdjustmentKind::gradientMap, AdjustmentKind::grain};
+// Swift's rawValue: the new layer's name and the manifest's word.
+QString rawValue(AdjustmentKind kind);
+// The kind a manifest names, or none.
+std::optional<AdjustmentKind> adjustmentKind(const QString &text);
+// The filter panel that edits it; Levels and Hue/Saturation: none.
+std::optional<FilterKind> filterKind(AdjustmentKind kind);
+
+// An adjustment layer's settings; its kind reads only its own.
+struct LayerAdjustment {
+    AdjustmentKind kind;
+    double hue = 0;
+    double saturation = 0;
+    double lightness = 0;
+    bool colorize = false;
+    // Absent in projects from before range-aware Hue/Saturation.
+    std::optional<HueSaturationSettings> hsvSettings = std::nullopt;
+    LevelsSettings levels{};
+    CurvesSettings curves{};
+    // Absent unless set, so older kinds save exactly as before.
+    std::optional<ExposureSettings> exposureSettings = std::nullopt;
+    std::optional<GradientMapSettings> gradientMapSettings = std::nullopt;
+    std::optional<GrainSettings> grainSettings = std::nullopt;
+    HueSaturationSettings resolvedHSV() const;
+    // Swift's computed properties over the optional settings.
+    ExposureSettings exposure() const;
+    void setExposure(const ExposureSettings &value);
+    GradientMapSettings gradientMap() const;
+    void setGradientMap(const GradientMapSettings &value);
+    GrainSettings grain() const;
+    void setGrain(const GrainSettings &value);
+    bool isValid() const;
+    // `region`: the document `image` covers, whole pixels by default.
+    QImage apply(const QImage &image, std::optional<QRectF> region = std::nullopt) const;
+    friend bool operator==(const LayerAdjustment &, const LayerAdjustment &) = default;
+};

src/Document/LayerAppearance+Session.h

@@ -0,0 +1,12 @@
+// Swift's LayerAppearance extension: opacity and blend modes.
+public:
+    LayerBlendMode displayedBlendMode(const ImageLayer &layer) const;
+    void previewBlendMode(std::optional<LayerBlendMode> mode, std::optional<QUuid> id);
+    bool canEditAppearance() const;
+    // A slider drag: many values, one undo step.
+    void beginOpacityEdit();
+    void finishOpacityEdit();
+    void setLayerOpacity(double opacity);
+    void setSelectedLayersOpacity(double opacity);
+    void cycleBlendMode(bool forward);
+    void setLayerBlendMode(LayerBlendMode mode);

src/Document/LayerAppearance.cpp

@@ -0,0 +1,130 @@
+#include "Document/LayerAppearance.h"
+#include "Document/EditorSession.h"
+#include <algorithm>
+#include <cmath>
+#include <array>
+#include <utility>
+
+namespace {
+const std::array<std::pair<LayerBlendMode, const char *>, 13> names{{
+    {LayerBlendMode::normal, "Normal"}, {LayerBlendMode::multiply, "Multiply"}, {LayerBlendMode::screen, "Screen"},
+    {LayerBlendMode::overlay, "Overlay"}, {LayerBlendMode::darken, "Darken"}, {LayerBlendMode::lighten, "Lighten"},
+    {LayerBlendMode::difference, "Difference"}, {LayerBlendMode::colorDodge, "Color Dodge"},
+    {LayerBlendMode::colorBurn, "Color Burn"}, {LayerBlendMode::hue, "Hue"}, {LayerBlendMode::saturation, "Saturation"},
+    {LayerBlendMode::color, "Color"}, {LayerBlendMode::luminosity, "Luminosity"},
+}};
+}
+
+QString rawValue(LayerBlendMode mode)
+{
+    return QString::fromLatin1(names.at(size_t(mode)).second);
+}
+
+std::optional<LayerBlendMode> layerBlendMode(const QString &rawValue)
+{
+    for (const auto &[mode, name] : names) {
+        if (rawValue == QLatin1String(name))
+            return mode;
+    }
+    return std::nullopt;
+}
+
+LayerBlendMode EditorSession::displayedBlendMode(const ImageLayer &layer) const
+{
+    if (m_blendPreview && m_blendPreview->layerID == layer.id && m_activeLayerID == layer.id)
+        return m_blendPreview->mode;
+    return layer.blendMode;
+}
+
+void EditorSession::previewBlendMode(std::optional<LayerBlendMode> mode, std::optional<QUuid> id)
+{
+    std::optional<BlendPreview> preview;
+    if (mode && id && id == m_activeLayerID && canEditAppearance())
+        preview = BlendPreview{*id, *mode};
+    // Hovering over the same entry again says nothing.
+    if (preview == m_blendPreview)
+        return;
+    m_blendPreview = preview;
+    notify();
+}
+
+bool EditorSession::canEditAppearance() const
+{
+    const std::optional<ImageLayer> active = activeLayer();
+    return canEditLayers() && m_selectedLayerIDs.size() == 1 && active && !active->isGroup;
+}
+
+void EditorSession::beginOpacityEdit()
+{
+    if (!canEditAppearance() || m_opacityEditLayerID)
+        return;
+    beginEdit(QStringLiteral("Layer Opacity"));
+    m_opacityEditLayerID = m_activeLayerID;
+}
+
+void EditorSession::finishOpacityEdit()
+{
+    if (!m_opacityEditLayerID)
+        return;
+    m_opacityEditLayerID = std::nullopt;
+    endEdit();
+}
+
+void EditorSession::setLayerOpacity(double opacity)
+{
+    if (!std::isfinite(opacity) || !canEditAppearance())
+        return;
+    const QUuid id = m_opacityEditLayerID.value_or(*m_activeLayerID);
+    const auto layer = std::find_if(m_document->layers.begin(), m_document->layers.end(), [&](const ImageLayer &each) { return each.id == id; });
+    if (layer == m_document->layers.end())
+        return;
+    // Inside a drag this nests in the drag's one step.
+    beginEdit(QStringLiteral("Layer Opacity"));
+    layer->opacity = std::clamp(opacity, 0.0, 1.0);
+    endEdit();
+}
+
+void EditorSession::setSelectedLayersOpacity(double opacity)
+{
+    if (!std::isfinite(opacity) || !canEditLayers())
+        return;
+    // Folders have no opacity of their own.
+    const double value = std::clamp(opacity, 0.0, 1.0);
+    const auto changes = [&](const ImageLayer &layer) { return m_selectedLayerIDs.contains(layer.id) && !layer.isGroup && layer.opacity != value; };
+    if (std::none_of(m_document->layers.begin(), m_document->layers.end(), changes))
+        return;
+    finishOpacityEdit();
+    beginEdit(QStringLiteral("Layer Opacity"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (changes(layer))
+            layer.opacity = value;
+    }
+    endEdit();
+}
+
+void EditorSession::cycleBlendMode(bool forward)
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    if (!canEditAppearance() || !layer)
+        return;
+    const auto found = std::find(allLayerBlendModes.begin(), allLayerBlendModes.end(), layer->blendMode);
+    const size_t count = allLayerBlendModes.size(), index = size_t(found - allLayerBlendModes.begin());
+    setLayerBlendMode(allLayerBlendModes[(index + (forward ? 1 : count - 1)) % count]);
+}
+
+void EditorSession::setLayerBlendMode(LayerBlendMode mode)
+{
+    if (m_blendPreview) {
+        m_blendPreview = std::nullopt;
+        notify();
+    }
+    if (!canEditAppearance())
+        return;
+    finishOpacityEdit();
+    beginEdit(QStringLiteral("Layer Blend Mode"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (layer.id == m_activeLayerID)
+            layer.blendMode = mode;
+    }
+    endEdit();
+}

src/Document/LayerAppearance.h

@@ -0,0 +1,20 @@
+#pragma once
+#include <QString>
+#include <array>
+#include <optional>
+
+enum class LayerBlendMode {
+    normal, multiply, screen, overlay, darken, lighten, difference,
+    colorDodge, colorBurn, hue, saturation, color, luminosity
+};
+
+// Swift's `allCases`: the order of the blend menu.
+inline constexpr std::array allLayerBlendModes{
+    LayerBlendMode::normal, LayerBlendMode::multiply, LayerBlendMode::screen, LayerBlendMode::overlay,
+    LayerBlendMode::darken, LayerBlendMode::lighten, LayerBlendMode::difference, LayerBlendMode::colorDodge,
+    LayerBlendMode::colorBurn, LayerBlendMode::hue, LayerBlendMode::saturation, LayerBlendMode::color,
+    LayerBlendMode::luminosity};
+
+// The name users see and the manifest stores.
+QString rawValue(LayerBlendMode mode);
+std::optional<LayerBlendMode> layerBlendMode(const QString &rawValue);

src/Document/LayerEffects+Renderer.cpp

@@ -0,0 +1,126 @@
+#include "Document/LayerEffects+Renderer.h"
+#include "Document/BrushStroke.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/LayerEffectsKernel.h"
+#include <QMutex>
+#include <QPainter>
+#include <cmath>
+#include <vector>
+
+// Swift's LayerEffectsRenderer, on the kernel alone.
+namespace {
+struct Entry {
+    QImage image;
+    std::optional<QImage> mask;
+    LayerEffects effects;
+    LayerEffectsRenderer::Rendered result;
+};
+
+qint64 bytes(const Entry &entry)
+{
+    return entry.result.image.sizeInBytes() + entry.image.sizeInBytes() + (entry.mask ? entry.mask->sizeInBytes() : 0);
+}
+
+// Swift's `===`: the same pixels, not equal ones.
+bool same(const std::optional<QImage> &lhs, const std::optional<QImage> &rhs)
+{
+    return lhs.has_value() == rhs.has_value() && (!lhs || lhs->cacheKey() == rhs->cacheKey());
+}
+
+QMutex lock;
+std::vector<Entry> entries;
+
+// The layer's pixels through its mask, as they show.
+QImage masked(const QImage &image, const std::optional<QImage> &mask)
+{
+    if (!mask)
+        return image;
+    const QRectF bounds(0, 0, image.width(), image.height());
+    QImage result = BrushRaster::context(image.width(), image.height(), false);
+    QPainter painter(&result);
+    BrushRaster::draw(image, bounds, painter);
+    // CoreGraphics scales a clip mask smoothly to its bounds.
+    painter.setRenderHint(QPainter::SmoothPixmapTransform);
+    painter.setCompositionMode(QPainter::CompositionMode_DestinationIn);
+    painter.drawImage(bounds, BrushRaster::alphaView(*mask));
+    return result;
+}
+}
+
+std::optional<LayerEffectsRenderer::Rendered> LayerEffectsRenderer::cached(const QImage &image, const std::optional<QImage> &mask,
+                                                                           const std::optional<LayerEffects> &effects)
+{
+    const LayerEffects shown = effects.value_or(LayerEffects()).visible();
+    if (shown.isEmpty() || !shown.isValid())
+        return std::nullopt;
+    {
+        const QMutexLocker locked(&lock);
+        for (const Entry &entry : entries) {
+            if (entry.image.cacheKey() == image.cacheKey() && same(entry.mask, mask) && entry.effects == shown)
+                return entry.result;
+        }
+    }
+    try {
+        const Entry made{image, mask, shown, render(image, mask, shown)};
+        // About 64 MiB of results and sources, eight at most.
+        constexpr qint64 budget = 64 * 1024 * 1024;
+        const QMutexLocker locked(&lock);
+        if (bytes(made) <= budget) {
+            entries.push_back(made);
+            qint64 total = 0;
+            for (const Entry &entry : entries)
+                total += bytes(entry);
+            while (entries.size() > 8 || total > budget) {
+                total -= bytes(entries.front());
+                entries.erase(entries.begin());
+            }
+        }
+        return made.result;
+    } catch (const ProjectError &error) {
+        qCWarning(lcRendering) << "a layer's effects could not be made:" << error.what();
+    } catch (const ExportError &error) {
+        qCWarning(lcRendering) << "a layer's effects could not be made:" << error.what();
+    }
+    return std::nullopt;
+}
+
+LayerTransform LayerEffectsRenderer::placed(const LayerTransform &transform, const QImage &image, double inset)
+{
+    const double width = image.width(), height = image.height();
+    if (!(width > inset * 2) || !(height > inset * 2))
+        return transform;
+    LayerTransform grown = transform;
+    grown.size = QSizeF(transform.size.width() * width / (width - inset * 2), transform.size.height() * height / (height - inset * 2));
+    grown.origin = transform.center() - QPointF(grown.size.width() / 2, grown.size.height() / 2);
+    return grown;
+}
+
+double LayerEffectsRenderer::margin(const LayerEffects &all)
+{
+    const LayerEffects effects = all.visible();
+    double margin = 0;
+    if (effects.stroke && !effects.stroke->inside)
+        margin = std::max(margin, effects.stroke->size);
+    if (effects.shadow)
+        margin = std::max(margin, effects.shadow->distance + effects.shadow->blur * 3);
+    return std::ceil(margin) + 2;
+}
+
+LayerEffectsRenderer::Rendered LayerEffectsRenderer::render(const QImage &image, const std::optional<QImage> &mask, const LayerEffects &all)
+{
+    const LayerEffects effects = all.visible();
+    if (!effects.isValid())
+        throw ProjectError(ProjectError::Kind::invalid);
+    const double inset = margin(effects);
+    const qint64 width = image.width() + qint64(inset) * 2, height = image.height() + qint64(inset) * 2;
+    if (width <= 0 || height <= 0 || width * height > 100'000'000)
+        throw ProjectError(ProjectError::Kind::tooLarge);
+    // The pixels with room round them, then the effects.
+    QImage padded = BrushRaster::context(int(width), int(height), false);
+    {
+        QPainter painter(&padded);
+        BrushRaster::draw(masked(image, mask), QRectF(inset, inset, image.width(), image.height()), painter);
+    }
+    return {LayerEffectsKernel::render(padded, effects), inset};
+}

src/Document/LayerEffects+Renderer.h

@@ -0,0 +1,21 @@
+#pragma once
+#include "Document/LayerEffects.h"
+#include "Document/LayerTransform.h"
+#include <QImage>
+#include <optional>
+
+// Swift's LayerEffectsRenderer: a layer with its effects round it.
+namespace LayerEffectsRenderer {
+// Swift's tuple: the grown image, the room round the pixels.
+struct Rendered {
+    QImage image;
+    double inset;
+};
+// Reused for the same pixels, mask and effects; eight kept.
+std::optional<Rendered> cached(const QImage &image, const std::optional<QImage> &mask, const std::optional<LayerEffects> &effects);
+// The transform grown by the margin, the image in place.
+LayerTransform placed(const LayerTransform &transform, const QImage &image, double inset);
+double margin(const LayerEffects &effects);
+// `mask` hides pixels first, so effects follow what shows.
+Rendered render(const QImage &image, const std::optional<QImage> &mask, const LayerEffects &effects);
+}

src/Document/LayerEffects+Session.cpp

@@ -0,0 +1,217 @@
+#include "Document/EditorSession.h"
+
+// Swift's session extension in LayerEffects.swift.
+namespace {
+// Swift's switch: one kind's effect, present or not.
+void take(LayerEffects &into, const LayerEffects &from, LayerEffectKind kind)
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: into.stroke = from.stroke; return;
+    case LayerEffectKind::shadow: into.shadow = from.shadow; return;
+    case LayerEffectKind::colorOverlay: into.colorOverlay = from.colorOverlay; return;
+    case LayerEffectKind::innerShadow: into.innerShadow = from.innerShadow; return;
+    }
+    throw std::logic_error("unknown effect kind");
+}
+}
+
+// Swift's isGroup terms are dead: folders never hold pixels.
+bool EditorSession::canEditEffects() const
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    return canEditLayers() && layer && layer->asset;
+}
+
+LayerEffects EditorSession::activeEffects() const
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    return layer && layer->effects ? *layer->effects : LayerEffects();
+}
+
+LayerEffects EditorSession::editingEffects() const
+{
+    const int index = m_document && m_effectsEditing ? indexOf(m_document->layers, m_effectsEditing->layerID) : -1;
+    return index >= 0 ? m_document->layers[size_t(index)].effects.value_or(LayerEffects()) : LayerEffects();
+}
+
+std::optional<LayerEffectSelection> EditorSession::selectedEffect() const
+{
+    if (!m_effectSelection || m_effectSelection->layerID != m_activeLayerID || !activeEffects().contains(m_effectSelection->kind))
+        return std::nullopt;
+    return m_effectSelection;
+}
+
+void EditorSession::addEffect(LayerEffectKind kind)
+{
+    if (!canEditEffects())
+        return;
+    const QUuid id = m_activeLayerID.value();
+    if (m_effectsEditing == LayerEffectSelection{id, kind})
+        return;
+    finishEffectsEditing(false);
+    const LayerEffects original = activeEffects();
+    LayerEffects effects = original;
+    // A new stroke or overlay takes the background colour.
+    const PaletteColor ground = m_backgroundColor;
+    switch (kind) {
+    case LayerEffectKind::stroke:
+        if (!effects.stroke)
+            effects.stroke = StrokeEffect{.red = ground.red, .green = ground.green, .blue = ground.blue};
+        break;
+    case LayerEffectKind::shadow:
+        if (!effects.shadow)
+            effects.shadow = ShadowEffect();
+        break;
+    case LayerEffectKind::colorOverlay:
+        if (!effects.colorOverlay)
+            effects.colorOverlay = ColorOverlayEffect{.red = ground.red, .green = ground.green, .blue = ground.blue};
+        break;
+    case LayerEffectKind::innerShadow:
+        if (!effects.innerShadow)
+            effects.innerShadow = InnerShadowEffect();
+        break;
+    }
+    setEffects(effects, id, QStringLiteral("Add ") + rawValue(kind));
+    selectEffect(kind, id, true);
+    m_effectsEditingOriginal = original;
+    notify();
+}
+
+void EditorSession::selectEffect(LayerEffectKind kind, QUuid id, bool editing)
+{
+    const int index = m_document ? indexOf(m_document->layers, id) : -1;
+    if (!canEditLayers() || index < 0 || !m_document->layers[size_t(index)].effects || !m_document->layers[size_t(index)].effects->contains(kind))
+        return;
+    const LayerEffectSelection selection{id, kind};
+    if (editing && m_effectsEditing != selection)
+        finishEffectsEditing(false);
+    // Swift's `selectedLayerIDs = [id]`: selectLayer sets it, unrefused here.
+    selectLayer(id);
+    m_isMaskSelected = false;
+    m_effectSelection = selection;
+    if (editing && m_effectsEditing != selection) {
+        m_effectsEditingOriginal = m_document->layers[size_t(indexOf(m_document->layers, id))].effects.value_or(LayerEffects());
+        m_effectsEditing = selection;
+    }
+    notify();
+}
+
+void EditorSession::finishEffectsEditing(bool commit)
+{
+    if (!m_effectsEditing)
+        return;
+    const LayerEffectSelection editing = *m_effectsEditing;
+    closeEffectColorPicker(commit);
+    const int index = m_document ? indexOf(m_document->layers, editing.layerID) : -1;
+    if (!commit && m_effectsEditingOriginal && index >= 0 && m_document->layers[size_t(index)].effects) {
+        LayerEffects effects = *m_document->layers[size_t(index)].effects;
+        take(effects, *m_effectsEditingOriginal, editing.kind);
+        setEffects(effects, editing.layerID, QStringLiteral("Cancel ") + rawValue(editing.kind));
+    }
+    m_effectsEditing.reset();
+    m_effectsEditingOriginal.reset();
+    if (!selectedEffect())
+        m_effectSelection.reset();
+    notify();
+}
+
+void EditorSession::setEffects(const LayerEffects &effects, std::optional<QUuid> id, const QString &name)
+{
+    const int index = m_document ? indexOf(m_document->layers, id ? id : m_activeLayerID) : -1;
+    if (!canEditLayers() || !effects.isValid() || index < 0)
+        return;
+    const ImageLayer &layer = m_document->layers[size_t(index)];
+    const std::optional<LayerEffects> stored = effects.isEmpty() ? std::nullopt : std::optional(effects);
+    if (!layer.asset || layer.effects == stored)
+        return;
+    finishOpacityEdit();
+    beginEdit(name);
+    m_document->layers[size_t(index)].effects = stored;
+    endEdit();
+}
+
+void EditorSession::changeEffects(const std::function<void(LayerEffects &)> &change)
+{
+    const int index = m_document && m_effectsEditing ? indexOf(m_document->layers, m_effectsEditing->layerID) : -1;
+    if (index < 0 || !m_document->layers[size_t(index)].effects || !m_document->layers[size_t(index)].effects->contains(m_effectsEditing->kind))
+        return;
+    LayerEffects effects = *m_document->layers[size_t(index)].effects;
+    change(effects);
+    setEffects(effects, m_effectsEditing->layerID, QStringLiteral("Edit ") + rawValue(m_effectsEditing->kind));
+}
+
+bool EditorSession::canCopyEffect(LayerEffectKind kind, QUuid source, QUuid target) const
+{
+    const int from = m_document ? indexOf(m_document->layers, source) : -1, to = m_document ? indexOf(m_document->layers, target) : -1;
+    if (!canEditLayers() || source == target || from < 0 || to < 0)
+        return false;
+    const ImageLayer &sourceLayer = m_document->layers[size_t(from)], &targetLayer = m_document->layers[size_t(to)];
+    return sourceLayer.effects && sourceLayer.effects->contains(kind) && targetLayer.asset;
+}
+
+void EditorSession::copyEffect(LayerEffectKind kind, QUuid source, QUuid target)
+{
+    if (!canCopyEffect(kind, source, target))
+        return;
+    const LayerEffects original = m_document->layers[size_t(indexOf(m_document->layers, source))].effects.value();
+    // Its editor closes first: a later Cancel keeps the copy.
+    if (m_effectsEditing == LayerEffectSelection{target, kind})
+        finishEffectsEditing(true);
+    LayerEffects effects = m_document->layers[size_t(indexOf(m_document->layers, target))].effects.value_or(LayerEffects());
+    take(effects, original, kind);
+    setEffects(effects, target, QStringLiteral("Copy ") + rawValue(kind));
+    selectEffect(kind, target);
+}
+
+void EditorSession::toggleEffect(LayerEffectKind kind, QUuid id)
+{
+    const int index = m_document ? indexOf(m_document->layers, id) : -1;
+    if (index < 0 || !m_document->layers[size_t(index)].effects)
+        return;
+    LayerEffects effects = *m_document->layers[size_t(index)].effects;
+    const bool enabled = effects.isEnabled(kind);
+    effects.setEnabled(!enabled, kind);
+    setEffects(effects, id, (enabled ? QStringLiteral("Hide ") : QStringLiteral("Show ")) + rawValue(kind));
+}
+
+void EditorSession::removeSelectedEffect()
+{
+    const std::optional<LayerEffectSelection> selected = selectedEffect();
+    const int index = m_document && selected ? indexOf(m_document->layers, selected->layerID) : -1;
+    if (!selected || !canEditLayers() || index < 0 || !m_document->layers[size_t(index)].effects)
+        return;
+    LayerEffects effects = *m_document->layers[size_t(index)].effects;
+    if (m_effectsEditing == selected) {
+        closeEffectColorPicker(false);
+        m_effectsEditing.reset();
+        m_effectsEditingOriginal.reset();
+    }
+    effects.remove(selected->kind);
+    setEffects(effects, selected->layerID, QStringLiteral("Remove ") + rawValue(selected->kind));
+    m_effectSelection.reset();
+    notify();
+}
+
+void EditorSession::setEffectsEditing(std::optional<LayerEffectSelection> selection)
+{
+    if (std::exchange(m_effectsEditing, selection) != selection)
+        notify();
+}
+
+void EditorSession::setEffectsEditingOriginal(std::optional<LayerEffects> original)
+{
+    if (std::exchange(m_effectsEditingOriginal, original) != original)
+        notify();
+}
+
+void EditorSession::dropEffectSelection()
+{
+    if (std::exchange(m_effectSelection, std::nullopt))
+        notify();
+}
+
+void EditorSession::closeEffectColorPicker(bool commit)
+{
+    if (m_colorPicker && m_colorPicker->target.kind == ColorPickerTarget::Kind::effect)
+        closeColorPicker(commit);
+}

src/Document/LayerEffects+Session.h

@@ -0,0 +1,27 @@
+// Swift's LayerEffects extension: effects kept with their layer.
+public:
+    bool canEditEffects() const;
+    LayerEffects activeEffects() const;
+    // The effects of the layer whose panel is open.
+    LayerEffects editingEffects() const;
+    // The chosen effect, if active and still on its layer.
+    std::optional<LayerEffectSelection> selectedEffect() const;
+    // Adds the effect with its defaults, then opens its panel.
+    void addEffect(LayerEffectKind kind);
+    void selectEffect(LayerEffectKind kind, QUuid id, bool editing = false);
+    // Cancel puts back this panel's effect alone.
+    void finishEffectsEditing(bool commit);
+    // One step, on the active layer unless another is named.
+    void setEffects(const LayerEffects &effects, std::optional<QUuid> id = std::nullopt, const QString &name = QStringLiteral("Layer Effects"));
+    // Panel edits stay with the layer that opened the panel.
+    void changeEffects(const std::function<void(LayerEffects &)> &change);
+    bool canCopyEffect(LayerEffectKind kind, QUuid source, QUuid target) const;
+    void copyEffect(LayerEffectKind kind, QUuid source, QUuid target);
+    void toggleEffect(LayerEffectKind kind, QUuid id);
+    void removeSelectedEffect();
+    // Swift's `effectSelection = nil`, before any guard; announced.
+    void dropEffectSelection();
+
+private:
+    // An open effect colour picker goes with its panel.
+    void closeEffectColorPicker(bool commit);

src/Document/LayerEffects.cpp

@@ -0,0 +1,178 @@
+#include "Document/LayerEffects.h"
+#include <cmath>
+#include <stdexcept>
+
+namespace {
+bool within(double value, double low, double high)
+{
+    return value >= low && value <= high;
+}
+
+bool colourIsValid(double red, double green, double blue)
+{
+    return within(red, 0, 1) && within(green, 0, 1) && within(blue, 0, 1);
+}
+
+// Away from the light; a layer's pixels count y downward.
+QSizeF fallingFrom(double angle, double distance)
+{
+    const double radians = angle * M_PI / 180;
+    return QSizeF(-std::cos(radians) * distance, std::sin(radians) * distance);
+}
+
+bool shadowIsValid(double angle, double distance, double blur, double opacity)
+{
+    return within(angle, -360, 360) && within(distance, 0, 5000) && within(blur, 0, 500) && within(opacity, 0, 1);
+}
+}
+
+bool StrokeEffect::isValid() const
+{
+    return within(size, 0, maxSize) && within(opacity, 0, 1) && colourIsValid(red, green, blue);
+}
+
+QSizeF ShadowEffect::offset() const
+{
+    return fallingFrom(angle, distance);
+}
+
+bool ShadowEffect::isValid() const
+{
+    return shadowIsValid(angle, distance, blur, opacity) && colourIsValid(red, green, blue);
+}
+
+bool ColorOverlayEffect::isValid() const
+{
+    return within(opacity, 0, 1) && colourIsValid(red, green, blue);
+}
+
+QSizeF InnerShadowEffect::offset() const
+{
+    return fallingFrom(angle, distance);
+}
+
+bool InnerShadowEffect::isValid() const
+{
+    return shadowIsValid(angle, distance, blur, opacity) && colourIsValid(red, green, blue);
+}
+
+QString rawValue(LayerEffectKind kind)
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: return QStringLiteral("Stroke");
+    case LayerEffectKind::shadow: return QStringLiteral("Drop Shadow");
+    case LayerEffectKind::colorOverlay: return QStringLiteral("Color Overlay");
+    case LayerEffectKind::innerShadow: return QStringLiteral("Inner Shadow");
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+bool LayerEffects::isValid() const
+{
+    return (!stroke || stroke->isValid()) && (!shadow || shadow->isValid()) && (!colorOverlay || colorOverlay->isValid())
+        && (!innerShadow || innerShadow->isValid());
+}
+
+std::vector<LayerEffectKind> LayerEffects::kinds() const
+{
+    std::vector<LayerEffectKind> result;
+    for (const LayerEffectKind kind : allLayerEffectKinds) {
+        if (contains(kind))
+            result.push_back(kind);
+    }
+    return result;
+}
+
+bool LayerEffects::contains(LayerEffectKind kind) const
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: return stroke.has_value();
+    case LayerEffectKind::shadow: return shadow.has_value();
+    case LayerEffectKind::colorOverlay: return colorOverlay.has_value();
+    case LayerEffectKind::innerShadow: return innerShadow.has_value();
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+bool LayerEffects::isEnabled(LayerEffectKind kind) const
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: return stroke && stroke->isEnabled();
+    case LayerEffectKind::shadow: return shadow && shadow->isEnabled();
+    case LayerEffectKind::colorOverlay: return colorOverlay && colorOverlay->isEnabled();
+    case LayerEffectKind::innerShadow: return innerShadow && innerShadow->isEnabled();
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+std::optional<PaletteColor> LayerEffects::color(LayerEffectKind kind) const
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: return stroke ? std::optional(stroke->color()) : std::nullopt;
+    case LayerEffectKind::shadow: return shadow ? std::optional(shadow->color()) : std::nullopt;
+    case LayerEffectKind::colorOverlay: return colorOverlay ? std::optional(colorOverlay->color()) : std::nullopt;
+    case LayerEffectKind::innerShadow: return innerShadow ? std::optional(innerShadow->color()) : std::nullopt;
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+namespace {
+// Swift's optional chaining: a missing effect takes nothing.
+template <typename Effect> void paint(std::optional<Effect> &effect, const PaletteColor &color)
+{
+    if (!effect)
+        return;
+    effect->red = color.red;
+    effect->green = color.green;
+    effect->blue = color.blue;
+}
+
+template <typename Effect> void enable(std::optional<Effect> &effect, bool enabled)
+{
+    if (effect)
+        effect->enabled = enabled;
+}
+
+template <typename Effect> std::optional<Effect> shown(const std::optional<Effect> &effect)
+{
+    return effect && effect->isEnabled() ? effect : std::nullopt;
+}
+}
+
+void LayerEffects::setColor(const PaletteColor &color, LayerEffectKind kind)
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: paint(stroke, color); return;
+    case LayerEffectKind::shadow: paint(shadow, color); return;
+    case LayerEffectKind::colorOverlay: paint(colorOverlay, color); return;
+    case LayerEffectKind::innerShadow: paint(innerShadow, color); return;
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+void LayerEffects::remove(LayerEffectKind kind)
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: stroke.reset(); return;
+    case LayerEffectKind::shadow: shadow.reset(); return;
+    case LayerEffectKind::colorOverlay: colorOverlay.reset(); return;
+    case LayerEffectKind::innerShadow: innerShadow.reset(); return;
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+void LayerEffects::setEnabled(bool enabled, LayerEffectKind kind)
+{
+    switch (kind) {
+    case LayerEffectKind::stroke: enable(stroke, enabled); return;
+    case LayerEffectKind::shadow: enable(shadow, enabled); return;
+    case LayerEffectKind::colorOverlay: enable(colorOverlay, enabled); return;
+    case LayerEffectKind::innerShadow: enable(innerShadow, enabled); return;
+    }
+    throw std::logic_error("unknown effect kind");
+}
+
+LayerEffects LayerEffects::visible() const
+{
+    return LayerEffects{shown(stroke), shown(shadow), shown(colorOverlay), shown(innerShadow)};
+}

src/Document/LayerEffects.h

@@ -0,0 +1,108 @@
+#pragma once
+#include "Document/ColorPalette.h"
+#include <QSizeF>
+#include <QString>
+#include <QUuid>
+#include <array>
+#include <optional>
+#include <vector>
+
+// A line round what the layer shows, outside or in.
+struct StrokeEffect {
+    // Swift's supported width in document pixels.
+    static constexpr double maxSize = 500;
+    // Absent in older projects, which means shown.
+    std::optional<bool> enabled = std::nullopt;
+    double size = 4;
+    double red = 0;
+    double green = 0;
+    double blue = 0;
+    double opacity = 1;
+    bool inside = false;
+    bool isEnabled() const { return enabled.value_or(true); }
+    PaletteColor color() const { return {red, green, blue}; }
+    bool isValid() const;
+    friend bool operator==(const StrokeEffect &, const StrokeEffect &) = default;
+};
+
+// The layer's shape behind it, offset and softened.
+struct ShadowEffect {
+    std::optional<bool> enabled = std::nullopt;
+    // Where the light comes from, counterclockwise from the right.
+    double angle = 90;
+    double distance = 20;
+    double blur = 20;
+    double red = 0;
+    double green = 0;
+    double blue = 0;
+    double opacity = 0.5;
+    bool isEnabled() const { return enabled.value_or(true); }
+    PaletteColor color() const { return {red, green, blue}; }
+    // In layer pixels, y down: away from the light.
+    QSizeF offset() const;
+    bool isValid() const;
+    friend bool operator==(const ShadowEffect &, const ShadowEffect &) = default;
+};
+
+// A flat colour over everything the layer shows.
+struct ColorOverlayEffect {
+    std::optional<bool> enabled = std::nullopt;
+    double red = 0;
+    double green = 0;
+    double blue = 0;
+    double opacity = 1;
+    bool isEnabled() const { return enabled.value_or(true); }
+    PaletteColor color() const { return {red, green, blue}; }
+    bool isValid() const;
+    friend bool operator==(const ColorOverlayEffect &, const ColorOverlayEffect &) = default;
+};
+
+// A shadow inside the layer's own edges.
+struct InnerShadowEffect {
+    std::optional<bool> enabled = std::nullopt;
+    double angle = 90;
+    double distance = 10;
+    double blur = 10;
+    double red = 0;
+    double green = 0;
+    double blue = 0;
+    double opacity = 0.5;
+    bool isEnabled() const { return enabled.value_or(true); }
+    PaletteColor color() const { return {red, green, blue}; }
+    QSizeF offset() const;
+    bool isValid() const;
+    friend bool operator==(const InnerShadowEffect &, const InnerShadowEffect &) = default;
+};
+
+// Swift's raw values are the panel's and the menu's words.
+enum class LayerEffectKind { stroke, shadow, colorOverlay, innerShadow };
+inline constexpr std::array allLayerEffectKinds{LayerEffectKind::stroke, LayerEffectKind::shadow, LayerEffectKind::colorOverlay,
+                                                LayerEffectKind::innerShadow};
+QString rawValue(LayerEffectKind kind);
+
+// What a layer draws round itself; its pixels stay untouched.
+struct LayerEffects {
+    std::optional<StrokeEffect> stroke = std::nullopt;
+    std::optional<ShadowEffect> shadow = std::nullopt;
+    std::optional<ColorOverlayEffect> colorOverlay = std::nullopt;
+    std::optional<InnerShadowEffect> innerShadow = std::nullopt;
+    bool isEmpty() const { return !stroke && !shadow && !colorOverlay && !innerShadow; }
+    bool isValid() const;
+    std::vector<LayerEffectKind> kinds() const;
+    bool contains(LayerEffectKind kind) const;
+    bool isEnabled(LayerEffectKind kind) const;
+    std::optional<PaletteColor> color(LayerEffectKind kind) const;
+    void setColor(const PaletteColor &color, LayerEffectKind kind);
+    void remove(LayerEffectKind kind);
+    void setEnabled(bool enabled, LayerEffectKind kind);
+    // The effects that are switched on.
+    LayerEffects visible() const;
+    friend bool operator==(const LayerEffects &, const LayerEffects &) = default;
+};
+
+// One effect of one layer: the panel's and the list's.
+struct LayerEffectSelection {
+    QUuid layerID;
+    LayerEffectKind kind;
+    friend bool operator==(const LayerEffectSelection &, const LayerEffectSelection &) = default;
+};

src/Document/LayerFlip+Session.h

@@ -0,0 +1,5 @@
+// Swift's LayerFlip extension.
+public:
+    void flipLayers(bool horizontally);
+    // Every layer, placed mask and the selection across the middle.
+    void flipCanvas(bool horizontally);

src/Document/LayerFlip.cpp

@@ -0,0 +1,71 @@
+#include "Document/EditorSession.h"
+
+LayerTransform LayerTransform::mirrored(bool horizontally, double axis) const
+{
+    LayerTransform result = *this;
+    // The middle crosses the line; picture and angle turn over.
+    if (horizontally) {
+        result.flipX = !flipX;
+        result.origin.setX(2 * axis - center().x() - size.width() / 2);
+    } else {
+        result.flipY = !flipY;
+        result.origin.setY(2 * axis - center().y() - size.height() / 2);
+    }
+    result.rotation = -rotation;
+    return result;
+}
+
+// A layer about its middle; a group about its box.
+void EditorSession::flipLayers(bool horizontally)
+{
+    commitTransform();
+    if (!canTransform())
+        return;
+    std::vector<ImageLayer> members;
+    double axis = 0;
+    if (transformsAsGroup()) {
+        const LayerTransform box = groupTransformBox().value();
+        members = groupTransformMembers();
+        axis = horizontally ? box.center().x() : box.center().y();
+    } else {
+        const ImageLayer layer = activeLayer().value();
+        members = {layer};
+        axis = horizontally ? layer.transform.center().x() : layer.transform.center().y();
+    }
+    QSet<QUuid> ids;
+    for (const ImageLayer &member : members)
+        ids.insert(member.id);
+    beginEdit(horizontally ? QStringLiteral("Flip Horizontal") : QStringLiteral("Flip Vertical"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (!ids.contains(layer.id))
+            continue;
+        const LayerTransform flipped = layer.transform.mirrored(horizontally, axis);
+        // A linked mask flips along; an unlinked one stays.
+        if (layer.mask)
+            layer.mask->placement = layer.mask->placementMovingLayer(layer.transform, flipped);
+        layer.transform = flipped;
+    }
+    endEdit();
+}
+
+// One step; Swift's finishOpacityEdit ran in commitTransform already.
+void EditorSession::flipCanvas(bool horizontally)
+{
+    commitTransform();
+    cancelCrop();
+    if (!canEditLayers())
+        return;
+    const QSizeF size = m_document->size();
+    const double axis = horizontally ? size.width() / 2 : size.height() / 2;
+    beginEdit(horizontally ? QStringLiteral("Flip Canvas Horizontal") : QStringLiteral("Flip Canvas Vertical"));
+    for (ImageLayer &layer : m_document->layers) {
+        layer.transform = layer.transform.mirrored(horizontally, axis);
+        if (layer.mask && layer.mask->placement)
+            layer.mask->placement = layer.mask->placement->mirrored(horizontally, axis);
+    }
+    if (m_document->selection) {
+        const QTransform mirror = horizontally ? QTransform(-1, 0, 0, 1, size.width(), 0) : QTransform(1, 0, 0, -1, 0, size.height());
+        m_document->selection->path = mirror.map(m_document->selection->path);
+    }
+    endEdit();
+}

src/Document/LayerGroups+Session.h

@@ -0,0 +1,13 @@
+// Swift's LayerGroups extension: folders and the multi-selection.
+public:
+    QSet<QUuid> descendantIDs(QUuid id) const;
+    void addGroup();
+    void toggleGroupExpansion(QUuid id);
+    // The primary layer becomes active when the document holds it.
+    void selectLayers(const QSet<QUuid> &ids, std::optional<QUuid> primary);
+    void extendSelection(QUuid id);
+    void groupSelectedLayers();
+    std::vector<LayerHierarchy::Entry> layerRows() const;
+    bool canPlaceLayer(QUuid id, std::optional<QUuid> parent) const;
+    bool placeLayer(QUuid id, std::optional<QUuid> parent, std::optional<QUuid> above = std::nullopt, bool atBottom = false);
+    void moveActiveLayerOutOfGroup();

src/Document/LayerGroups.cpp

@@ -0,0 +1,405 @@
+#include "Document/LayerGroups.h"
+#include "Document/EditorSession.h"
+#include <QHash>
+#include <algorithm>
+#include <map>
+#include <stdexcept>
+
+namespace {
+using Children = std::map<std::optional<QUuid>, std::vector<const ProjectLayerRecord *>>;
+
+void visit(const Children &children, const std::optional<QUuid> &parent, int depth, bool visible, bool topFirst,
+           const QSet<QUuid> &collapsed, std::vector<LayerHierarchy::Entry> &result)
+{
+    if (depth > 64)
+        return;
+    const auto found = children.find(parent);
+    if (found == children.end())
+        return;
+    std::vector<const ProjectLayerRecord *> siblings = found->second;
+    if (topFirst)
+        std::reverse(siblings.begin(), siblings.end());
+    for (const ProjectLayerRecord *layer : siblings) {
+        const bool effective = visible && layer->isVisible;
+        result.push_back({*layer, depth, effective});
+        if (layer->isGroup == true && !collapsed.contains(layer->id))
+            visit(children, layer->id, depth + 1, effective, topFirst, collapsed, result);
+    }
+}
+
+// Swift's `try?`: a refused hierarchy changes nothing.
+bool isValidHierarchy(const std::vector<ImageLayer> &layers)
+{
+    std::vector<ProjectLayerRecord> records;
+    for (const ImageLayer &layer : layers)
+        records.push_back(layer.hierarchyRecord());
+    try {
+        LayerHierarchy::validate(records);
+    } catch (const ProjectError &) {
+        return false;
+    }
+    return true;
+}
+}
+
+std::vector<LayerHierarchy::Entry> LayerHierarchy::entries(const std::vector<ProjectLayerRecord> &layers,
+                                                           bool topFirst, const QSet<QUuid> &collapsed)
+{
+    Children children;
+    for (const ProjectLayerRecord &layer : layers)
+        children[layer.parentID].push_back(&layer);
+    std::vector<Entry> result;
+    visit(children, std::nullopt, 0, true, topFirst, collapsed, result);
+    return result;
+}
+
+std::vector<ProjectLayerRecord> LayerHierarchy::visibleLayers(const std::vector<ProjectLayerRecord> &layers)
+{
+    std::vector<ProjectLayerRecord> result;
+    for (const Entry &entry : entries(layers)) {
+        if (entry.visible && entry.layer.isGroup != true)
+            result.push_back(entry.layer);
+    }
+    return result;
+}
+
+void LayerHierarchy::validate(const std::vector<ProjectLayerRecord> &layers)
+{
+    QHash<QUuid, const ProjectLayerRecord *> byID;
+    for (const ProjectLayerRecord &layer : layers) {
+        if (byID.contains(layer.id) || (layer.isGroup == true && layer.imageFile))
+            throw ProjectError(ProjectError::Kind::invalid);
+        byID.insert(layer.id, &layer);
+    }
+    for (const ProjectLayerRecord &layer : layers) {
+        QSet<QUuid> seen{layer.id};
+        std::optional<QUuid> parent = layer.parentID;
+        while (parent) {
+            const ProjectLayerRecord *node = byID.value(*parent);
+            if (seen.size() > 64 || seen.contains(*parent) || !node || node->isGroup != true)
+                throw ProjectError(ProjectError::Kind::invalid);
+            seen.insert(*parent);
+            parent = node->parentID;
+        }
+        if (layer.isGroup == true && seen.size() > 64)
+            throw ProjectError(ProjectError::Kind::invalid);
+    }
+}
+
+ProjectLayerRecord ImageLayer::hierarchyRecord() const
+{
+    const QString file = id.toString(QUuid::WithoutBraces).toUpper();
+    const std::optional<LayerShape> live = liveShape();
+    const std::optional<LayerText> typed = liveText();
+    return {.id = id,
+            .name = name,
+            .isVisible = isVisible,
+            .transform = transform,
+            .imageFile = asset ? std::optional(file + ".png") : std::nullopt,
+            .parentID = parentID,
+            .isGroup = isGroup,
+            .opacity = opacity,
+            .blendMode = blendMode,
+            .maskFile = mask ? std::optional(file + ".mask.png") : std::nullopt,
+            .maskEnabled = mask ? std::optional(mask->isEnabled) : std::nullopt,
+            .maskSourceID = maskSourceID,
+            .adjustment = adjustment,
+            .maskPlacement = mask ? mask->placement : std::nullopt,
+            .maskLinked = mask ? std::optional(mask->isLinked) : std::nullopt,
+            .shape = live ? std::optional(live->style) : std::nullopt,
+            .effects = effects,
+            .text = typed ? std::optional(typed->style) : std::nullopt};
+}
+
+std::vector<LayerHierarchy::Entry> CanvasDocument::hierarchyEntries() const
+{
+    std::vector<ProjectLayerRecord> records;
+    for (const ImageLayer &layer : layers)
+        records.push_back(layer.hierarchyRecord());
+    return LayerHierarchy::entries(records);
+}
+
+QSet<QUuid> CanvasDocument::effectiveVisibleIDs() const
+{
+    QSet<QUuid> result;
+    for (const LayerHierarchy::Entry &entry : hierarchyEntries()) {
+        if (entry.visible)
+            result.insert(entry.layer.id);
+    }
+    return result;
+}
+
+std::vector<ImageLayer> CanvasDocument::renderLayers() const
+{
+    QHash<QUuid, const ImageLayer *> byID;
+    for (const ImageLayer &layer : layers) {
+        if (byID.contains(layer.id))
+            throw std::logic_error("two layers share an id");
+        byID.insert(layer.id, &layer);
+    }
+    std::vector<ImageLayer> result;
+    for (const LayerHierarchy::Entry &entry : hierarchyEntries()) {
+        if (entry.visible && entry.layer.isGroup != true)
+            result.push_back(*byID.value(entry.layer.id));
+    }
+    return result;
+}
+
+void EditorSession::selectLayers(const QSet<QUuid> &ids, std::optional<QUuid> primary)
+{
+    dropEffectSelection();
+    if (ids != m_selectedLayerIDs && !finishText())
+        return;
+    if (m_brushStroke)
+        return;
+    QSet<QUuid> valid;
+    if (m_document) {
+        for (const ImageLayer &layer : m_document->layers) {
+            if (ids.contains(layer.id))
+                valid.insert(layer.id);
+        }
+    }
+    if (valid != m_selectedLayerIDs) {
+        commitTransform();
+        resolveGradient();
+    }
+    // The primary layer is active; any other of them otherwise.
+    const std::optional<QUuid> active = primary && valid.contains(*primary) ? primary
+        : valid.isEmpty()                                                   ? std::nullopt
+                                                                            : std::optional(*valid.begin());
+    setActiveLayerID(active);
+    m_selectedLayerIDs = valid;
+    notify();
+}
+
+void EditorSession::extendSelection(QUuid id)
+{
+    const auto known = [&](const ImageLayer &layer) { return layer.id == id; };
+    // Cmd-Shift-click works while a transform is open too.
+    if ((!canEditLayers() && !m_transformEdit) || std::none_of(m_document->layers.begin(), m_document->layers.end(), known))
+        return;
+    QSet<QUuid> ids = m_selectedLayerIDs;
+    if (ids.contains(id) && ids.size() > 1) {
+        // Taking the active layer out hands its role on.
+        ids.remove(id);
+        selectLayers(ids, m_activeLayerID == id ? std::optional(*ids.begin()) : m_activeLayerID);
+    } else {
+        ids.insert(id);
+        selectLayers(ids, id);
+    }
+}
+
+void EditorSession::groupSelectedLayers()
+{
+    if (!canEditLayers() || m_document->layers.size() >= 10'000)
+        return;
+    const std::vector<ImageLayer> &existing = m_document->layers;
+    std::map<QUuid, const ImageLayer *> byID;
+    for (const ImageLayer &layer : existing)
+        byID[layer.id] = &layer;
+    // The folders around a layer, nearest first.
+    using Chain = std::vector<QUuid>;
+    const auto ancestors = [&](QUuid id) {
+        Chain result;
+        for (std::optional<QUuid> parent = byID.at(id)->parentID; parent; parent = byID.at(*parent)->parentID)
+            result.push_back(*parent);
+        return result;
+    };
+    // A selected folder carries its contents: they stay inside it.
+    QSet<QUuid> rootIDs;
+    for (const QUuid id : m_selectedLayerIDs) {
+        if (!byID.count(id))
+            continue;
+        const Chain chain = ancestors(id);
+        if (std::none_of(chain.begin(), chain.end(), [&](QUuid above) { return m_selectedLayerIDs.contains(above); }))
+            rootIDs.insert(id);
+    }
+    std::vector<QUuid> ordered;
+    std::vector<Chain> chains;
+    for (const LayerHierarchy::Entry &entry : m_document->hierarchyEntries()) {
+        if (rootIDs.contains(entry.layer.id)) {
+            ordered.push_back(entry.layer.id);
+            chains.push_back(ancestors(entry.layer.id));
+        }
+    }
+    // The nearest folder they all share; the root when none.
+    std::optional<QUuid> parent;
+    for (const QUuid candidate : chains.empty() ? Chain() : chains.front()) {
+        const auto holds = [&](const Chain &chain) { return std::find(chain.begin(), chain.end(), candidate) != chain.end(); };
+        if (std::all_of(chains.begin(), chains.end(), holds)) {
+            parent = candidate;
+            break;
+        }
+    }
+    int number = 1;
+    const auto taken = [&](const QString &name) {
+        return std::any_of(existing.begin(), existing.end(), [&](const ImageLayer &layer) { return layer.name == name; });
+    };
+    while (taken(QStringLiteral("Folder %1").arg(number)))
+        ++number;
+    ImageLayer group(QStringLiteral("Folder %1").arg(number), m_document->size());
+    group.isGroup = true;
+    group.parentID = parent;
+    // It sits at the topmost selected branch under that parent.
+    QSet<QUuid> branches;
+    for (QUuid branch : ordered) {
+        while (byID.at(branch)->parentID && byID.at(branch)->parentID != parent)
+            branch = *byID.at(branch)->parentID;
+        branches.insert(branch);
+    }
+    const auto isKept = [&](const ImageLayer &layer) { return !rootIDs.contains(layer.id); };
+    const auto highest = std::find_if(existing.rbegin(), existing.rend(), [&](const ImageLayer &layer) { return branches.contains(layer.id); });
+    // With nothing selected the folder goes on top.
+    const auto insertion = highest == existing.rend() ? std::ptrdiff_t(existing.size()) : std::count_if(existing.begin(), highest.base(), isKept);
+    std::vector<ImageLayer> layers;
+    std::copy_if(existing.begin(), existing.end(), std::back_inserter(layers), isKept);
+    layers.insert(layers.begin() + insertion, group);
+    for (const QUuid id : ordered) {
+        ImageLayer child = *byID.at(id);
+        child.parentID = group.id;
+        layers.push_back(child);
+    }
+    if (!isValidHierarchy(layers))
+        return;
+    beginEdit(QStringLiteral("Group Layers"));
+    m_document->layers = layers;
+    setActiveLayerID(group.id);
+    if (parent)
+        m_collapsedGroupIDs.remove(*parent);
+    endEdit();
+}
+
+std::vector<LayerHierarchy::Entry> EditorSession::layerRows() const
+{
+    std::vector<ProjectLayerRecord> records;
+    if (m_document) {
+        for (const ImageLayer &layer : m_document->layers)
+            records.push_back(layer.hierarchyRecord());
+    }
+    return LayerHierarchy::entries(records, true, m_collapsedGroupIDs);
+}
+
+QSet<QUuid> EditorSession::descendantIDs(QUuid id) const
+{
+    QSet<QUuid> result;
+    if (!m_document)
+        return result;
+    // One lookup of children by parent, then the walk.
+    QMultiHash<QUuid, QUuid> children;
+    for (const ImageLayer &layer : m_document->layers) {
+        if (layer.parentID)
+            children.insert(*layer.parentID, layer.id);
+    }
+    std::vector<QUuid> pending{id};
+    while (!pending.empty()) {
+        const QUuid parent = pending.back();
+        pending.pop_back();
+        for (const QUuid child : children.values(parent)) {
+            if (!result.contains(child)) {
+                result.insert(child);
+                pending.push_back(child);
+            }
+        }
+    }
+    return result;
+}
+
+void EditorSession::addGroup()
+{
+    if (!canEditLayers() || m_document->layers.size() >= 10'000)
+        return;
+    int number = 1;
+    const auto taken = [&](const QString &name) {
+        return std::any_of(m_document->layers.begin(), m_document->layers.end(), [&](const ImageLayer &layer) { return layer.name == name; });
+    };
+    while (taken(QStringLiteral("Folder %1").arg(number)))
+        ++number;
+    ImageLayer group(QStringLiteral("Folder %1").arg(number), m_document->size());
+    group.isGroup = true;
+    const std::optional<ImageLayer> active = activeLayer();
+    group.parentID = active && active->isGroup ? m_activeLayerID : active ? active->parentID : std::nullopt;
+    std::vector<ImageLayer> layers = m_document->layers;
+    const auto place = std::find_if(layers.begin(), layers.end(), [&](const ImageLayer &layer) { return m_activeLayerID && layer.id == *m_activeLayerID; });
+    layers.insert(place == layers.end() ? place : place + 1, group);
+    // A folder nested too deep is refused.
+    if (!isValidHierarchy(layers))
+        return;
+    beginEdit(QStringLiteral("New Folder"));
+    m_document->layers = layers;
+    setActiveLayerID(group.id);
+    if (group.parentID)
+        m_collapsedGroupIDs.remove(*group.parentID);
+    endEdit();
+}
+
+void EditorSession::toggleGroupExpansion(QUuid id)
+{
+    const auto isFolder = [&](const ImageLayer &layer) { return layer.id == id && layer.isGroup; };
+    if (m_isProjectBusy || !m_document || std::none_of(m_document->layers.begin(), m_document->layers.end(), isFolder))
+        return;
+    if (m_collapsedGroupIDs.contains(id)) {
+        m_collapsedGroupIDs.remove(id);
+    } else {
+        // A folded folder takes the selection from what it hides.
+        if (m_activeLayerID && descendantIDs(id).contains(*m_activeLayerID))
+            selectLayer(id);
+        m_collapsedGroupIDs.insert(id);
+    }
+    notify();
+}
+
+bool EditorSession::canPlaceLayer(QUuid id, std::optional<QUuid> parent) const
+{
+    const auto holds = [&](QUuid wanted, bool folder) {
+        return std::any_of(m_document->layers.begin(), m_document->layers.end(),
+                           [&](const ImageLayer &layer) { return layer.id == wanted && (!folder || layer.isGroup); });
+    };
+    if (!canEditLayers() || !holds(id, false))
+        return false;
+    // Into a folder: never itself nor one of its contents.
+    return !parent || (*parent != id && !descendantIDs(id).contains(*parent) && holds(*parent, true));
+}
+
+bool EditorSession::placeLayer(QUuid id, std::optional<QUuid> parent, std::optional<QUuid> above, bool atBottom)
+{
+    if (!canPlaceLayer(id, parent))
+        return false;
+    std::vector<ImageLayer> layers = m_document->layers;
+    const auto from = std::find_if(layers.begin(), layers.end(), [&](const ImageLayer &layer) { return layer.id == id; });
+    ImageLayer layer = *from;
+    layers.erase(from);
+    layer.parentID = parent;
+    auto insertion = atBottom ? layers.begin() : layers.end();
+    // The layer itself is gone by now: no target.
+    if (above) {
+        const auto target = std::find_if(layers.begin(), layers.end(),
+                                         [&](const ImageLayer &each) { return each.id == *above && each.parentID == parent; });
+        if (target == layers.end())
+            return false;
+        insertion = target + 1;
+    }
+    layers.insert(insertion, layer);
+    // Dropped into a clipping stack it joins; broken ones release.
+    adoptClipping(id, layers);
+    releaseDetachedClipping(layers);
+    if (!isValidHierarchy(layers))
+        return false;
+    beginEdit(QStringLiteral("Move Layer"));
+    m_document->layers = layers;
+    setActiveLayerID(id);
+    if (parent)
+        m_collapsedGroupIDs.remove(*parent);
+    endEdit();
+    return true;
+}
+
+void EditorSession::moveActiveLayerOutOfGroup()
+{
+    const std::optional<ImageLayer> layer = activeLayer();
+    if (!layer || !layer->parentID)
+        return;
+    const auto group = std::find_if(m_document->layers.begin(), m_document->layers.end(),
+                                    [&](const ImageLayer &each) { return each.id == *layer->parentID; });
+    if (group != m_document->layers.end())
+        placeLayer(layer->id, group->parentID, group->id);
+}

src/Document/LayerGroups.h

@@ -0,0 +1,17 @@
+#pragma once
+#include "IO/ProjectStore.h"
+#include <QSet>
+#include <vector>
+
+namespace LayerHierarchy {
+struct Entry {
+    ProjectLayerRecord layer;
+    int depth;
+    bool visible;
+};
+
+std::vector<Entry> entries(const std::vector<ProjectLayerRecord> &layers, bool topFirst = false,
+                           const QSet<QUuid> &collapsed = {});
+std::vector<ProjectLayerRecord> visibleLayers(const std::vector<ProjectLayerRecord> &layers);
+void validate(const std::vector<ProjectLayerRecord> &layers);
+}

src/Document/LayerMask+Session.h

@@ -0,0 +1,20 @@
+// Swift's LayerMask extension: masks on layers and folders.
+public:
+    void selectLayerTarget(QUuid id, bool mask);
+    // Layers and folders alike take a mask.
+    bool canEditMask() const;
+    void addLayerMask(bool revealing = true);
+    void toggleLayerMask();
+    void deleteLayerMask();
+    bool canCopyMask(QUuid source, QUuid target) const;
+    void copyMask(QUuid source, QUuid target);
+    void toggleMaskLink(QUuid id);
+    // Where a mask shows now; nil while covering its layer.
+    std::optional<LayerTransform> displayedMaskPlacement(const ImageLayer &layer) const;
+    // An unlinked mask distorted alone, in the layer's grid.
+    std::optional<QImage> maskDistortPreview(const ImageLayer &layer) const;
+    // The selection made a mask; without one, a solid mask.
+    void addMask(bool revealing = true);
+
+private:
+    void commitMaskTransform();

src/Document/LayerMask.cpp

@@ -0,0 +1,493 @@
+#include "Document/Distort.h"
+#include "Document/LayerMask.h"
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/DownsampleCache.h"
+#include <QHash>
+#include "IO/ProjectStore.h"
+#include <algorithm>
+#include <cmath>
+#include <stdexcept>
+
+LayerMask::LayerMask(ImportedImage asset, bool isEnabled, std::optional<LayerTransform> placement, bool isLinked)
+    : asset(std::move(asset)), isEnabled(isEnabled), placement(placement), isLinked(isLinked)
+{
+}
+
+std::optional<QImage> LayerMask::enabledImage() const
+{
+    return isEnabled ? std::optional(asset.image()) : std::nullopt;
+}
+
+std::optional<LayerMask> ProjectSnapshot::mask(const ProjectLayerRecord &layer) const
+{
+    const auto asset = masks.find(layer.id);
+    if (!layer.maskFile || asset == masks.end())
+        return std::nullopt;
+    return LayerMask(asset->second, layer.maskEnabled.value_or(true), layer.maskPlacement, layer.maskLinked.value_or(true));
+}
+
+bool operator==(const LayerMask &lhs, const LayerMask &rhs)
+{
+    return lhs.asset.identity() == rhs.asset.identity() && lhs.isEnabled == rhs.isEnabled
+        && lhs.placement == rhs.placement && lhs.isLinked == rhs.isLinked;
+}
+
+LayerMask LayerMask::replacing(ImportedImage replacement) const
+{
+    return LayerMask(std::move(replacement), isEnabled, placement, isLinked);
+}
+
+bool LayerMask::isValid(const QImage &image)
+{
+    return image.format() == QImage::Format_Grayscale8;
+}
+
+LayerMask LayerMask::solid(bool revealing)
+{
+    QImage image = BrushRaster::context(1, 1, true);
+    image.fill(revealing ? 255 : 0);
+    return LayerMask(ImportedImage(image, image, QStringLiteral("Layer Mask")));
+}
+
+ImportedImage LayerMask::assetFrom(const QImage &image)
+{
+    if (!isValid(image))
+        throw ProjectError(ProjectError::Kind::invalid);
+    const double factor = std::min(1.0, 96.0 / std::max(image.width(), image.height()));
+    const QSize size(std::max(1, int(image.width() * factor)), std::max(1, int(image.height() * factor)));
+    const QImage thumbnail = image.scaled(size, Qt::IgnoreAspectRatio, Qt::SmoothTransformation);
+    if (thumbnail.isNull())
+        throw ExportError(ExportError::Kind::render);
+    return ImportedImage(image, thumbnail, QStringLiteral("Layer Mask"));
+}
+
+std::optional<LayerTransform> LayerMask::placementMovingLayer(const LayerTransform &old,
+                                                              const LayerTransform &updated) const
+{
+    // A uniform mask looks the same wherever it sits.
+    if (asset.size().width() <= 1 && asset.size().height() <= 1)
+        return std::nullopt;
+    std::optional<LayerTransform> moved;
+    if (!isLinked)
+        moved = placement.value_or(old);
+    else if (placement)
+        moved = placement->following(old, updated);
+    if (moved && moved->samePlacement(updated))
+        return std::nullopt;
+    return moved;
+}
+
+double LayerMask::background(const QImage &thumbnail)
+{
+    const int width = thumbnail.width(), height = thumbnail.height();
+    if (width <= 0 || height <= 0)
+        return 1;
+    if (!isValid(thumbnail))
+        throw std::logic_error("a mask thumbnail must be 8-bit gray");
+    int total = 0, count = 0;
+    for (int y = 0; y < height; ++y) {
+        for (int x = 0; x < width; ++x) {
+            if (y != 0 && y != height - 1 && x != 0 && x != width - 1)
+                continue;
+            total += thumbnail.constScanLine(y)[x];
+            count += 1;
+        }
+    }
+    return total * 2 >= count * 255 ? 1 : 0;
+}
+
+QImage LayerMask::placed(int width, int height, const LayerTransform &layer, const LayerTransform &placement,
+                         int maskWidth, int maskHeight, double background,
+                         const std::function<void(QPainter &)> &compose)
+{
+    QImage surface = BrushRaster::context(width, height, true);
+    surface.fill(qRound(background * 255));
+    QPainter context(&surface);
+    context.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
+    context.setTransform(BrushRaster::pixelToDocument(placement, maskWidth, maskHeight)
+                         * BrushRaster::pixelToDocument(layer, width, height).inverted());
+    compose(context);
+    context.end();
+    return surface;
+}
+
+void LayerMask::drawSmooth(const QImage &image, const QRectF &rect, QPainter &context)
+{
+    context.save();
+    context.setRenderHint(QPainter::SmoothPixmapTransform, true);
+    context.drawImage(rect, image);
+    context.restore();
+}
+
+std::optional<QImage> LayerMask::clipImage(const std::optional<LayerTransform> &placement, const LayerTransform &layer,
+                                           int width, int height, std::optional<double> limit) const
+{
+    const std::optional<QImage> image = enabledImage();
+    if (!image)
+        return std::nullopt;
+    if (!placement || placement->samePlacement(layer) || width <= 0 || height <= 0)
+        return image;
+    const double factor = limit ? std::min(1.0, std::max(1.0, *limit) / std::max(width, height)) : 1;
+    const int w = std::max(1, int(std::ceil(width * factor))), h = std::max(1, int(std::ceil(height * factor)));
+    return MaskPlacementCache::shared().image(*image, *placement, layer, w, h, [&]() -> std::optional<QImage> {
+        // Drawn from a sharp halving near the covered size.
+        const double covered = placement->size.width() / std::max(1.0, layer.size.width()) * w;
+        const QImage source = DownsampleCache::shared().imageDrawnAt(*image, covered / image->width());
+        try {
+            return placed(w, h, layer, *placement, image->width(), image->height(), background(asset.thumbnail),
+                          [&](QPainter &context) { drawSmooth(source, QRectF(0, 0, image->width(), image->height()), context); });
+        } catch (const ExportError &error) {
+            qCWarning(lcRendering) << "a placed mask could not be rendered:" << error.what();
+            return std::nullopt;
+        }
+    });
+}
+
+MaskPlacementCache &MaskPlacementCache::shared()
+{
+    static MaskPlacementCache cache;
+    return cache;
+}
+
+std::optional<QImage> MaskPlacementCache::image(const QImage &mask, const LayerTransform &placement,
+                                                const LayerTransform &layer, int width, int height,
+                                                const std::function<std::optional<QImage>()> &build)
+{
+    {
+        const std::lock_guard<std::mutex> guard(m_lock);
+        m_clock += 1;
+        for (Entry &entry : m_entries) {
+            if (entry.mask.cacheKey() == mask.cacheKey() && entry.placement == placement && entry.layer == layer
+                && entry.width == width && entry.height == height) {
+                entry.lastUse = m_clock;
+                return entry.image;
+            }
+        }
+    }
+    const std::optional<QImage> image = build();
+    if (!image)
+        return std::nullopt;
+    constexpr qint64 pixelBudget = 64'000'000;
+    if (qint64(width) * height > pixelBudget)
+        return image;
+    const std::lock_guard<std::mutex> guard(m_lock);
+    m_entries.push_back({mask, placement, layer, width, height, *image, m_clock});
+    const auto pixels = [&] {
+        qint64 total = 0;
+        for (const Entry &entry : m_entries)
+            total += qint64(entry.width) * entry.height;
+        return total;
+    };
+    while (m_entries.size() > 8 || pixels() > pixelBudget) {
+        m_entries.erase(std::min_element(m_entries.begin(), m_entries.end(),
+                                         [](const Entry &lhs, const Entry &rhs) { return lhs.lastUse < rhs.lastUse; }));
+    }
+    return image;
+}
+
+void FolderMaskClip::apply(QPointF center, const QPainter &context, QImage &coverage, double scale) const
+{
+    const double width = transform.size.width() * scale, height = transform.size.height() * scale;
+    QImage veil(coverage.size(), QImage::Format_Alpha8);
+    if (veil.isNull())
+        throw ExportError(ExportError::Kind::render);
+    veil.fill(0);
+    {
+        QTransform placement;
+        placement.translate(center.x(), center.y());
+        placement.rotateRadians(transform.radians());
+        placement.scale(transform.flipX ? -1 : 1, transform.flipY ? -1 : 1);
+        QPainter masking(&veil);
+        masking.setRenderHint(QPainter::SmoothPixmapTransform, quality(transform.sampling) != InterpolationQuality::none);
+        masking.setRenderHint(QPainter::Antialiasing, true);
+        masking.setTransform(placement * context.deviceTransform());
+        masking.drawImage(QRectF(-width / 2, -height / 2, width, height), BrushRaster::alphaView(image));
+    }
+    // A failed copy of shared coverage must not pass.
+    QPainter multiplying(&coverage);
+    if (!multiplying.isActive())
+        throw ExportError(ExportError::Kind::render);
+    multiplying.setCompositionMode(QPainter::CompositionMode_DestinationIn);
+    multiplying.drawImage(QRectF(coverage.rect()), veil);
+}
+
+void FolderMaskClip::draw(const std::vector<QUuid> &ids, const std::function<std::optional<QUuid>(QUuid)> &parent,
+                          const std::function<std::optional<Applier>(QUuid)> &clip, const QPainter &context,
+                          const std::function<void(QUuid, const QImage &clip)> &drawLayer)
+{
+    // Coverage through a folder and its ancestors; null for none.
+    QHash<QUuid, QImage> covered;
+    std::function<QImage(std::optional<QUuid>, int)> through = [&](std::optional<QUuid> folder, int depth) -> QImage {
+        if (!folder || depth >= 64)
+            return QImage();
+        if (const auto found = covered.constFind(*folder); found != covered.constEnd())
+            return *found;
+        QImage coverage = through(parent(*folder), depth + 1);
+        if (const std::optional<Applier> applier = clip(*folder)) {
+            if (coverage.isNull()) {
+                coverage = QImage(context.device()->width(), context.device()->height(), QImage::Format_Alpha8);
+                if (coverage.isNull())
+                    throw ExportError(ExportError::Kind::render);
+                coverage.fill(255);
+            }
+            (*applier)(context, coverage);
+        }
+        covered.insert(*folder, coverage);
+        return coverage;
+    };
+    for (const QUuid &id : ids)
+        drawLayer(id, through(parent(id), 0));
+}
+
+LayerTransform ImageLayer::maskTransform() const
+{
+    return mask && mask->placement ? *mask->placement : transform;
+}
+
+bool EditorSession::canEditMask() const
+{
+    return canEditLayers() && m_selectedLayerIDs.size() == 1 && activeLayer().has_value();
+}
+
+void EditorSession::selectLayerTarget(QUuid id, bool mask)
+{
+    dropEffectSelection();
+    if (m_isProjectBusy || m_isImporting || m_brushStroke)
+        return;
+    resolveGradient();
+    selectLayer(id);
+    const std::optional<ImageLayer> active = activeLayer();
+    m_isMaskSelected = mask && active && active->mask;
+    notify();
+}
+
+void EditorSession::addMask(bool revealing)
+{
+    const std::optional<DocumentSelection> current = selection();
+    if (!current) {
+        addLayerMask(revealing);
+        return;
+    }
+    const std::optional<ImageLayer> active = activeLayer();
+    if (!canEditMask() || active->mask)
+        return;
+    const int index = indexOf(m_document->layers, active->id);
+    // Mask pixels cover the layer's own pixel grid.
+    const int width = active->asset ? active->asset->size().width() : int(std::round(active->size().width()));
+    const int height = active->asset ? active->asset->size().height() : int(std::round(active->size().height()));
+    try {
+        if (width <= 0 || height <= 0 || qint64(width) * height > 100'000'000)
+            throw ProjectError(ProjectError::Kind::tooLarge);
+        QImage image = BrushRaster::context(width, height, true);
+        image.fill(revealing ? Qt::white : Qt::black);
+        const QTransform toPixels = BrushRaster::pixelToDocument(active->transform, width, height).inverted();
+        QPainter painter(&image);
+        painter.setRenderHint(QPainter::Antialiasing, current->antialiased);
+        painter.fillPath(toPixels.map(current->path), revealing ? Qt::black : Qt::white);
+        const LayerMask mask(LayerMask::assetFrom(image));
+        finishOpacityEdit();
+        beginEdit(QStringLiteral("Add Mask from Selection"));
+        m_document->layers[size_t(index)].mask = mask;
+        m_document->selection = std::nullopt;
+        m_isMaskSelected = true;
+        endEdit();
+    } catch (const std::runtime_error &error) {
+        qCWarning(lcApp).noquote() << "cannot make a mask from the selection:" << error.what();
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+}
+
+void EditorSession::addLayerMask(bool revealing)
+{
+    const std::optional<ImageLayer> active = activeLayer();
+    if (!canEditMask() || active->mask)
+        return;
+    finishOpacityEdit();
+    beginEdit(revealing ? QStringLiteral("Add Reveal-All Mask") : QStringLiteral("Add Hide-All Mask"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (layer.id == active->id)
+            layer.mask = LayerMask::solid(revealing);
+    }
+    m_isMaskSelected = true;
+    endEdit();
+}
+
+void EditorSession::toggleLayerMask()
+{
+    const std::optional<ImageLayer> active = activeLayer();
+    if (!canEditMask() || !active->mask)
+        return;
+    finishOpacityEdit();
+    beginEdit(active->mask->isEnabled ? QStringLiteral("Disable Layer Mask") : QStringLiteral("Enable Layer Mask"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (layer.id == active->id)
+            layer.mask->isEnabled = !layer.mask->isEnabled;
+    }
+    endEdit();
+}
+
+void EditorSession::deleteLayerMask()
+{
+    const std::optional<ImageLayer> active = activeLayer();
+    if (!canEditMask() || !active->mask)
+        return;
+    finishOpacityEdit();
+    beginEdit(QStringLiteral("Delete Layer Mask"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (layer.id == active->id)
+            layer.mask = std::nullopt;
+    }
+    m_isMaskSelected = false;
+    endEdit();
+}
+
+bool EditorSession::canCopyMask(QUuid source, QUuid target) const
+{
+    if (!canEditLayers() || source == target)
+        return false;
+    const auto holds = [&](QUuid id, bool asSource) {
+        return std::any_of(m_document->layers.begin(), m_document->layers.end(), [&](const ImageLayer &layer) {
+            return layer.id == id && (asSource ? layer.mask.has_value() : !layer.isGroup);
+        });
+    };
+    return holds(source, true) && holds(target, false);
+}
+
+void EditorSession::copyMask(QUuid source, QUuid target)
+{
+    if (!canCopyMask(source, target))
+        return;
+    commitTransform();
+    const auto find = [&](QUuid id) {
+        return std::find_if(m_document->layers.begin(), m_document->layers.end(), [&](const ImageLayer &layer) { return layer.id == id; });
+    };
+    // The copy sits where the mask sits on the document.
+    LayerMask mask = *find(source)->mask;
+    mask.placement = find(source)->maskTransform();
+    beginEdit(find(target)->mask ? QStringLiteral("Replace Layer Mask") : QStringLiteral("Copy Layer Mask"));
+    find(target)->mask = mask;
+    selectLayer(target);
+    m_isMaskSelected = true;
+    endEdit();
+}
+
+void EditorSession::toggleMaskLink(QUuid id)
+{
+    const auto holds = [&](const ImageLayer &layer) { return layer.id == id && layer.mask; };
+    if (!canEditLayers() || std::none_of(m_document->layers.begin(), m_document->layers.end(), holds))
+        return;
+    commitTransform();
+    const auto layer = std::find_if(m_document->layers.begin(), m_document->layers.end(), holds);
+    beginEdit(layer->mask->isLinked ? QStringLiteral("Unlink Layer Mask") : QStringLiteral("Link Layer Mask"));
+    layer->mask->isLinked = !layer->mask->isLinked;
+    endEdit();
+}
+
+std::optional<LayerTransform> EditorSession::displayedMaskPlacement(const ImageLayer &layer) const
+{
+    if (!layer.mask)
+        return std::nullopt;
+    // Previewed on a grown layer, the mask keeps its bounds.
+    if (m_filterEdit && m_filterEdit->grownTransform && m_filterEdit->previewImage(layer.id))
+        return layer.mask->placement.value_or(layer.transform);
+    if (m_transformEdit && m_transformEdit->group) {
+        const TransformGroup &group = *m_transformEdit->group;
+        const auto original = group.originals.constFind(layer.id);
+        if (original == group.originals.constEnd())
+            return layer.mask->placement;
+        if (m_transformEdit->corners)
+            return layer.mask->isLinked && !layer.mask->placement ? std::nullopt : std::optional(layer.mask->placement.value_or(layer.transform));
+        return layer.mask->placementMovingLayer(layer.transform, original->following(group.box, m_transformEdit->draft));
+    }
+    if (!m_transformEdit || m_transformEdit->layerID != layer.id)
+        return layer.mask->placement;
+    const LayerTransform &draft = m_transformEdit->draft;
+    if (m_transformEdit->mask)
+        return draft.samePlacement(layer.transform) ? std::nullopt : std::optional(draft);
+    // A distortion carries a linked layer's own mask; others stay.
+    if (m_transformEdit->corners)
+        return layer.mask->isLinked && !layer.mask->placement ? std::nullopt : std::optional(layer.mask->placement.value_or(layer.transform));
+    return layer.mask->placementMovingLayer(layer.transform, draft);
+}
+
+std::optional<QImage> EditorSession::maskDistortPreview(const ImageLayer &layer) const
+{
+    const std::optional<TransformEdit> &edit = m_transformEdit;
+    if (!edit || !edit->mask || edit->layerID != layer.id || !edit->corners || !layer.mask || !layer.mask->isEnabled)
+        return std::nullopt;
+    const LayerMask &owned = *layer.mask;
+    if (m_maskDistortPreviewCache && m_maskDistortPreviewCache->corners == *edit->corners && m_maskDistortPreviewCache->draft == edit->draft
+        && m_maskDistortPreviewCache->mask == owned.asset.identity() && m_maskDistortPreviewCache->layer == layer.transform)
+        return m_maskDistortPreviewCache->result;
+    const int width = layer.asset ? layer.asset->size().width() : int(std::round(layer.size().width()));
+    const int height = layer.asset ? layer.asset->size().height() : int(std::round(layer.size().height()));
+    std::optional<QImage> result;
+    try {
+        const DistortWarp::Warped moved = DistortWarp::warpMask(owned.asset.image(), edit->draft, *edit->corners, LayerMask::background(owned.asset.thumbnail), 2048);
+        result = LayerMask(ImportedImage(moved.image, owned.asset.thumbnail, owned.asset.name)).clipImage(moved.transform, layer.transform, width, height, 2048);
+    } catch (const ProjectError &) {
+    } catch (const ExportError &) {
+    }
+    m_maskDistortPreviewCache = MaskDistortPreviewCache{*edit->corners, edit->draft, owned.asset.identity(), layer.transform, result};
+    return result;
+}
+
+void EditorSession::commitMaskTransform()
+{
+    const TransformEdit edit = *m_transformEdit;
+    m_maskDistortPreviewCache.reset();
+    const int index = indexOf(m_document->layers, edit.layerID);
+    if (edit.corners && edit.draft.isValid() && index >= 0 && m_document->layers[index].mask) {
+        // A distorted mask is resampled; its background fills outside.
+        m_transformEdit = std::nullopt;
+        const ImageLayer layer = m_document->layers[index];
+        const LayerMask &mask = *layer.mask;
+        try {
+            const DistortWarp::Warped moved = DistortWarp::warpMask(mask.asset.image(), edit.draft, *edit.corners, LayerMask::background(mask.asset.thumbnail));
+            // Everything that can throw comes before the transaction opens.
+            const ImportedImage asset = moved.image.cacheKey() == mask.asset.image().cacheKey() ? mask.asset : LayerMask::assetFrom(moved.image);
+            beginEdit(QStringLiteral("Distort Layer Mask"));
+            m_document->layers[index].mask = LayerMask(asset, mask.isEnabled,
+                                                       moved.transform.samePlacement(layer.transform) ? std::nullopt : std::optional(moved.transform), mask.isLinked);
+            endEdit();
+        } catch (const ProjectError &error) {
+            setBrushError(QString::fromUtf8(error.what()));
+        } catch (const ExportError &error) {
+            setBrushError(QString::fromUtf8(error.what()));
+        }
+        return;
+    }
+    // The mask takes the placement; its pixels are untouched.
+    beginEdit(QStringLiteral("Transform Layer Mask"));
+    for (ImageLayer &layer : m_document->layers) {
+        if (layer.id == edit.layerID && layer.mask && edit.draft.isValid())
+            layer.mask->placement = edit.draft.samePlacement(layer.transform) ? std::nullopt : std::optional(edit.draft);
+    }
+    m_transformEdit = std::nullopt;
+    endEdit();
+}
+
+// The stroke's mask, in the layer's grid, preview-sized.
+std::optional<QImage> BrushStroke::placedMaskPreview(const LayerTransform &placement) const
+{
+    const int gridWidth = layer.asset ? layer.asset->size().width() : int(std::round(layer.size().width()));
+    const int gridHeight = layer.asset ? layer.asset->size().height() : int(std::round(layer.size().height()));
+    const double factor = std::min(1.0, 2048.0 / std::max({1, gridWidth, gridHeight}));
+    const int w = std::max(1, int(std::ceil(gridWidth * factor))), h = std::max(1, int(std::ceil(gridHeight * factor)));
+    const std::optional<ImportedImage> &old = layer.mask ? std::optional(layer.mask->asset) : std::nullopt;
+    const double perMaskPixel = placement.size.width() / std::max(1, width) * w / std::max(1.0, layer.transform.size.width());
+    try {
+        return LayerMask::placed(w, h, layer.transform, placement, width, height, old ? LayerMask::background(old->thumbnail) : 1, [&](QPainter &context) {
+            if (old)
+                LayerMask::drawSmooth(DownsampleCache::shared().imageDrawnAt(old->image(), perMaskPixel), sourceRect, context);
+            for (const BrushPatch &patch : patches())
+                LayerMask::drawSmooth(patch.image, patch.rect, context);
+        });
+    } catch (const ExportError &error) {
+        qCWarning(lcRendering) << "the placed mask's preview could not be made:" << error.what();
+        return std::nullopt;
+    }
+}

src/Document/LayerMask.h

@@ -0,0 +1,84 @@
+#pragma once
+#include "Document/LayerTransform.h"
+#include "IO/ImageImporter.h"
+#include <QPainter>
+#include <QUuid>
+#include <functional>
+#include <mutex>
+#include <optional>
+#include <vector>
+
+// Layer-local coverage: white reveals, black hides, gray is soft.
+struct LayerMask {
+    ImportedImage asset;
+    bool isEnabled = true;
+    // Set once the mask moves apart from its layer.
+    std::optional<LayerTransform> placement;
+    bool isLinked = true;
+
+    explicit LayerMask(ImportedImage asset, bool isEnabled = true,
+                       std::optional<LayerTransform> placement = std::nullopt, bool isLinked = true);
+
+    std::optional<QImage> enabledImage() const;
+    friend bool operator==(const LayerMask &lhs, const LayerMask &rhs);
+    LayerMask replacing(ImportedImage asset) const;
+    static bool isValid(const QImage &image);
+    static LayerMask solid(bool revealing);
+    static ImportedImage assetFrom(const QImage &image);
+
+    std::optional<LayerTransform> placementMovingLayer(const LayerTransform &old, const LayerTransform &updated) const;
+    static double background(const QImage &thumbnail);
+    static QImage placed(int width, int height, const LayerTransform &layer, const LayerTransform &placement,
+                         int maskWidth, int maskHeight, double background,
+                         const std::function<void(QPainter &)> &compose);
+    static void drawSmooth(const QImage &image, const QRectF &rect, QPainter &context);
+    // The mask over a layer's pixel grid; nil while disabled.
+    std::optional<QImage> clipImage(const std::optional<LayerTransform> &placement, const LayerTransform &layer,
+                                    int width, int height, std::optional<double> limit = std::nullopt) const;
+};
+
+// The canvas's last preview of an unlinked mask distorted alone.
+struct MaskDistortPreviewCache {
+    Corners corners;
+    LayerTransform draft;
+    ImageIdentity mask;
+    LayerTransform layer;
+    std::optional<QImage> result;
+};
+
+// Masks resampled into their layers' grids, least recently used dropped.
+class MaskPlacementCache {
+public:
+    static MaskPlacementCache &shared();
+    std::optional<QImage> image(const QImage &mask, const LayerTransform &placement, const LayerTransform &layer,
+                                int width, int height, const std::function<std::optional<QImage>()> &build);
+
+private:
+    struct Entry {
+        QImage mask;
+        LayerTransform placement;
+        LayerTransform layer;
+        int width;
+        int height;
+        QImage image;
+        quint64 lastUse;
+    };
+    std::vector<Entry> m_entries;
+    quint64 m_clock = 0;
+    std::mutex m_lock;
+};
+
+// A folder's mask, multiplied into every layer inside the folder.
+struct FolderMaskClip {
+    QImage image;
+    LayerTransform transform;
+
+    using Applier = std::function<void(const QPainter &context, QImage &coverage)>;
+
+    // Multiplies the mask, placed like a layer's own, into coverage.
+    void apply(QPointF center, const QPainter &context, QImage &coverage, double scale = 1) const;
+    // Draws ids in order, each through its folders' masks.
+    static void draw(const std::vector<QUuid> &ids, const std::function<std::optional<QUuid>(QUuid)> &parent,
+                     const std::function<std::optional<Applier>(QUuid)> &clip, const QPainter &context,
+                     const std::function<void(QUuid, const QImage &clip)> &drawLayer);
+};

src/Document/LayerMerge+Session.h

@@ -0,0 +1,8 @@
+// Swift's LayerMerge extension: selected layers made one.
+public:
+    bool canMergeLayers() const;
+    QString mergeTitle() const;
+    void mergeLayers();
+
+private:
+    std::optional<MergePlan> mergePlan() const;

src/Document/LayerMerge.cpp

@@ -0,0 +1,140 @@
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "Document/Filters.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+
+// What Ctrl+E merges, in stacking order, and its place.
+std::optional<EditorSession::MergePlan> EditorSession::mergePlan() const
+{
+    const std::optional<ImageLayer> active = activeLayer();
+    if (!canEditLayers() || !active)
+        return std::nullopt;
+    const std::vector<ImageLayer> &layers = m_document->layers;
+    if (m_selectedLayerIDs.size() > 1) {
+        // Several selected layers, with anything their folders hold.
+        QSet<QUuid> picked = m_selectedLayerIDs;
+        for (const QUuid &id : m_selectedLayerIDs)
+            picked.unite(descendantIDs(id));
+        std::vector<QUuid> ordered;
+        std::optional<ImageLayer> top;
+        bool pixels = false;
+        for (const ImageLayer &layer : layers) {
+            if (!picked.contains(layer.id))
+                continue;
+            ordered.push_back(layer.id);
+            pixels = pixels || !layer.isGroup;
+            if (m_selectedLayerIDs.contains(layer.id))
+                top = layer;
+        }
+        if (!pixels || !top)
+            return std::nullopt;
+        return MergePlan{ordered, picked, top->name, top->parentID, top->id, QStringLiteral("Merge Layers")};
+    }
+    if (active->isGroup) {
+        // A folder merges its contents, and the folder goes.
+        const QSet<QUuid> inside = descendantIDs(active->id);
+        std::vector<QUuid> ids;
+        bool pixels = false;
+        for (const ImageLayer &layer : layers) {
+            if (inside.contains(layer.id) || layer.id == active->id)
+                ids.push_back(layer.id);
+            pixels = pixels || (inside.contains(layer.id) && !layer.isGroup);
+        }
+        if (!pixels)
+            return std::nullopt;
+        return MergePlan{ids, QSet<QUuid>(ids.begin(), ids.end()), active->name, active->parentID, active->id, QStringLiteral("Merge Group")};
+    }
+    // One layer merges with the one beneath it.
+    for (int index = indexOf(layers, active->id) - 1; index >= 0; --index) {
+        const ImageLayer &below = layers[index];
+        if (below.parentID != active->parentID)
+            continue;
+        if (below.isGroup)
+            return std::nullopt;
+        return MergePlan{{below.id, active->id}, {below.id, active->id}, below.name, active->parentID, active->id, QStringLiteral("Merge Down")};
+    }
+    return std::nullopt;
+}
+
+bool EditorSession::canMergeLayers() const
+{
+    return mergePlan().has_value();
+}
+
+QString EditorSession::mergeTitle() const
+{
+    const std::optional<MergePlan> plan = mergePlan();
+    return plan ? plan->action : QStringLiteral("Merge Down");
+}
+
+// The layers as shown, baked into one pixel layer.
+void EditorSession::mergeLayers()
+{
+    commitTransform();
+    const std::optional<MergePlan> plan = mergePlan();
+    if (!plan)
+        return;
+    const std::vector<ImageLayer> &layers = m_document->layers;
+    const QSet<QUuid> kept(plan->ids.begin(), plan->ids.end());
+    // Only the merged layers, cut loose from the rest.
+    CanvasDocument flat(m_document->width, m_document->height);
+    flat.id = m_document->id;
+    flat.resolution = m_document->resolution;
+    for (const ImageLayer &layer : layers) {
+        if (!kept.contains(layer.id))
+            continue;
+        ImageLayer copy = layer;
+        if (copy.parentID && !kept.contains(*copy.parentID))
+            copy.parentID = std::nullopt;
+        if (copy.maskSourceID && !kept.contains(*copy.maskSourceID))
+            copy.maskSourceID = std::nullopt;
+        flat.layers.push_back(copy);
+    }
+    std::optional<ImageLayer> merged;
+    try {
+        QImage full = BrushRaster::context(m_document->width, m_document->height, false);
+        {
+            QPainter context(&full);
+            drawLiveComposite(flat, context);
+        }
+        const LayerTransform canvas{.origin = {0, 0}, .size = m_document->size()};
+        const PixelFilter::Trimmed trimmed = PixelFilter::trimmed(full, canvas);
+        // At 1:1 the trimmed size is the crop's: the constructor's.
+        merged = ImageLayer(ImportedImage(trimmed.image, PixelAdjust::thumbnail(trimmed.image), plan->name), trimmed.transform.origin);
+    } catch (const ExportError &error) {
+        qCWarning(lcRendering) << "merge: the layers could not be rendered:" << error.what();
+        return;
+    }
+    merged->parentID = plan->parent;
+    std::vector<ImageLayer> next;
+    int insertion = 0;
+    for (const ImageLayer &layer : layers) {
+        if (plan->removed.contains(layer.id))
+            continue;
+        next.push_back(layer);
+        // Layers clipped to anything merged now clip to the result.
+        if (next.back().maskSourceID && plan->removed.contains(*next.back().maskSourceID))
+            next.back().maskSourceID = merged->id;
+    }
+    for (const ImageLayer &layer : layers) {
+        if (layer.id == plan->anchor)
+            break;
+        insertion += plan->removed.contains(layer.id) ? 0 : 1;
+    }
+    next.insert(next.begin() + insertion, *merged);
+    std::vector<ProjectLayerRecord> records;
+    for (const ImageLayer &layer : next)
+        records.push_back(layer.hierarchyRecord());
+    try {
+        LayerHierarchy::validate(records);
+    } catch (const ProjectError &error) {
+        qCWarning(lcRendering) << "merge: the result is no valid hierarchy:" << error.what();
+        return;
+    }
+    beginEdit(plan->action);
+    m_document->layers = std::move(next);
+    setActiveLayerID(merged->id);
+    endEdit();
+}

src/Document/LayerTransform.cpp

@@ -0,0 +1,282 @@
+#include "Document/LayerTransform.h"
+#include "Document/BrushStroke.h"
+#include <algorithm>
+#include <cmath>
+#include <numbers>
+#include <stdexcept>
+
+namespace {
+bool sameSize(QSizeF lhs, QSizeF rhs)
+{
+    return lhs.width() == rhs.width() && lhs.height() == rhs.height();
+}
+
+struct Shift {
+    double move = 0;
+    std::optional<double> target;
+};
+
+Shift shift(const std::array<double, 3> &guides, const std::vector<double> &targets, double tolerance)
+{
+    Shift best;
+    for (double guide : guides) {
+        for (double target : targets) {
+            const double move = target - guide;
+            if (std::abs(move) > tolerance)
+                continue;
+            if (best.target && std::abs(best.move) <= std::abs(move))
+                continue;
+            best = {move, target};
+        }
+    }
+    return best;
+}
+}
+
+QString rawValue(LayerSampling sampling)
+{
+    switch (sampling) {
+    case LayerSampling::nearest:
+        return QStringLiteral("Nearest");
+    case LayerSampling::smooth:
+        return QStringLiteral("Smooth");
+    case LayerSampling::high:
+        return QStringLiteral("High quality");
+    }
+    throw std::logic_error("unknown LayerSampling");
+}
+
+std::optional<LayerSampling> layerSampling(const QString &rawValue)
+{
+    for (const LayerSampling sampling : {LayerSampling::nearest, LayerSampling::smooth, LayerSampling::high}) {
+        if (rawValue == ::rawValue(sampling))
+            return sampling;
+    }
+    return std::nullopt;
+}
+
+InterpolationQuality quality(LayerSampling sampling)
+{
+    switch (sampling) {
+    case LayerSampling::nearest:
+        return InterpolationQuality::none;
+    case LayerSampling::smooth:
+        return InterpolationQuality::low;
+    case LayerSampling::high:
+        return InterpolationQuality::high;
+    }
+    throw std::logic_error("unknown LayerSampling");
+}
+
+const std::array<QPointF, 8> LayerTransform::handles = {
+    QPointF(0, 0), QPointF(0.5, 0), QPointF(1, 0), QPointF(1, 0.5),
+    QPointF(1, 1), QPointF(0.5, 1), QPointF(0, 1), QPointF(0, 0.5)};
+
+bool operator==(const LayerTransform &lhs, const LayerTransform &rhs)
+{
+    return lhs.origin.x() == rhs.origin.x() && lhs.origin.y() == rhs.origin.y()
+        && sameSize(lhs.size, rhs.size) && lhs.rotation == rhs.rotation
+        && lhs.flipX == rhs.flipX && lhs.flipY == rhs.flipY && lhs.sampling == rhs.sampling;
+}
+
+QPointF LayerTransform::center() const
+{
+    return {origin.x() + size.width() / 2, origin.y() + size.height() / 2};
+}
+
+double LayerTransform::radians() const
+{
+    return std::fmod(rotation, 360) * std::numbers::pi / 180;
+}
+
+bool LayerTransform::isValid() const
+{
+    const std::array<double, 5> values = {origin.x(), origin.y(), size.width(), size.height(), rotation};
+    return std::all_of(values.begin(), values.end(), [](double value) { return std::isfinite(value); })
+        && size.width() >= 1 && size.width() <= 300'000 && size.height() >= 1 && size.height() <= 300'000
+        && std::abs(origin.x()) <= 1'000'000 && std::abs(origin.y()) <= 1'000'000;
+}
+
+QPointF LayerTransform::point(QPointF unit) const
+{
+    const double x = (unit.x() - 0.5) * size.width(), y = (unit.y() - 0.5) * size.height();
+    const QPointF middle = center();
+    return {middle.x() + x * std::cos(radians()) - y * std::sin(radians()),
+            middle.y() + x * std::sin(radians()) + y * std::cos(radians())};
+}
+
+bool LayerTransform::contains(QPointF point) const
+{
+    const double x = point.x() - center().x(), y = point.y() - center().y();
+    return std::abs(x * std::cos(radians()) + y * std::sin(radians())) <= size.width() / 2
+        && std::abs(-x * std::sin(radians()) + y * std::cos(radians())) <= size.height() / 2;
+}
+
+double LayerTransform::scalePercent(QSizeF pixelSize) const
+{
+    return size.width() / std::max(1.0, pixelSize.width()) * 100;
+}
+
+LayerTransform LayerTransform::scaled(double toPercent, QSizeF pixelSize) const
+{
+    LayerTransform result = *this;
+    result.size = {pixelSize.width() * toPercent / 100, pixelSize.height() * toPercent / 100};
+    result.origin = {center().x() - result.size.width() / 2, center().y() - result.size.height() / 2};
+    return result;
+}
+
+LayerTransform LayerTransform::rounded() const
+{
+    LayerTransform result = *this;
+    result.origin = {std::round(origin.x()), std::round(origin.y())};
+    result.size = {std::max(1.0, std::round(size.width())), std::max(1.0, std::round(size.height()))};
+    result.rotation = std::round(rotation);
+    return result;
+}
+
+QTransform LayerTransform::unitToDocument() const
+{
+    return BrushRaster::pixelToDocument(*this, 1, 1);
+}
+
+LayerTransform LayerTransform::placing(const QTransform &map) const
+{
+    const double sign = flipX ? -1 : 1;
+    const double angle = std::atan2(map.m12() * sign, map.m11() * sign);
+    const double along = -map.m21() * std::sin(angle) + map.m22() * std::cos(angle);
+    const QPointF middle = map.map(QPointF(0.5, 0.5));
+    LayerTransform result = *this;
+    result.size = {std::hypot(map.m11(), map.m12()), std::abs(along)};
+    const double degrees = angle * 180 / std::numbers::pi;
+    result.rotation = degrees + std::round((rotation - degrees) / 360) * 360;
+    result.flipY = along < 0;
+    result.origin = {middle.x() - result.size.width() / 2, middle.y() - result.size.height() / 2};
+    return result;
+}
+
+LayerTransform LayerTransform::following(const LayerTransform &old, const LayerTransform &updated) const
+{
+    if (old == updated)
+        return *this;
+    if (sameSize(old.size, updated.size) && old.rotation == updated.rotation
+        && old.flipX == updated.flipX && old.flipY == updated.flipY) {
+        LayerTransform moved = *this;
+        moved.origin += updated.origin - old.origin;
+        return moved;
+    }
+    return placing(unitToDocument() * old.unitToDocument().inverted() * updated.unitToDocument());
+}
+
+bool LayerTransform::samePlacement(const LayerTransform &other) const
+{
+    LayerTransform copy = *this;
+    copy.sampling = other.sampling;
+    return copy == other;
+}
+
+std::optional<Corners> TransformDrag::corners(QPointF to, bool shift) const
+{
+    if (!originalCorners)
+        return std::nullopt;
+    double dx = to.x() - start.x(), dy = to.y() - start.y();
+    if (shift) {
+        if (std::abs(dx) >= std::abs(dy))
+            dy = 0;
+        else
+            dx = 0;
+    }
+    std::vector<int> moved;
+    if (mode.kind == Kind::distort) {
+        const int corner = mode.index / 2;
+        moved = mode.index % 2 == 0 ? std::vector<int>{corner} : std::vector<int>{corner, (corner + 1) % 4};
+    } else if (mode.kind == Kind::move) {
+        moved = {0, 1, 2, 3};
+    } else {
+        return std::nullopt;
+    }
+    Corners result = *originalCorners;
+    for (int corner : moved)
+        result[corner] += QPointF(dx, dy);
+    return result;
+}
+
+LayerTransform TransformDrag::updated(QPointF to, bool lockRatio, bool shift, bool option) const
+{
+    LayerTransform result = original;
+    const double radians = original.radians();
+    switch (mode.kind) {
+    case Kind::distort:
+        break;
+    case Kind::move: {
+        double dx = to.x() - start.x(), dy = to.y() - start.y();
+        if (shift) {
+            if (std::abs(dx) >= std::abs(dy))
+                dy = 0;
+            else
+                dx = 0;
+        }
+        result.origin += QPointF(dx, dy);
+        break;
+    }
+    case Kind::rotate: {
+        const QPointF center = original.center();
+        const double delta = std::atan2(to.y() - center.y(), to.x() - center.x())
+            - std::atan2(start.y() - center.y(), start.x() - center.x());
+        result.rotation += delta * 180 / std::numbers::pi;
+        if (shift)
+            result.rotation = std::round(result.rotation / 15) * 15;
+        break;
+    }
+    case Kind::resize: {
+        const QPointF handle = LayerTransform::handles[mode.index];
+        const QPointF anchorUnit = option ? QPointF(0.5, 0.5) : QPointF(1 - handle.x(), 1 - handle.y());
+        const QPointF anchor = original.point(anchorUnit);
+        const QPointF initialHandle = original.point(handle);
+        const double dx = initialHandle.x() + to.x() - start.x() - anchor.x();
+        const double dy = initialHandle.y() + to.y() - start.y() - anchor.y();
+        const double span = option ? 2 : 1;
+        const double localX = (dx * std::cos(radians) + dy * std::sin(radians)) * span;
+        const double localY = (-dx * std::sin(radians) + dy * std::cos(radians)) * span;
+        const double sx = handle.x() * 2 - 1, sy = handle.y() * 2 - 1;
+        const double originalWidth = original.size.width(), originalHeight = original.size.height();
+        const double rawWidth = sx == 0 ? originalWidth : localX * sx;
+        const double rawHeight = sy == 0 ? originalHeight : localY * sy;
+        const bool mirroredX = rawWidth < 0, mirroredY = rawHeight < 0;
+        double width = std::max(1.0, std::abs(rawWidth));
+        double height = std::max(1.0, std::abs(rawHeight));
+        if (lockRatio != shift) {
+            double factor;
+            if (sx == 0)
+                factor = height / originalHeight;
+            else if (sy == 0)
+                factor = width / originalWidth;
+            else
+                factor = std::max(1 / std::min(originalWidth, originalHeight),
+                                  (localX * sx * originalWidth + localY * sy * originalHeight)
+                                      / (originalWidth * originalWidth + originalHeight * originalHeight));
+            width = originalWidth * factor;
+            height = originalHeight * factor;
+        }
+        result.size = {width, height};
+        if (mirroredX)
+            result.flipX = !result.flipX;
+        if (mirroredY)
+            result.flipY = !result.flipY;
+        const double offsetX = (0.5 - anchorUnit.x()) * width * (mirroredX ? -1 : 1);
+        const double offsetY = (0.5 - anchorUnit.y()) * height * (mirroredY ? -1 : 1);
+        const QPointF center(anchor.x() + offsetX * std::cos(radians) - offsetY * std::sin(radians),
+                             anchor.y() + offsetX * std::sin(radians) + offsetY * std::cos(radians));
+        result.origin = {center.x() - width / 2, center.y() - height / 2};
+        break;
+    }
+    }
+    return result.isValid() ? result : original;
+}
+
+TransformSnap::Offset TransformSnap::offset(const QRectF &box, const std::vector<double> &xs,
+                                            const std::vector<double> &ys, double tolerance)
+{
+    const Shift horizontal = shift({box.left(), box.center().x(), box.right()}, xs, tolerance);
+    const Shift vertical = shift({box.top(), box.center().y(), box.bottom()}, ys, tolerance);
+    return {QSizeF(horizontal.move, vertical.move), horizontal.target, vertical.target};
+}

src/Document/LayerTransform.h

@@ -0,0 +1,111 @@
+#pragma once
+#include <QHash>
+#include <QPointF>
+#include <QRectF>
+#include <QSizeF>
+#include <QString>
+#include <QTransform>
+#include <QUuid>
+#include <array>
+#include <memory>
+#include <optional>
+#include <vector>
+
+enum class LayerSampling { nearest, smooth, high };
+// The name users see and the manifest stores.
+QString rawValue(LayerSampling sampling);
+std::optional<LayerSampling> layerSampling(const QString &rawValue);
+
+// CoreGraphics interpolation qualities the renderer asks for.
+enum class InterpolationQuality { none, low, high };
+InterpolationQuality quality(LayerSampling sampling);
+
+using Corners = std::array<QPointF, 4>;
+
+// Unrotated document bounds; rotation is clockwise about the center.
+struct LayerTransform {
+    QPointF origin;
+    QSizeF size;
+    double rotation = 0;
+    bool flipX = false;
+    bool flipY = false;
+    LayerSampling sampling = LayerSampling::high;
+
+    static const std::array<QPointF, 8> handles;
+
+    friend bool operator==(const LayerTransform &lhs, const LayerTransform &rhs);
+    QPointF center() const;
+    double radians() const;
+    bool isValid() const;
+    QPointF point(QPointF unit) const;
+    bool contains(QPointF point) const;
+    double scalePercent(QSizeF pixelSize) const;
+    LayerTransform scaled(double toPercent, QSizeF pixelSize) const;
+    LayerTransform rounded() const;
+    QTransform unitToDocument() const;
+    LayerTransform placing(const QTransform &map) const;
+    LayerTransform following(const LayerTransform &old, const LayerTransform &updated) const;
+    bool samePlacement(const LayerTransform &other) const;
+    // Mirrored across a line at `axis`; defined in LayerFlip.cpp.
+    LayerTransform mirrored(bool horizontally, double axis) const;
+};
+
+// Several layers under one box; each follows it.
+struct TransformGroup {
+    LayerTransform box;
+    QHash<QUuid, LayerTransform> originals;
+};
+
+// A pending transform: the layers change on commit.
+struct FloatingTransform;
+
+struct TransformEdit {
+    QUuid layerID;
+    LayerTransform draft;
+    std::optional<TransformGroup> group;
+    // An unlinked mask, selected: the edit places it alone.
+    bool mask = false;
+    // A drag's edit ends with it; Ctrl+T's waits for Apply.
+    bool persistent = true;
+    // Set once a handle is Ctrl-dragged: the corners move freely.
+    std::optional<Corners> corners;
+    // Ctrl+T on selected pixels: they float, then merge back.
+    std::shared_ptr<const FloatingTransform> floating;
+};
+
+// Where a move has just snapped: guides while it lasts.
+struct SnapGuides {
+    std::vector<double> xs;
+    std::vector<double> ys;
+    friend bool operator==(const SnapGuides &, const SnapGuides &) = default;
+};
+
+struct TransformDrag {
+    enum class Kind { move, resize, rotate, distort };
+    struct Mode {
+        Kind kind;
+        int index = 0;
+    };
+
+    LayerTransform original;
+    QPointF start;
+    Mode mode;
+    std::optional<Corners> originalCorners;
+
+    std::optional<Corners> corners(QPointF to, bool shift = false) const;
+    LayerTransform updated(QPointF to, bool lockRatio, bool shift, bool option = false) const;
+};
+
+namespace TransformSnap {
+// Screen points within which a guide snaps.
+inline constexpr double distance = 10;
+
+struct Offset {
+    QSizeF offset;
+    std::optional<double> x;
+    std::optional<double> y;
+};
+
+Offset offset(const QRectF &box, const std::vector<double> &xs, const std::vector<double> &ys,
+              double tolerance);
+}

src/Document/Levels+Session.h

@@ -0,0 +1,17 @@
+// Swift's Levels extension: one Levels edit at a time.
+public:
+    void beginLevels();
+    void updateLevels(const LevelsSettings &settings, bool preview);
+    // The sheet's eyedroppers: the point a click sets, or none.
+    void setLevelsSampleMode(std::optional<LevelsSample> mode);
+    void cancelLevels();
+    // Off the UI thread; `done` follows the step or refusal.
+    void commitLevels(std::function<void()> done = {});
+
+private:
+    // Off the UI thread, for the open edit.
+    void countLevelsHistogram();
+    void renderLevelsPreview();
+    void finishLevelsPreview();
+    void finishLevelsHistogram();
+    void finishLevelsCommit();

src/Document/Levels.cpp

@@ -0,0 +1,369 @@
+#include "Document/Levels.h"
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/RasterSnapshot.h"
+#include <QPainter>
+#include <QtConcurrent>
+#include <algorithm>
+#include <cmath>
+#include <stdexcept>
+#include <vector>
+extern "C" {
+#include "LevelsPixels.h"
+}
+
+QString rawValue(LevelsChannel channel)
+{
+    switch (channel) {
+    case LevelsChannel::rgb: return QStringLiteral("RGB");
+    case LevelsChannel::red: return QStringLiteral("Red");
+    case LevelsChannel::green: return QStringLiteral("Green");
+    case LevelsChannel::blue: return QStringLiteral("Blue");
+    }
+    throw std::logic_error("no such channel");
+}
+
+namespace {
+double clamped(double value, double low, double high, double fallback)
+{
+    return std::isfinite(value) ? std::min(high, std::max(low, value)) : fallback;
+}
+}
+
+LevelRange LevelRange::normalized() const
+{
+    LevelRange result = *this;
+    result.black = clamped(black, 0, 254, 0);
+    result.white = clamped(white, result.black + 1, 255, 255);
+    result.gamma = clamped(gamma, 0.1, 9.99, 1);
+    result.outputBlack = clamped(outputBlack, 0, 255, 0);
+    result.outputWhite = clamped(outputWhite, 0, 255, 255);
+    return result;
+}
+
+double LevelRange::apply(double value) const
+{
+    const LevelRange s = normalized();
+    const double input = std::min(1.0, std::max(0.0, (value * 255 - s.black) / (s.white - s.black)));
+    return (s.outputBlack + std::pow(input, 1 / s.gamma) * (s.outputWhite - s.outputBlack)) / 255;
+}
+
+LevelRange LevelsSettings::current() const
+{
+    return ranges[size_t(channel)];
+}
+
+void LevelsSettings::setCurrent(const LevelRange &range)
+{
+    ranges[size_t(channel)] = range.normalized();
+}
+
+bool LevelsSettings::isIdentity() const
+{
+    return std::all_of(ranges.begin(), ranges.end(), [](const LevelRange &range) { return range.normalized() == LevelRange(); });
+}
+
+double LevelsSettings::apply(double value, LevelsChannel channel) const
+{
+    return ranges[0].apply(ranges[size_t(channel)].apply(value));
+}
+
+double LevelsHistogramDisplay::scale(const std::array<double, 256> &bins)
+{
+    const auto counted = [](double bin) { return std::isfinite(bin) && bin > 0; };
+    double peak = 0;
+    for (const double bin : bins) {
+        if (counted(bin))
+            peak = std::max(peak, bin);
+    }
+    // The ends and lone spikes cap at four typical peaks.
+    std::vector<double> interior;
+    std::copy_if(bins.begin() + 1, bins.end() - 1, std::back_inserter(interior), counted);
+    if (interior.empty())
+        return peak;
+    std::sort(interior.begin(), interior.end());
+    const double typicalPeak = interior[size_t(double(interior.size() - 1) * 0.95)];
+    return std::min(peak, typicalPeak * 4);
+}
+
+namespace {
+// The job's image in a premultiplied context, rows packed.
+QImage drawn(const QImage &image)
+{
+    QImage context = BrushRaster::context(image.width(), image.height(), false);
+    QPainter painter(&context);
+    BrushRaster::draw(image, QRectF(0, 0, image.width(), image.height()), painter);
+    return context;
+}
+}
+
+QImage LevelsFilter::run(const LevelsJob &job)
+{
+    if (job.settings.isIdentity())
+        return job.image;
+    QImage context = drawn(job.image);
+    // On the stack: a heap failure would escape the catch.
+    std::array<float, 3 * 256> tables;
+    for (size_t channel = 0; channel < 3; ++channel) {
+        for (size_t value = 0; value <= 255; ++value)
+            tables[channel * 256 + value] = float(job.settings.apply(double(value) / 255, allLevelsChannels[channel + 1]));
+    }
+    // The kernel unpremultiplies each edge itself (Swift's cc5aac4 does twice).
+    levels_apply(context.bits(), size_t(context.width()) * size_t(context.height()), tables.data());
+    if (job.selection)
+        return PixelAdjust::blend(context, job.image, *job.selection, job.mapping, false);
+    return context;
+}
+
+LevelsHistogram LevelsFilter::histogram(const LevelsJob &job)
+{
+    const int width = job.image.width(), height = job.image.height();
+    const QImage context = drawn(job.image);
+    const std::optional<QImage> mask = job.selection ? std::optional(PixelAdjust::coverage(*job.selection, width, height, job.mapping)) : std::nullopt;
+    // The kernel accumulates: row by row, since Qt pads masks.
+    std::array<double, 1024> bins{};
+    for (int y = 0; y < height; ++y)
+        levels_histogram(context.constScanLine(y), mask ? mask->constScanLine(y) : nullptr, size_t(width), bins.data());
+    LevelsHistogram result{};
+    for (size_t channel = 0; channel < 4; ++channel)
+        std::copy_n(bins.begin() + ptrdiff_t(channel * 256), 256, result[channel].begin());
+    return result;
+}
+
+LevelsEdit::LevelsEdit(const ImageLayer &layer, std::optional<SelectionClip> selection)
+    : layerID(layer.id), original(layer.asset.value()), transform(layer.transform), selection(std::move(selection)),
+      mapping(BrushRaster::pixelToDocument(transform, original.size().width(), original.size().height()))
+{
+    const QSize size = original.size();
+    // A lookup a pixel is quick; smaller copies looked coarse.
+    const double factor = std::min(1.0, 8000.0 / std::max(size.width(), size.height()));
+    if (factor < 1) {
+        const int width = std::max(1, int(size.width() * factor)), height = std::max(1, int(size.height() * factor));
+        previewSource = BrushRaster::context(width, height, false);
+        QPainter painter(&previewSource);
+        const QRectF rect(0, 0, width, height);
+        if (original.raster)
+            original.raster->draw(rect, painter);
+        else
+            BrushRaster::draw(original.image(), rect, painter);
+        previewMapping = BrushRaster::pixelToDocument(transform, width, height);
+    } else {
+        previewSource = original.image();
+        previewMapping = mapping;
+    }
+}
+
+std::optional<QImage> LevelsEdit::previewImage(QUuid layer) const
+{
+    return layer == layerID ? preparedPreview : std::nullopt;
+}
+
+LevelsJob LevelsEdit::previewJob() const
+{
+    return LevelsJob{previewSource, settings, selection, previewMapping};
+}
+
+// Swift's session extension in Levels.swift.
+
+void EditorSession::beginLevels()
+{
+    // `canAdjustColors` reads `levels`, not `hueSaturation`.
+    if (m_hueSaturation || !canAdjustColors())
+        return;
+    // A pending gradient lands first, then Levels begins.
+    if (m_gradientEdit) {
+        commitGradient([this] { beginLevels(); });
+        return;
+    }
+    commitTransform();
+    cancelCrop();
+    cancelLasso();
+    const std::optional<DocumentSelection> current = selection();
+    try {
+        m_levels.emplace(activeLayer().value(), current ? std::optional(current->clip(m_document->size())) : std::nullopt);
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+        return;
+    }
+    countLevelsHistogram();
+    notify();
+}
+
+void EditorSession::countLevelsHistogram()
+{
+    const LevelsJob job = m_levels.value().previewJob();
+    m_levelsHistogram.setFuture(QtConcurrent::run([job]() -> std::optional<LevelsHistogram> {
+        try {
+            return LevelsFilter::histogram(job);
+        } catch (const ExportError &error) {
+            qCWarning(lcApp) << "the Levels histogram cannot be made:" << error.what();
+            return std::nullopt;
+        }
+    }));
+}
+
+void EditorSession::finishLevelsHistogram()
+{
+    const std::optional<LevelsHistogram> result = m_levelsHistogram.result();
+    // A commit cancels it, as Swift's; new edits replace it.
+    if (!m_levels || m_levels->committing)
+        return;
+    // A failed count leaves the bins empty, as Swift's try?.
+    if (result)
+        m_levels->histogram = *result;
+    m_levels->histogramReady = true;
+    notify();
+}
+
+void EditorSession::updateLevels(const LevelsSettings &settings, bool preview)
+{
+    if (!m_levels || m_levels->committing)
+        return;
+    LevelsEdit &edit = *m_levels;
+    edit.settings = settings;
+    edit.preview = preview;
+    // An adjustment layer takes the settings; its pixels stay.
+    if (previewAdjustmentEditing(preview)) {
+        notify();
+        return;
+    }
+    if (!preview || settings.isIdentity()) {
+        // Swift cancels the running preview: its result drops.
+        m_levelsPreviewFor = QUuid();
+        edit.pending = std::nullopt;
+        edit.preparedPreview = std::nullopt;
+        ++m_brushRevision;
+        notify();
+        return;
+    }
+    edit.pending = edit.previewJob();
+    renderLevelsPreview();
+    notify();
+}
+
+// Renders the newest job; one that arrives mid-render waits.
+void EditorSession::renderLevelsPreview()
+{
+    if (!m_levels || !m_levels->pending || m_levelsRendering)
+        return;
+    const LevelsJob job = *std::exchange(m_levels->pending, std::nullopt);
+    m_levelsPreviewFor = m_levels->id;
+    m_levelsRendering = true;
+    m_levelsPreview.setFuture(QtConcurrent::run([job]() -> std::optional<QImage> {
+        try {
+            return LevelsFilter::run(job);
+        } catch (const ExportError &error) {
+            qCWarning(lcApp) << "the Levels preview cannot be made:" << error.what();
+            return std::nullopt;
+        }
+    }));
+}
+
+void EditorSession::finishLevelsPreview()
+{
+    m_levelsRendering = false;
+    const std::optional<QImage> result = m_levelsPreview.result();
+    // A cancelled run, or another edit's, lands nowhere.
+    if (m_levels && m_levels->id == std::exchange(m_levelsPreviewFor, QUuid())) {
+        m_levels->preparedPreview = result;
+        ++m_brushRevision;
+        notify();
+    }
+    renderLevelsPreview();
+}
+
+void EditorSession::setLevelsSampleMode(std::optional<LevelsSample> mode)
+{
+    if (!m_levels)
+        return;
+    m_levels->sampleMode = mode;
+    ++m_brushRevision;
+    notify();
+}
+
+void EditorSession::cancelLevels()
+{
+    if (finishAdjustmentEditing(false) || !m_levels || m_levels->committing)
+        return;
+    m_levels.reset();
+    ++m_brushRevision;
+    resumeFileRequests();
+    notify();
+}
+
+void EditorSession::commitLevels(std::function<void()> done)
+{
+    // The caller resumes from the event loop, as after await.
+    const auto finish = [this, done] {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    };
+    if (finishAdjustmentEditing(true) || !m_levels || m_levels->committing) {
+        finish();
+        return;
+    }
+    if (m_levels->settings.isIdentity()) {
+        cancelLevels();
+        finish();
+        return;
+    }
+    LevelsEdit &edit = *m_levels;
+    edit.committing = true;
+    // Swift cancels the preview: its result and queue drop.
+    m_levelsPreviewFor = QUuid();
+    edit.pending = std::nullopt;
+    m_committingLevels = Committing{edit.id, std::move(done)};
+    setIsProjectBusy(true);
+    // A painted layer flattens there, where a failure is caught.
+    m_levelsCommit.setFuture(QtConcurrent::run([original = edit.original, settings = edit.settings, selection = edit.selection,
+                                                mapping = edit.mapping]() -> Leveled {
+        try {
+            const QImage image = LevelsFilter::run(LevelsJob{original.image(), settings, selection, mapping});
+            return Leveled{ImportedImage(image, PixelAdjust::thumbnail(image), QStringLiteral("Levels")), std::nullopt};
+        } catch (const ExportError &error) {
+            return Leveled{std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::finishLevelsCommit()
+{
+    const Leveled made = m_levelsCommit.result();
+    const LevelsEdit edit = std::exchange(m_levels, std::nullopt).value();
+    const Committing pending = std::exchange(m_committingLevels, std::nullopt).value();
+    // Swift's defer: the edit goes, the project frees, redraw.
+    const auto finish = [&] {
+        ++m_brushRevision;
+        setIsProjectBusy(false);
+        if (pending.done)
+            QMetaObject::invokeMethod(this, pending.done, Qt::QueuedConnection);
+    };
+    if (made.failure) {
+        setBrushError(made.failure);
+        finish();
+        return;
+    }
+    const int index = m_document ? indexOf(m_document->layers, edit.layerID) : -1;
+    if (index < 0) {
+        finish();
+        return;
+    }
+    const ImageLayer current = m_document->layers[index];
+    if (!current.asset || current.asset->identity() != edit.original.identity() || current.transform != edit.transform) {
+        finish();
+        return;
+    }
+    beginEdit(QStringLiteral("Levels"));
+    ImageLayer layer = current;
+    layer.asset = made.asset;
+    // Swift rebuilds the layer without shape, effects and text.
+    layer.shape = std::nullopt;
+    layer.effects = std::nullopt;
+    layer.text = std::nullopt;
+    m_document->layers[index] = layer;
+    endEdit();
+    finish();
+}

src/Document/Levels.h

@@ -0,0 +1,98 @@
+#pragma once
+#include "Document/LayerTransform.h"
+#include "Document/Selection.h"
+#include "IO/ImageImporter.h"
+#include <QImage>
+#include <QTransform>
+#include <QUuid>
+#include <array>
+#include <optional>
+
+struct ImageLayer;
+// Defined with the automatic levels, as Swift's.
+enum class LevelsSample;
+
+// The composite first, then each colour.
+enum class LevelsChannel { rgb, red, green, blue };
+inline constexpr std::array<LevelsChannel, 4> allLevelsChannels{LevelsChannel::rgb, LevelsChannel::red, LevelsChannel::green, LevelsChannel::blue};
+// Swift's rawValue: the channel picker's title.
+QString rawValue(LevelsChannel channel);
+
+// One channel's input range, gamma and output range.
+struct LevelRange {
+    double black = 0;
+    double gamma = 1;
+    double white = 255;
+    double outputBlack = 0;
+    double outputWhite = 255;
+    // Clamped into range; a value that is no number resets.
+    LevelRange normalized() const;
+    double apply(double value) const;
+    friend bool operator==(const LevelRange &, const LevelRange &) = default;
+};
+
+struct LevelsSettings {
+    LevelsChannel channel = LevelsChannel::rgb;
+    std::array<LevelRange, 4> ranges{};
+    LevelRange current() const;
+    // Stored normalized, as Swift's setter.
+    void setCurrent(const LevelRange &range);
+    bool isIdentity() const;
+    // The channel's range first, then the composite's.
+    double apply(double value, LevelsChannel channel) const;
+    // Swift's extension in LevelsAutomatic: a sample calibrates each channel.
+    LevelsSettings sampling(const std::array<double, 3> &rgb, LevelsSample mode) const;
+    friend bool operator==(const LevelsSettings &, const LevelsSettings &) = default;
+};
+
+// The composite's bins, then red's, green's and blue's.
+using LevelsHistogram = std::array<std::array<double, 256>, 4>;
+
+// Swift's LevelsHistogramDisplay: spikes cannot flatten the graph.
+namespace LevelsHistogramDisplay {
+double scale(const std::array<double, 256> &bins);
+}
+
+struct LevelsJob {
+    QImage image;
+    LevelsSettings settings;
+    std::optional<SelectionClip> selection;
+    QTransform mapping;
+};
+
+// Swift's LevelsFilter.
+namespace LevelsFilter {
+// The settings over the image, through the selection.
+QImage run(const LevelsJob &job);
+// Alpha-weighted; RGB is the mean of the channels, not luminance.
+LevelsHistogram histogram(const LevelsJob &job);
+}
+
+// An open Levels edit: the layer's pixels, settings and preview.
+class LevelsEdit {
+public:
+    LevelsEdit(const ImageLayer &layer, std::optional<SelectionClip> selection);
+
+    // The prepared preview, for this edit's own layer.
+    std::optional<QImage> previewImage(QUuid layer) const;
+    LevelsJob previewJob() const;
+
+    // Swift's object identity: results land on their own edit.
+    const QUuid id = QUuid::createUuid();
+    const QUuid layerID;
+    const ImportedImage original;
+    const LayerTransform transform;
+    const std::optional<SelectionClip> selection;
+    const QTransform mapping;
+    // Full size to 8000 pixels a side, else scaled down.
+    QImage previewSource;
+    QTransform previewMapping;
+    std::optional<LevelsSample> sampleMode;
+    LevelsSettings settings;
+    bool preview = true;
+    bool committing = false;
+    LevelsHistogram histogram{};
+    bool histogramReady = false;
+    std::optional<QImage> preparedPreview;
+    std::optional<LevelsJob> pending;
+};

src/Document/LevelsAutomatic+Session.h

@@ -0,0 +1,5 @@
+// Swift's LevelsAutomatic extension: automatic settings and sampling.
+public:
+    void autoLevels(LevelsAuto mode);
+    // The chosen eyedropper sets its point from the layer's pixel.
+    void sampleLevels(QPointF point);

src/Document/LevelsAutomatic.cpp

@@ -0,0 +1,153 @@
+#include "Document/LevelsAutomatic.h"
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include <QPainter>
+#include <algorithm>
+#include <cmath>
+#include <numeric>
+#include <stdexcept>
+
+QString rawValue(LevelsSample sample)
+{
+    switch (sample) {
+    case LevelsSample::black: return QStringLiteral("Black");
+    case LevelsSample::gray: return QStringLiteral("Gray");
+    case LevelsSample::white: return QStringLiteral("White");
+    }
+    throw std::logic_error("no such sample");
+}
+
+QString rawValue(LevelsAuto mode)
+{
+    switch (mode) {
+    case LevelsAuto::contrast: return QStringLiteral("Contrast");
+    case LevelsAuto::color: return QStringLiteral("Color");
+    case LevelsAuto::neutral: return QStringLiteral("Color + neutral midtones");
+    }
+    throw std::logic_error("no such mode");
+}
+
+namespace {
+// The bins holding all but a thousandth at either end.
+std::optional<std::pair<double, double>> endpoints(const std::array<double, 256> &bins)
+{
+    const double total = std::accumulate(bins.begin(), bins.end(), 0.0);
+    if (!(total > 0))
+        return std::nullopt;
+    double sum = 0;
+    int low = 0, high = 255;
+    for (int index = 0; index < 256; ++index) {
+        sum += bins[size_t(index)];
+        if (sum > total * 0.001) {
+            low = index;
+            break;
+        }
+    }
+    sum = 0;
+    for (int index = 255; index >= 0; --index) {
+        sum += bins[size_t(index)];
+        if (sum > total * 0.001) {
+            high = index;
+            break;
+        }
+    }
+    return low < high ? std::optional(std::pair(double(low), double(high))) : std::nullopt;
+}
+}
+
+LevelsSettings settings(LevelsAuto mode, const LevelsHistogram &histogram)
+{
+    LevelsSettings result;
+    if (mode == LevelsAuto::contrast) {
+        // A shared interval preserves channel relationships.
+        std::optional<double> low, high;
+        for (size_t channel = 1; channel < 4; ++channel) {
+            if (const std::optional<std::pair<double, double>> limits = endpoints(histogram[channel])) {
+                low = std::min(low.value_or(limits->first), limits->first);
+                high = std::max(high.value_or(limits->second), limits->second);
+            }
+        }
+        // Each channel's low lies below its high: so do theirs.
+        if (low)
+            result.ranges[0] = LevelRange{*low, 1, high.value(), 0, 255};
+        return result;
+    }
+    for (size_t channel = 1; channel < 4; ++channel) {
+        const std::optional<std::pair<double, double>> limits = endpoints(histogram[channel]);
+        if (!limits)
+            continue;
+        LevelRange range{limits->first, 1, limits->second, 0, 255};
+        if (mode == LevelsAuto::neutral) {
+            const double total = std::accumulate(histogram[channel].begin(), histogram[channel].end(), 0.0);
+            double weighted = 0;
+            for (size_t index = 0; index < 256; ++index)
+                weighted += range.apply(double(index) / 255) * histogram[channel][index];
+            // Weight at both ends keeps the mean inside, under 9.99.
+            range.gamma = std::max(0.1, std::log(weighted / total) / std::log(0.5));
+        }
+        result.ranges[channel] = range;
+    }
+    return result;
+}
+
+// Samples are unpremultiplied RGB; the channels calibrate together.
+LevelsSettings LevelsSettings::sampling(const std::array<double, 3> &rgb, LevelsSample mode) const
+{
+    LevelsSettings result = *this;
+    result.ranges[0] = LevelRange();
+    for (size_t channel = 1; channel < 4; ++channel) {
+        LevelRange range = result.ranges[channel];
+        const double value = rgb[channel - 1] * 255;
+        if (mode == LevelsSample::black) {
+            range.black = std::min(range.white - 1, value);
+        } else if (mode == LevelsSample::white) {
+            range.white = std::max(range.black + 1, value);
+        } else {
+            const double fraction = (value - range.black) / (range.white - range.black);
+            if (!(fraction > 0 && fraction < 1))
+                continue;
+            range.gamma = std::log(fraction) / std::log(0.5);
+        }
+        range.outputBlack = 0;
+        range.outputWhite = 255;
+        result.ranges[channel] = range.normalized();
+    }
+    return result;
+}
+
+// Swift's session extension in LevelsAutomatic.swift.
+
+void EditorSession::autoLevels(LevelsAuto mode)
+{
+    if (!m_levels || !m_levels->histogramReady || m_levels->committing)
+        return;
+    m_levels->sampleMode = std::nullopt;
+    updateLevels(::settings(mode, m_levels->histogram), m_levels->preview);
+}
+
+void EditorSession::sampleLevels(QPointF point)
+{
+    // Positive bounds: a point that is no number samples nothing.
+    if (!m_levels || !m_levels->sampleMode || m_levels->committing || !m_document
+        || !(point.x() >= 0 && point.y() >= 0 && point.x() < m_document->width && point.y() < m_document->height))
+        return;
+    const LevelsEdit &edit = *m_levels;
+    const QPointF pixel = edit.mapping.inverted().map(point);
+    const QSize size = edit.original.size();
+    if (!(pixel.x() >= 0 && pixel.y() >= 0 && pixel.x() < size.width() && pixel.y() < size.height()))
+        return;
+    try {
+        QImage context = BrushRaster::context(1, 1, false);
+        {
+            QPainter painter(&context);
+            BrushRaster::draw(edit.original.image(), QRectF(0, 0, 1, 1), painter, QRectF(std::floor(pixel.x()), std::floor(pixel.y()), 1, 1));
+        }
+        const uchar *bytes = context.constBits();
+        if (bytes[3] == 0)
+            return;
+        const auto channel = [bytes](int index) { return std::min(1.0, double(bytes[index]) / bytes[3]); };
+        updateLevels(edit.settings.sampling({channel(0), channel(1), channel(2)}, *edit.sampleMode), edit.preview);
+    } catch (const ExportError &error) {
+        setBrushError(QString::fromUtf8(error.what()));
+    }
+}

src/Document/LevelsAutomatic.h

@@ -0,0 +1,13 @@
+#pragma once
+#include "Document/Levels.h"
+
+// What a Levels eyedropper sets: black, gray or white point.
+enum class LevelsSample { black, gray, white };
+inline constexpr std::array<LevelsSample, 3> allLevelsSamples{LevelsSample::black, LevelsSample::gray, LevelsSample::white};
+QString rawValue(LevelsSample sample);
+
+// Swift's LevelsAuto: settings read from a histogram.
+enum class LevelsAuto { contrast, color, neutral };
+inline constexpr std::array<LevelsAuto, 3> allLevelsAutos{LevelsAuto::contrast, LevelsAuto::color, LevelsAuto::neutral};
+QString rawValue(LevelsAuto mode);
+LevelsSettings settings(LevelsAuto mode, const LevelsHistogram &histogram);

src/Document/LiveLayerMask+Session.h

@@ -0,0 +1,22 @@
+// Swift's LiveLayerMask extension: clipping and live masks.
+public:
+    // The layers as the export draws them, in document pixels.
+    void drawLiveComposite(const CanvasDocument &document, QPainter &context, bool onSurface = false) const;
+    // A layer clipped to `source` shows through that layer's alpha.
+    bool canLinkMask(QUuid source, QUuid target) const;
+    bool linkMask(QUuid source, QUuid target);
+    void removeLiveMask(QUuid target);
+    // Alt-click clips a layer to its lower sibling, or frees.
+    bool canToggleClippingMask(QUuid id) const;
+    void toggleClippingMask(QUuid id);
+    static void adoptClipping(QUuid id, std::vector<ImageLayer> &layers);
+    static void releaseDetachedClipping(std::vector<ImageLayer> &layers);
+    // Asks: bake, unlink or cancel. False when nothing depends.
+    bool deleteWithLiveMaskChoice(const std::vector<QUuid> &ids);
+    using BakedImages = std::map<QUuid, ImportedImage>;
+    void finishDeletingLayer(QUuid id, const BakedImages &baked);
+    void finishDeletingLayers(const std::vector<QUuid> &ids, const BakedImages &baked);
+
+private:
+    std::optional<QUuid> clippingBase(const ImageLayer &layer) const;
+    void finishBake();

src/Document/LiveLayerMask.cpp

@@ -0,0 +1,384 @@
+#include "Document/LiveLayerMask.h"
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "Document/LayerEffects+Renderer.h"
+#include "Document/PixelAdjust.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/AdjustmentSurface.h"
+#include "Rendering/LayerRenderer.h"
+#include "Rendering/LiveMaskRenderer.h"
+#include <QHash>
+#include <QMessageBox>
+#include <QPushButton>
+#include <QSet>
+#include <QtConcurrent>
+#include <map>
+#include <stdexcept>
+
+void EditorSession::drawLiveComposite(const CanvasDocument &document, QPainter &context, bool onSurface) const
+{
+    // Adjustments read the pixels beneath: a surface of their own.
+    if (!onSurface && std::any_of(document.layers.begin(), document.layers.end(), [](const ImageLayer &layer) { return layer.adjustment.has_value(); })) {
+        AdjustmentSurface::draw(context, [&](QPainter &surface) { drawLiveComposite(document, surface, true); });
+        return;
+    }
+    std::map<QUuid, ImageLayer> records;
+    for (const ImageLayer &layer : document.layers) {
+        if (!records.emplace(layer.id, layer).second)
+            throw std::logic_error("a document repeats a layer id");
+    }
+    const auto parent = [&](QUuid id) { return records.contains(id) ? records.at(id).parentID : std::nullopt; };
+    LiveMaskRenderer live([&](QUuid id) { return records.contains(id) ? records.at(id).maskSourceID : std::nullopt; },
+                          [&](QUuid id, QPainter &target, const QImage &clip) {
+                              if (!records.contains(id) || !records.at(id).asset)
+                                  return;
+                              const ImageLayer &layer = records.at(id);
+                              const QImage image = layer.asset->image();
+                              const LayerTransform transform = displayedTransform(layer);
+                              const std::optional<QImage> mask =
+                                  layer.mask ? layer.mask->clipImage(displayedMaskPlacement(layer), transform, image.width(), image.height()) : std::nullopt;
+                              // With effects the mask is in their image already.
+                              if (const auto effects = LayerEffectsRenderer::cached(image, mask, layer.effects)) {
+                                  const LayerTransform grown = LayerEffectsRenderer::placed(transform, effects->image, effects->inset);
+                                  LayerRenderer::draw(effects->image, grown, grown.center(), target,
+                                                      {.opacity = layer.opacity, .blendMode = displayedBlendMode(layer), .clip = clip});
+                                  return;
+                              }
+                              LayerRenderer::draw(image, transform, transform.center(), target,
+                                                  {.opacity = layer.opacity, .blendMode = displayedBlendMode(layer), .mask = mask.value_or(QImage()), .clip = clip});
+                          });
+    live.adjustment = [&](QUuid id) { return records.contains(id) ? records.at(id).adjustment : std::nullopt; };
+    live.adjustmentOpacity = [&](QUuid id) { return records.at(id).opacity; };
+    live.adjustmentClip = [&](QUuid id, const QPainter &painter, QImage &coverage) {
+        const ImageLayer &layer = records.at(id);
+        if (const std::optional<QImage> image = layer.mask ? layer.mask->enabledImage() : std::nullopt)
+            FolderMaskClip{*image, layer.transform}.apply(layer.transform.center(), painter, coverage);
+    };
+    std::vector<QUuid> ids;
+    for (const ImageLayer &layer : document.renderLayers())
+        ids.push_back(layer.id);
+    live.prepareStacks(ids, parent, [&](QUuid id) { return records.contains(id) ? displayedBlendMode(records.at(id)) : LayerBlendMode::normal; });
+    FolderMaskClip::draw(ids, parent, [&](QUuid id) -> std::optional<FolderMaskClip::Applier> {
+        if (!records.contains(id) || !records.at(id).mask)
+            return std::nullopt;
+        const ImageLayer &folder = records.at(id);
+        const std::optional<QImage> image = folder.mask->enabledImage();
+        if (!image)
+            return std::nullopt;
+        const FolderMaskClip clip{*image, displayedTransform(folder)};
+        return [clip](const QPainter &painter, QImage &coverage) { clip.apply(clip.transform.center(), painter, coverage); };
+    }, context, [&](QUuid id, const QImage &clip) { live.drawComposite(id, context, clip); });
+}
+
+void LiveMaskGraph::validate(const std::vector<ProjectLayerRecord> &layers)
+{
+    QHash<QUuid, const ProjectLayerRecord *> records;
+    for (const ProjectLayerRecord &layer : layers) {
+        if (records.contains(layer.id))
+            throw ProjectError(ProjectError::Kind::invalid);
+        records.insert(layer.id, &layer);
+    }
+    for (const ProjectLayerRecord &layer : layers) {
+        QSet<QUuid> path;
+        std::optional<QUuid> current = layer.id;
+        while (current) {
+            const ProjectLayerRecord *record = records.value(*current);
+            if (path.size() >= 256 || path.contains(*current) || !record)
+                throw ProjectError(ProjectError::Kind::invalid);
+            path.insert(*current);
+            if (record->maskSourceID) {
+                const ProjectLayerRecord *source = records.value(*record->maskSourceID);
+                // Folders never clip; nothing clips to an adjustment.
+                if (record->isGroup == true || !source || source->isGroup == true || source->adjustment)
+                    throw ProjectError(ProjectError::Kind::invalid);
+            }
+            current = record->maskSourceID;
+        }
+    }
+}
+
+std::optional<ImportedImage> LiveMaskBaker::bake(const ProjectSnapshot &snapshot, QUuid target)
+{
+    const std::vector<ProjectLayerRecord> &layers = snapshot.manifest.layers;
+    const size_t found = std::find_if(layers.begin(), layers.end(), [&](const ProjectLayerRecord &layer) { return layer.id == target; }) - layers.begin();
+    if (found == layers.size() || !snapshot.images.contains(target))
+        return std::nullopt;
+    // Checked reads: a lost guard aborts or throws.
+    const ImportedImage &original = snapshot.images.at(target);
+    const int width = original.size().width(), height = original.size().height();
+    QImage surface = BrushRaster::context(width, height, false);
+    const QTransform inverse = BrushRaster::pixelToDocument(layers[found].transform, width, height).inverted();
+    // Swift's dictionary traps on an id that repeats.
+    QHash<QUuid, const ProjectLayerRecord *> records;
+    for (const ProjectLayerRecord &layer : layers) {
+        if (records.contains(layer.id))
+            throw std::logic_error("a snapshot to bake repeats a layer id");
+        records.insert(layer.id, &layer);
+    }
+    LiveMaskRenderer live([&](QUuid id) { return records.contains(id) ? records.value(id)->maskSourceID : std::nullopt; },
+                          [&](QUuid id, QPainter &context, const QImage &clip) {
+                              // `at` is checked: end() would read wild memory.
+                              if (!records.contains(id) || !snapshot.images.contains(id))
+                                  return;
+                              const QImage pixels = snapshot.images.at(id).image();
+                              // Only the live dependency bakes: mask and appearance stay.
+                              if (id == target) {
+                                  LayerRenderer::draw(pixels, {.origin = {0, 0}, .size = QSizeF(width, height)}, QPointF(width / 2.0, height / 2.0),
+                                                      context, {.clip = clip});
+                                  return;
+                              }
+                              const ProjectLayerRecord &layer = *records.value(id);
+                              const std::optional<LayerMask> mask = snapshot.mask(layer);
+                              const std::optional<QImage> shown =
+                                  mask ? mask->clipImage(mask->placement, layer.transform, pixels.width(), pixels.height()) : std::nullopt;
+                              context.save();
+                              context.setTransform(inverse, true);
+                              LayerRenderer::draw(pixels, layer.transform, layer.transform.center(), context,
+                                                  {.opacity = layer.opacity.value_or(1), .mask = shown.value_or(QImage()), .clip = clip});
+                              context.restore();
+                          });
+    {
+        QPainter painter(&surface);
+        live.draw(target, painter);
+    }
+    return ImportedImage(surface, PixelAdjust::thumbnail(surface), original.name);
+}
+
+bool EditorSession::canLinkMask(QUuid source, QUuid target) const
+{
+    if (!canEditLayers())
+        return false;
+    bool found = false;
+    std::vector<ProjectLayerRecord> records;
+    for (const ImageLayer &layer : m_document->layers) {
+        records.push_back(layer.hierarchyRecord());
+        if (layer.id == target) {
+            records.back().maskSourceID = source;
+            found = true;
+        }
+    }
+    if (!found)
+        return false;
+    // The graph refuses itself, folders, strangers and rings.
+    try {
+        LiveMaskGraph::validate(records);
+    } catch (const ProjectError &) {
+        return false;
+    }
+    return true;
+}
+
+bool EditorSession::linkMask(QUuid source, QUuid target)
+{
+    if (!canLinkMask(source, target))
+        return false;
+    ImageLayer &layer = m_document->layers[indexOf(m_document->layers, target)];
+    if (layer.maskSourceID == source)
+        return true;
+    beginEdit(QStringLiteral("Create Clipping Mask"));
+    layer.maskSourceID = source;
+    endEdit();
+    return true;
+}
+
+void EditorSession::adoptClipping(QUuid id, std::vector<ImageLayer> &layers)
+{
+    const int moved = indexOf(layers, id);
+    if (moved < 0 || layers[moved].isGroup)
+        return;
+    std::vector<const ImageLayer *> siblings;
+    for (const ImageLayer &layer : layers) {
+        if (layer.parentID == layers[moved].parentID)
+            siblings.push_back(&layer);
+    }
+    const size_t own = std::find_if(siblings.begin(), siblings.end(), [&](const ImageLayer *layer) { return layer->id == id; }) - siblings.begin();
+    // Between a base or its stack and a clipped layer.
+    if (own == 0 || own + 1 == siblings.size())
+        return;
+    const std::optional<QUuid> source = siblings[own + 1]->maskSourceID;
+    const ImageLayer &below = *siblings[own - 1];
+    if (source && *source != id && (below.id == *source || below.maskSourceID == source))
+        layers[moved].maskSourceID = source;
+}
+
+void EditorSession::removeLiveMask(QUuid target)
+{
+    if (!canEditLayers())
+        return;
+    const int clipped = indexOf(m_document->layers, target);
+    if (clipped < 0 || !m_document->layers[clipped].maskSourceID)
+        return;
+    // Those above it on the same base let go too.
+    const std::optional<QUuid> source = m_document->layers[clipped].maskSourceID, parent = m_document->layers[clipped].parentID;
+    beginEdit(QStringLiteral("Release Clipping Mask"));
+    bool reached = false;
+    for (ImageLayer &layer : m_document->layers) {
+        if (layer.parentID != parent)
+            continue;
+        reached = reached || layer.id == target;
+        if (reached && layer.id != target && layer.maskSourceID != source)
+            break;
+        if (reached)
+            layer.maskSourceID = std::nullopt;
+    }
+    endEdit();
+}
+
+bool EditorSession::canToggleClippingMask(QUuid id) const
+{
+    if (!canEditLayers())
+        return false;
+    const std::vector<ImageLayer> &layers = m_document->layers;
+    const int layer = indexOf(layers, id);
+    if (layer < 0)
+        return false;
+    if (layers[layer].maskSourceID)
+        return true;
+    const std::optional<QUuid> base = clippingBase(layers[layer]);
+    return base && canLinkMask(*base, id);
+}
+
+std::optional<QUuid> EditorSession::clippingBase(const ImageLayer &layer) const
+{
+    // The next lower sibling, or the base that one shares.
+    const ImageLayer *below = nullptr;
+    for (const ImageLayer &each : m_document->layers) {
+        if (each.id == layer.id)
+            break;
+        if (each.parentID == layer.parentID)
+            below = &each;
+    }
+    return below ? std::optional(below->maskSourceID.value_or(below->id)) : std::nullopt;
+}
+
+void EditorSession::toggleClippingMask(QUuid id)
+{
+    if (!canEditLayers())
+        return;
+    const std::vector<ImageLayer> &layers = m_document->layers;
+    const int layer = indexOf(layers, id);
+    if (layer < 0)
+        return;
+    if (layers[layer].maskSourceID) {
+        removeLiveMask(id);
+        return;
+    }
+    if (const std::optional<QUuid> base = clippingBase(layers[layer]))
+        linkMask(*base, id);
+}
+
+void EditorSession::releaseDetachedClipping(std::vector<ImageLayer> &layers)
+{
+    // A clipped layer sits in one run above its base.
+    std::map<std::optional<QUuid>, std::optional<QUuid>> bases;
+    for (ImageLayer &layer : layers) {
+        std::optional<QUuid> &base = bases[layer.parentID];
+        if (!layer.maskSourceID) {
+            base = layer.id;
+        } else if (layer.maskSourceID != base) {
+            layer.maskSourceID = std::nullopt;
+            base = layer.id;
+        }
+    }
+}
+
+bool EditorSession::deleteWithLiveMaskChoice(const std::vector<QUuid> &ids)
+{
+    if (!m_document)
+        return false;
+    QSet<QUuid> removed;
+    for (const QUuid id : ids) {
+        removed.unite(descendantIDs(id));
+        removed.insert(id);
+    }
+    std::vector<QUuid> targets;
+    for (const ImageLayer &layer : m_document->layers) {
+        if (!removed.contains(layer.id) && layer.maskSourceID && removed.contains(*layer.maskSourceID))
+            targets.push_back(layer.id);
+    }
+    if (targets.empty())
+        return false;
+    QMessageBox alert;
+    alert.setIcon(QMessageBox::Warning);
+    alert.setText(ids.size() == 1 ? QStringLiteral("This layer supplies a live mask") : QStringLiteral("These layers supply live masks"));
+    alert.setInformativeText(QStringLiteral("Bake keeps the current masked appearance in the dependent layers’ pixels. "
+                                            "Remove Links reveals their pixels. You can undo either choice."));
+    const QPushButton *bake = alert.addButton(QStringLiteral("Bake and Delete"), QMessageBox::AcceptRole);
+    alert.addButton(QStringLiteral("Cancel"), QMessageBox::RejectRole);
+    const QPushButton *unlink = alert.addButton(QStringLiteral("Remove Links and Delete"), QMessageBox::DestructiveRole);
+    alert.exec();
+    if (alert.clickedButton() == unlink) {
+        finishDeletingLayers(ids, {});
+        return true;
+    }
+    const std::optional<ProjectSnapshot> snapshot = projectSnapshot();
+    if (alert.clickedButton() != bake || !snapshot)
+        return true;
+    qCInfo(lcApp) << "baking" << int(targets.size()) << "live masks before a deletion";
+    setIsProjectBusy(true);
+    m_bakingIDs = ids;
+    m_baker.setFuture(QtConcurrent::run([snapshot = *snapshot, targets]() -> Baked {
+        Baked result;
+        try {
+            for (const QUuid target : targets) {
+                if (const std::optional<ImportedImage> image = LiveMaskBaker::bake(snapshot, target))
+                    result.images.insert({target, *image});
+            }
+        } catch (const ExportError &error) {
+            result.failure = QString::fromUtf8(error.what());
+        }
+        return result;
+    }));
+    return true;
+}
+
+void EditorSession::finishBake()
+{
+    const Baked baked = m_baker.result();
+    if (baked.failure) {
+        qCWarning(lcApp).noquote() << "cannot bake the live masks:" << *baked.failure;
+        setBrushError(baked.failure);
+    } else {
+        finishDeletingLayers(m_bakingIDs, baked.images);
+    }
+    setIsProjectBusy(false);
+}
+
+void EditorSession::finishDeletingLayer(QUuid id, const BakedImages &baked)
+{
+    if (!m_document)
+        return;
+    std::vector<ImageLayer> &layers = m_document->layers;
+    const int index = indexOf(layers, id);
+    if (index < 0)
+        return;
+    QSet<QUuid> removed = descendantIDs(id);
+    removed.insert(id);
+    beginEdit(QStringLiteral("Delete Layer"));
+    std::erase_if(layers, [&](const ImageLayer &layer) { return removed.contains(layer.id); });
+    // Who was clipped to them is freed, baked where asked.
+    for (ImageLayer &layer : layers) {
+        if (!layer.maskSourceID || !removed.contains(*layer.maskSourceID))
+            continue;
+        layer.maskSourceID = std::nullopt;
+        if (const auto image = baked.find(layer.id); image != baked.end())
+            layer.asset = image->second;
+    }
+    if (m_activeLayerID && removed.contains(*m_activeLayerID))
+        setActiveLayerID(layers.empty() ? std::nullopt : std::optional(layers[std::min(index, int(layers.size()) - 1)].id));
+    endEdit();
+}
+
+void EditorSession::finishDeletingLayers(const std::vector<QUuid> &ids, const BakedImages &baked)
+{
+    if (ids.size() <= 1) {
+        if (!ids.empty())
+            finishDeletingLayer(ids.front(), baked);
+        return;
+    }
+    beginEdit(QStringLiteral("Delete Layers"));
+    for (const QUuid id : ids)
+        finishDeletingLayer(id, baked);
+    endEdit();
+}

src/Document/LiveLayerMask.h

@@ -0,0 +1,12 @@
+#pragma once
+#include "IO/ProjectStore.h"
+#include <vector>
+
+namespace LiveMaskGraph {
+void validate(const std::vector<ProjectLayerRecord> &layers);
+}
+
+namespace LiveMaskBaker {
+// The target's pixels through its live mask; nil without pixels.
+std::optional<ImportedImage> bake(const ProjectSnapshot &snapshot, QUuid target);
+}

src/Document/MagicWand+Session.h

@@ -0,0 +1,8 @@
+// Swift's MagicWand extension.
+public:
+    // Off the UI thread; `done` follows the step or refusal.
+    void magicWand(QPointF point, SelectionMode mode, std::function<void()> done = {});
+
+private:
+    void finishWand();
+    std::optional<QImage> wandSample(const CanvasDocument &document) const;

src/Document/MagicWand.cpp

@@ -0,0 +1,174 @@
+#include "Document/MagicWand.h"
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include "Rendering/LayerRenderer.h"
+#include <QtConcurrent>
+#include <algorithm>
+#include <cmath>
+#include <cstdlib>
+extern "C" {
+#include "WandPixels.h"
+}
+
+QString title(WandSampleSize size)
+{
+    switch (size) {
+    case WandSampleSize::point:
+        return QStringLiteral("Point Sample");
+    case WandSampleSize::threeByThree:
+        return QStringLiteral("3 by 3 Average");
+    case WandSampleSize::fiveByFive:
+        return QStringLiteral("5 by 5 Average");
+    }
+    throw std::logic_error("unknown sample size");
+}
+
+int radius(WandSampleSize size)
+{
+    return int(size);
+}
+
+MagicWandError::MagicWandError(Kind kind)
+    : std::runtime_error(kind == Kind::tooDetailed
+                             ? "That selection is too detailed to outline. Try a different Tolerance, or turn on Contiguous."
+                             : "There isn’t enough memory to make that selection."),
+      kind(kind)
+{
+}
+
+std::optional<QPainterPath> MagicWand::select(const QImage &image, QPointF point, const WandSettings &settings)
+{
+    const int width = image.width(), height = image.height();
+    if (!std::isfinite(point.x()) || !std::isfinite(point.y()))
+        return std::nullopt;
+    const double fx = std::floor(point.x()), fy = std::floor(point.y());
+    if (fx < 0 || fy < 0 || fx >= width || fy >= height)
+        return std::nullopt;
+    // Premultiplied RGBA, rows top down, as the kernel reads.
+    QImage rgba = BrushRaster::context(width, height, false);
+    QPainter painter(&rgba);
+    BrushRaster::draw(image, QRectF(0, 0, width, height), painter);
+    painter.end();
+    std::vector<uchar> selected(size_t(width) * size_t(height));
+    const long count = wand_mask(rgba.constBits(), size_t(width), size_t(height), size_t(rgba.bytesPerLine()), size_t(fx), size_t(fy),
+                                 size_t(radius(settings.sampleSize)), std::clamp(settings.tolerance, 0, 255), settings.contiguous ? 1 : 0, selected.data());
+    if (count < 0)
+        throw MagicWandError(MagicWandError::Kind::memory);
+    if (count == 0)
+        return std::nullopt;
+    return outline(selected, width, height);
+}
+
+std::optional<QPainterPath> MagicWand::outline(const std::vector<uchar> &mask, int width, int height)
+{
+    if (width <= 0 || height <= 0 || mask.size() != size_t(width) * size_t(height))
+        return std::nullopt;
+    int32_t *points = nullptr, *loops = nullptr;
+    size_t pointCount = 0, loopCount = 0;
+    const int status = wand_trace(mask.data(), size_t(width), size_t(height), &points, &pointCount, &loops, &loopCount);
+    const std::unique_ptr<int32_t, decltype(&std::free)> ownedPoints(points, &std::free), ownedLoops(loops, &std::free);
+    if (status == -2)
+        throw MagicWandError(MagicWandError::Kind::tooDetailed);
+    if (status != 0)
+        throw MagicWandError(MagicWandError::Kind::memory);
+    if (loopCount == 0)
+        return std::nullopt;
+    // Outer loops clockwise, holes the other way: winding fills them.
+    QPainterPath path;
+    path.setFillRule(Qt::WindingFill);
+    size_t index = 0;
+    for (size_t loop = 0; loop < loopCount; ++loop) {
+        const size_t length = size_t(loops[loop]);
+        path.moveTo(points[index * 2], points[index * 2 + 1]);
+        for (size_t corner = index + 1; corner < index + length; ++corner)
+            path.lineTo(points[corner * 2], points[corner * 2 + 1]);
+        path.closeSubpath();
+        index += length;
+    }
+    return path;
+}
+
+void EditorSession::setWandSettings(const WandSettings &settings)
+{
+    m_wandSettings = settings;
+    notify();
+}
+
+// What the wand reads: composite, or the active layer's pixels.
+std::optional<QImage> EditorSession::wandSample(const CanvasDocument &document) const
+{
+    try {
+        QImage context = BrushRaster::context(document.width, document.height, false);
+        QPainter painter(&context);
+        // A painted asset flattens here; the catch covers it too.
+        if (m_wandSettings.sampleAllLayers) {
+            drawLiveComposite(document, painter);
+        } else if (const std::optional<ImageLayer> layer = activeLayer(); layer && !layer->isGroup && layer->asset) {
+            const LayerTransform transform = displayedTransform(*layer);
+            LayerRenderer::draw(layer->asset->image(), transform, transform.center(), painter, {});
+        }
+        painter.end();
+        return context;
+    } catch (const ExportError &error) {
+        qCWarning(lcApp) << "the wand cannot sample the canvas:" << error.what();
+        return std::nullopt;
+    }
+}
+
+void EditorSession::magicWand(QPointF point, SelectionMode mode, std::function<void()> done)
+{
+    // The caller resumes from the event loop, as after await.
+    const auto finish = [this, done] {
+        if (done)
+            QMetaObject::invokeMethod(this, done, Qt::QueuedConnection);
+    };
+    const std::optional<CanvasDocument> &document = m_document;
+    // Swift's positive bounds refuse a point that is no number.
+    const bool inside = document && point.x() >= 0 && point.y() >= 0 && point.x() < document->width && point.y() < document->height;
+    if (!canEditSelection() || m_isProjectBusy || m_selectionMoveOrigin || !inside) {
+        finish();
+        return;
+    }
+    const std::optional<QImage> sample = wandSample(*document);
+    if (!sample) {
+        finish();
+        return;
+    }
+    setIsProjectBusy(true);
+    m_wanding = Wanding{mode, document->id, std::move(done)};
+    m_wand.setFuture(QtConcurrent::run([sample = *sample, point, settings = m_wandSettings]() -> Wanded {
+        try {
+            return Wanded{MagicWand::select(sample, point, settings), std::nullopt};
+        } catch (const MagicWandError &error) {
+            return Wanded{std::nullopt, QString::fromUtf8(error.what())};
+        } catch (const ExportError &error) {
+            return Wanded{std::nullopt, QString::fromUtf8(error.what())};
+        }
+    }));
+}
+
+void EditorSession::finishWand()
+{
+    const Wanded result = m_wand.result();
+    const Wanding pending = std::exchange(m_wanding, std::nullopt).value();
+    setIsProjectBusy(false);
+    if (result.failure) {
+        qCWarning(lcApp).noquote() << "the wand could not select:" << *result.failure;
+        setBrushError(result.failure);
+    } else if (m_document && m_document->id == pending.documentID) {
+        if (!result.path) {
+            // Nothing matched: New clears, as a lasso click enclosing nothing.
+            if (pending.mode == SelectionMode::replace)
+                deselect();
+        } else if (pending.mode == SelectionMode::replace) {
+            // A traced outline lies on the canvas: no costly clip.
+            setSelection(DocumentSelection{*result.path, m_selectionAntialiased}, QStringLiteral("Magic Wand"));
+        } else {
+            applySelection(*result.path, pending.mode, QStringLiteral("Magic Wand"));
+        }
+    }
+    if (pending.done)
+        QMetaObject::invokeMethod(this, pending.done, Qt::QueuedConnection);
+}

src/Document/MagicWand.h

@@ -0,0 +1,40 @@
+#pragma once
+#include <QImage>
+#include <QPainterPath>
+#include <QPointF>
+#include <QString>
+#include <optional>
+#include <stdexcept>
+#include <vector>
+
+// Pixels either side of the click, averaged into the reference.
+enum class WandSampleSize { point, threeByThree, fiveByFive };
+QString title(WandSampleSize size);
+int radius(WandSampleSize size);
+
+// The Magic Wand's options-bar settings.
+struct WandSettings {
+    // How far each channel may differ, 0–255, and still match.
+    int tolerance = 32;
+    WandSampleSize sampleSize = WandSampleSize::point;
+    // Only similar pixels connected to the clicked one.
+    bool contiguous = true;
+    // The visible composite rather than the active layer.
+    bool sampleAllLayers = false;
+    friend bool operator==(const WandSettings &, const WandSettings &) = default;
+};
+
+class MagicWandError : public std::runtime_error {
+public:
+    enum class Kind { tooDetailed, memory };
+    explicit MagicWandError(Kind kind);
+    const Kind kind;
+};
+
+// Selects pixels like a clicked one; the C kernel matches.
+namespace MagicWand {
+// The outline of pixels matching the click; nil when none.
+std::optional<QPainterPath> select(const QImage &image, QPointF point, const WandSettings &settings);
+// The outline of a mask's nonzero pixels along their edges.
+std::optional<QPainterPath> outline(const std::vector<uchar> &mask, int width, int height);
+}

src/Document/MaskTracing+Session.h

@@ -0,0 +1,4 @@
+// Swift's MaskTracing extension: thumbnails loaded as selections.
+public:
+    void loadMaskSelection(QUuid layerID, SelectionMode mode = SelectionMode::replace);
+    void loadLayerSelection(QUuid layerID, SelectionMode mode = SelectionMode::replace);

src/Document/MaskTracing.cpp

@@ -0,0 +1,146 @@
+#include "Document/MaskTracing.h"
+#include "Document/BrushStroke.h"
+#include "Document/EditorSession.h"
+#include "IO/ImageExporter.h"
+#include "Logging.h"
+#include <QPainter>
+#include <functional>
+#include <map>
+#include <vector>
+
+namespace {
+// Outer loops run clockwise, holes counterclockwise, for winding fill.
+std::optional<QPainterPath> trace(const QImage &image, bool alpha, const std::function<bool(uchar)> &test)
+{
+    const int width = image.width(), height = image.height();
+    if (width <= 0 || height <= 0)
+        return std::nullopt;
+    const int channels = alpha ? 4 : 1;
+    // Drawn over a cleared bitmap, as the Swift tracer does.
+    QImage pixels(width, height, alpha ? QImage::Format_RGBA8888_Premultiplied : QImage::Format_Grayscale8);
+    if (pixels.isNull())
+        return std::nullopt;
+    pixels.fill(0);
+    QPainter(&pixels).drawImage(QRectF(0, 0, width, height), image);
+    const auto selected = [&](int x, int y) {
+        return x >= 0 && y >= 0 && x < width && y < height
+            && test(pixels.constScanLine(y)[x * channels + channels - 1]);
+    };
+    // Directed unit edges, keyed by their start vertex.
+    const int stride = width + 1;
+    std::map<int, std::vector<int>> outgoing;
+    const auto edge = [&](int x0, int y0, int x1, int y1) { outgoing[y0 * stride + x0].push_back(y1 * stride + x1); };
+    for (int y = 0; y < height; ++y) {
+        for (int x = 0; x < width; ++x) {
+            if (!selected(x, y))
+                continue;
+            if (!selected(x, y - 1))
+                edge(x, y, x + 1, y);
+            if (!selected(x + 1, y))
+                edge(x + 1, y, x + 1, y + 1);
+            if (!selected(x, y + 1))
+                edge(x + 1, y + 1, x, y + 1);
+            if (!selected(x - 1, y))
+                edge(x, y + 1, x, y);
+        }
+    }
+    if (outgoing.empty())
+        return std::nullopt;
+    QPainterPath path;
+    path.setFillRule(Qt::WindingFill);
+    while (!outgoing.empty()) {
+        const int start = outgoing.begin()->first;
+        std::vector<int> loop;
+        int current = start;
+        do {
+            const auto found = outgoing.find(current);
+            if (found == outgoing.end())
+                break;
+            const int end = found->second.back();
+            found->second.pop_back();
+            if (found->second.empty())
+                outgoing.erase(found);
+            loop.push_back(current);
+            current = end;
+        } while (current != start);
+        // Keep corners; drop vertices that continue straight.
+        std::vector<QPointF> corners;
+        const int count = int(loop.size());
+        for (int index = 0; index < count; ++index) {
+            const int vertex = loop[index], previous = loop[(index + count - 1) % count], following = loop[(index + 1) % count];
+            const int inX = vertex % stride - previous % stride, inY = vertex / stride - previous / stride;
+            const int outX = following % stride - vertex % stride, outY = following / stride - vertex / stride;
+            if (inX != outX || inY != outY)
+                corners.push_back(QPointF(vertex % stride, vertex / stride));
+        }
+        if (corners.size() < 3)
+            continue;
+        path.moveTo(corners.front());
+        for (std::size_t index = 1; index < corners.size(); ++index)
+            path.lineTo(corners[index]);
+        path.closeSubpath();
+    }
+    return path.isEmpty() ? std::nullopt : std::optional(path);
+}
+}
+
+std::optional<QPainterPath> MaskTracing::darkPixels(const QImage &image)
+{
+    return trace(image, false, [](uchar value) { return value < 128; });
+}
+
+std::optional<QPainterPath> MaskTracing::opaquePixels(const QImage &image)
+{
+    return trace(image, true, [](uchar value) { return value >= 128; });
+}
+
+void EditorSession::loadMaskSelection(QUuid layerID, SelectionMode mode)
+{
+    if (!canEditSelection())
+        return;
+    const int index = indexOf(m_document->layers, layerID);
+    if (index < 0 || !m_document->layers[size_t(index)].mask)
+        return;
+    const ImageLayer &layer = m_document->layers[size_t(index)];
+    QImage mask;
+    // A painted mask flattens here; a failure loads nothing.
+    try {
+        mask = layer.mask->asset.image();
+    } catch (const ExportError &error) {
+        qCWarning(lcApp).noquote() << "cannot read the mask of" << layer.name << ":" << error.what();
+        return;
+    }
+    const std::optional<QPainterPath> traced = MaskTracing::darkPixels(mask);
+    // Swift beeps: nothing black to select.
+    if (!traced) {
+        qCWarning(lcApp).noquote() << "no black to select in the mask of" << layer.name;
+        return;
+    }
+    applySelection(BrushRaster::pixelToDocument(layer.maskTransform(), mask.width(), mask.height()).map(*traced), mode,
+                   QStringLiteral("Load Mask Selection"));
+}
+
+void EditorSession::loadLayerSelection(QUuid layerID, SelectionMode mode)
+{
+    const int index = canEditSelection() ? indexOf(m_document->layers, layerID) : -1;
+    // Swift beeps: no pixels, or none opaque.
+    if (index < 0 || m_document->layers[size_t(index)].isGroup || !m_document->layers[size_t(index)].asset) {
+        qCWarning(lcApp) << "no pixels to select for layer" << layerID.toString();
+        return;
+    }
+    const ImageLayer &layer = m_document->layers[size_t(index)];
+    QImage image;
+    try {
+        image = layer.asset->image();
+    } catch (const ExportError &error) {
+        qCWarning(lcApp).noquote() << "cannot read the pixels of" << layer.name << ":" << error.what();
+        return;
+    }
+    const std::optional<QPainterPath> traced = MaskTracing::opaquePixels(image);
+    if (!traced) {
+        qCWarning(lcApp).noquote() << "no opaque pixels to select in" << layer.name;
+        return;
+    }
+    applySelection(BrushRaster::pixelToDocument(layer.transform, image.width(), image.height()).map(*traced), mode,
+                   QStringLiteral("Load Layer Selection"));
+}

src/Document/MaskTracing.h

@@ -0,0 +1,10 @@
+#pragma once
+#include <QImage>
+#include <QPainterPath>
+#include <optional>
+
+// Raster coverage traced into an outline along exact pixel edges.
+namespace MaskTracing {
+std::optional<QPainterPath> darkPixels(const QImage &image);
+std::optional<QPainterPath> opaquePixels(const QImage &image);
+}

src/Document/PixelAdjust.cpp

@@ -0,0 +1,236 @@
+#include "Document/PixelAdjust.h"
+#include "Rendering/PoolMap.h"
+#include "Document/BrushStroke.h"
+#include "IO/ImageExporter.h"
+#include <QtConcurrent>
+#include <algorithm>
+#include <cmath>
+#include <memory>
+#include <new>
+#include <numeric>
+#include <stdexcept>
+
+namespace {
+using Float4 = float __attribute__((vector_size(16)));
+
+// A pixel in floats: gray has one lane, colour four.
+template <typename Pixel> Pixel load(const uchar *bytes);
+template <> float load<float>(const uchar *bytes)
+{
+    return float(bytes[0]);
+}
+template <> Float4 load<Float4>(const uchar *bytes)
+{
+    return Float4{float(bytes[0]), float(bytes[1]), float(bytes[2]), float(bytes[3])};
+}
+
+// Rounded once, as a render to eight bits does.
+void store(float value, uchar *bytes)
+{
+    bytes[0] = uchar(std::min(255.0f, std::max(0.0f, value) + 0.5f));
+}
+void store(Float4 value, uchar *bytes)
+{
+    for (int c = 0; c < 4; ++c)
+        store(value[c], bytes + c);
+}
+
+// Normalised weights from the centre out, three sigmas long.
+std::vector<float> gaussian(double sigma)
+{
+    const int reach = int(std::ceil(3 * sigma));
+    std::vector<double> exact(size_t(reach) + 1);
+    double total = 0;
+    for (int i = 0; i <= reach; ++i) {
+        exact[size_t(i)] = std::exp(-double(i) * i / (2 * sigma * sigma));
+        total += i == 0 ? exact[0] : 2 * exact[size_t(i)];
+    }
+    std::vector<float> weights;
+    for (const double weight : exact)
+        weights.push_back(float(weight / total));
+    return weights;
+}
+
+// Rows for a parallel map; no memory: a render error.
+std::vector<int> rowNumbers(int height)
+{
+    try {
+        std::vector<int> rows(static_cast<size_t>(height));
+        std::iota(rows.begin(), rows.end(), 0);
+        return rows;
+    } catch (const std::bad_alloc &) {
+        throw ExportError(ExportError::Kind::render);
+    }
+}
+
+// Columns a block at a time keep the taps cached.
+constexpr int block = 64;
+
+template <typename Pixel> void blur(const QImage &image, const std::vector<float> &weights, bool clamped, Pixel *across, QImage &result)
+{
+    const int width = image.width(), height = image.height(), reach = int(weights.size()) - 1, size = int(sizeof(Pixel) / sizeof(float));
+    const uchar *const in = image.constBits();
+    uchar *const out = result.bits();
+    const qsizetype inStride = image.bytesPerLine(), outStride = result.bytesPerLine();
+    // Past the edge: the edge itself, or nothing.
+    const auto source = [&](int at, int limit) { return clamped ? std::clamp(at, 0, limit - 1) : at; };
+    std::vector<int> rows = rowNumbers(height);
+    PoolMap::blocking(rows, [&](int y) {
+        const uchar *row = in + y * inStride;
+        Pixel *line = across + size_t(y) * size_t(width);
+        for (int x = 0; x < width; ++x) {
+            Pixel sum = weights[0] * load<Pixel>(row + x * size);
+            // Away from the edges every tap is inside: no checks.
+            if (x >= reach && x + reach < width) {
+                for (int i = 1; i <= reach; ++i)
+                    sum += weights[size_t(i)] * (load<Pixel>(row + (x - i) * size) + load<Pixel>(row + (x + i) * size));
+            } else {
+                for (int i = 1; i <= reach; ++i) {
+                    const int left = source(x - i, width), right = source(x + i, width);
+                    if (left >= 0)
+                        sum += weights[size_t(i)] * load<Pixel>(row + left * size);
+                    if (right < width)
+                        sum += weights[size_t(i)] * load<Pixel>(row + right * size);
+                }
+            }
+            line[x] = sum;
+        }
+    });
+    PoolMap::blocking(rows, [&](int y) {
+        uchar *line = out + y * outStride;
+        for (int start = 0; start < width; start += block) {
+            const int count = std::min(block, width - start);
+            Pixel sums[block];
+            for (int x = 0; x < count; ++x)
+                sums[x] = weights[0] * across[size_t(y) * size_t(width) + size_t(start + x)];
+            for (int i = -reach; i <= reach; ++i) {
+                const int from = source(y + i, height);
+                if (i == 0 || from < 0 || from >= height)
+                    continue;
+                const Pixel *taps = across + size_t(from) * size_t(width) + size_t(start);
+                const float weight = weights[size_t(std::abs(i))];
+                for (int x = 0; x < count; ++x)
+                    sums[x] += weight * taps[x];
+            }
+            for (int x = 0; x < count; ++x)
+                store(sums[x], line + (start + x) * size);
+        }
+    });
+}
+}
+
+QImage PixelAdjust::thumbnail(const QImage &image)
+{
+    const double factor = std::min(1.0, 96.0 / std::max(image.width(), image.height()));
+    const int width = std::max(1, int(image.width() * factor)), height = std::max(1, int(image.height() * factor));
+    QImage result = BrushRaster::context(width, height, false);
+    QPainter painter(&result);
+    BrushRaster::draw(image, QRectF(0, 0, width, height), painter);
+    return result;
+}
+
+QImage PixelAdjust::coverage(const SelectionClip &selection, int width, int height, const QTransform &pixelToDocument)
+{
+    QImage mask = BrushRaster::context(width, height, true);
+    // An empty selection covers nothing.
+    if (!selection.coverage)
+        return mask;
+    QPainter painter(&mask);
+    painter.setTransform(pixelToDocument.inverted());
+    painter.setRenderHint(QPainter::SmoothPixmapTransform, true);
+    painter.drawImage(selection.rect, *selection.coverage);
+    return mask;
+}
+
+QImage PixelAdjust::blend(const QImage &adjusted, const QImage &original, const SelectionClip &selection,
+                          const QTransform &pixelToDocument, bool isMask)
+{
+    const int width = adjusted.width(), height = adjusted.height();
+    if (original.size() != adjusted.size() || adjusted.format() != original.format())
+        throw std::logic_error("blend needs two images of one size and format");
+    const QImage mask = coverage(selection, width, height, pixelToDocument);
+    QImage result = BrushRaster::context(width, height, isMask);
+    const int channels = isMask ? 1 : 4;
+    for (int y = 0; y < height; ++y) {
+        const uchar *top = adjusted.constScanLine(y), *bottom = original.constScanLine(y), *weight = mask.constScanLine(y);
+        uchar *target = result.scanLine(y);
+        for (int x = 0; x < width; ++x) {
+            const int m = weight[x];
+            for (int channel = 0; channel < channels; ++channel) {
+                const int index = x * channels + channel;
+                target[index] = uchar((top[index] * m + bottom[index] * (255 - m) + 127) / 255);
+            }
+        }
+    }
+    return result;
+}
+
+QImage PixelAdjust::gaussianBlur(const QImage &image, double sigma, bool clamped)
+{
+    const bool gray = image.format() == QImage::Format_Grayscale8;
+    if (!gray && image.format() != QImage::Format_RGBA8888_Premultiplied)
+        throw std::logic_error("a blur takes premultiplied RGBA or gray");
+    if (!(sigma > 0) || !std::isfinite(sigma))
+        throw std::logic_error("a blur needs a positive sigma");
+    const std::vector<float> weights = gaussian(sigma);
+    const size_t pixels = size_t(image.width()) * size_t(image.height());
+    QImage result = BrushRaster::context(image.width(), image.height(), gray);
+    // Allocated here, where a failure throws, before the workers run.
+    if (gray) {
+        const std::unique_ptr<float[]> across(new (std::nothrow) float[pixels]);
+        if (!across)
+            throw ExportError(ExportError::Kind::render);
+        blur<float>(image, weights, clamped, across.get(), result);
+    } else {
+        const std::unique_ptr<Float4[]> across(new (std::nothrow) Float4[pixels]);
+        if (!across)
+            throw ExportError(ExportError::Kind::render);
+        blur<Float4>(image, weights, clamped, across.get(), result);
+    }
+    return result;
+}
+
+QImage PixelAdjust::motionBlur(const QImage &image, double radius, double angle)
+{
+    if (image.format() != QImage::Format_RGBA8888_Premultiplied)
+        throw std::logic_error("a motion blur takes premultiplied RGBA");
+    if (!(radius > 0) || !std::isfinite(radius) || !std::isfinite(angle))
+        throw std::logic_error("a motion blur needs a finite positive radius and angle");
+    // Measured on a dot: a Gaussian taper of its radius.
+    const std::vector<float> weights = gaussian(radius);
+    const int width = image.width(), height = image.height(), reach = int(weights.size()) - 1;
+    // Core Image's y points up: counterclockwise climbs the screen.
+    const double dx = std::cos(angle), dy = -std::sin(angle);
+    QImage result = BrushRaster::context(width, height, false);
+    const uchar *const in = image.constBits();
+    uchar *const out = result.bits();
+    const qsizetype inStride = image.bytesPerLine(), outStride = result.bytesPerLine();
+    // Bilinear, clear past the edges, as an unclamped image reads.
+    const auto sample = [&](double x, double y) {
+        const double left = std::floor(x), top = std::floor(y);
+        const float across = float(x - left), down = float(y - top);
+        Float4 sum{0, 0, 0, 0};
+        for (int j = 0; j < 2; ++j) {
+            const int row = int(top) + j;
+            if (row < 0 || row >= height)
+                continue;
+            for (int i = 0; i < 2; ++i) {
+                const int column = int(left) + i;
+                if (column >= 0 && column < width)
+                    sum += (j ? down : 1 - down) * (i ? across : 1 - across) * load<Float4>(in + row * inStride + column * 4);
+            }
+        }
+        return sum;
+    };
+    std::vector<int> rows = rowNumbers(height);
+    PoolMap::blocking(rows, [&](int y) {
+        uchar *line = out + y * outStride;
+        for (int x = 0; x < width; ++x) {
+            Float4 sum = weights[0] * load<Float4>(in + y * inStride + x * 4);
+            for (int i = 1; i <= reach; ++i)
+                sum += weights[size_t(i)] * (sample(x + i * dx, y + i * dy) + sample(x - i * dx, y - i * dy));
+            store(sum, line + x * 4);
+        }
+    });
+    return result;
+}

src/Document/PixelAdjust.h

@@ -0,0 +1,19 @@
+#pragma once
+#include "Document/Selection.h"
+#include <QImage>
+#include <QTransform>
+
+// Shared plumbing for whole-image adjustments.
+namespace PixelAdjust {
+// A small preview for the Layers panel, like an import's.
+QImage thumbnail(const QImage &image);
+// Selection coverage on the image's own pixel grid.
+QImage coverage(const SelectionClip &selection, int width, int height, const QTransform &pixelToDocument);
+// coverage × adjusted + (1 − coverage) × original.
+QImage blend(const QImage &adjusted, const QImage &original, const SelectionClip &selection,
+             const QTransform &pixelToDocument, bool isMask);
+// Core Image's Gaussian on stored values; `clamped` repeats the edge.
+QImage gaussianBlur(const QImage &image, double sigma, bool clamped);
+// Core Image's motion blur: a Gaussian taper along the angle.
+QImage motionBlur(const QImage &image, double radius, double angle);
+}