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Filter › Dither, as Compositor 1.3.3 (12.15)

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雷电芽衣 committed 2026-09-30 07:55:57 -04:00
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kernels/DitherPixels.c
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#include "DitherPixels.h"
#include <math.h>
#include <stdlib.h>
static inline float clamp01(float v) { return v < 0 ? 0 : v > 1 ? 1 : v; }
// Density darkens (positive) or lightens as a gamma, so black and white stay put; contrast pivots on mid gray.
static inline float adjust_tone(float v, float gamma, float contrast) {
v = powf(clamp01(v), gamma);
return clamp01((v - 0.5f) * contrast + 0.5f);
}
// One error-diffusion kernel: neighbors to the right on this row and below, with their weights over `divisor`.
typedef struct { int dx, dy, weight; } Tap;
typedef struct { const Tap *taps; int count; float divisor; } Kernel;
static const Tap atkinson[] = { {1,0,1}, {2,0,1}, {-1,1,1}, {0,1,1}, {1,1,1}, {0,2,1} };
static const Tap floyd[] = { {1,0,7}, {-1,1,3}, {0,1,5}, {1,1,1} };
// Atkinson passes on only six eighths of the error, which is what gives the Mac's crisp, contrasty look.
static Kernel kernel_for(int style) {
return style == DITHER_ATKINSON ? (Kernel){ atkinson, 6, 8 } : (Kernel){ floyd, 4, 16 };
}
static inline float quantize(float v, int levels) {
float steps = (float)(levels - 1);
return roundf(clamp01(v) * steps) / steps;
}
// Diffuses each plane in serpentine order, so the error's drift doesn't streak to one side.
static void diffuse(float *plane, const uint8_t *alpha, size_t width, size_t height, const DitherParams *p) {
Kernel k = kernel_for(p->style);
for (size_t y = 0; y < height; ++y) {
int reverse = (int)(y & 1);
for (size_t i = 0; i < width; ++i) {
size_t x = reverse ? width - 1 - i : i;
size_t at = y * width + x;
if (!alpha[at]) continue;
float old = plane[at], q = quantize(old, p->levels);
plane[at] = q;
float error = (old - q) * p->diffusion / k.divisor;
for (int t = 0; t < k.count; ++t) {
long nx = (long)x + (reverse ? -k.taps[t].dx : k.taps[t].dx), ny = (long)y + k.taps[t].dy;
if (nx < 0 || nx >= (long)width || ny >= (long)height) continue;
plane[(size_t)ny * width + (size_t)nx] += error * (float)k.taps[t].weight;
}
}
}
}
static const uint8_t bayer8[64] = {
0, 32, 8, 40, 2, 34, 10, 42, 48, 16, 56, 24, 50, 18, 58, 26,
12, 44, 4, 36, 14, 46, 6, 38, 60, 28, 52, 20, 62, 30, 54, 22,
3, 35, 11, 43, 1, 33, 9, 41, 51, 19, 59, 27, 49, 17, 57, 25,
15, 47, 7, 39, 13, 45, 5, 37, 63, 31, 55, 23, 61, 29, 53, 21,
};
// The ordered threshold for a pixel, in [0, 1). Smaller Bayer matrices are the top-left corners of the 8 × 8 one,
// rescaled, which is how the recursive construction nests them.
static inline float ordered_threshold(int style, size_t x, size_t y) {
switch (style) {
case DITHER_BAYER_2: { static const uint8_t m[4] = { 0, 2, 3, 1 }; return ((float)m[(y & 1) * 2 + (x & 1)] + 0.5f) / 4; }
case DITHER_BAYER_4: {
static const uint8_t m[16] = { 0, 8, 2, 10, 12, 4, 14, 6, 3, 11, 1, 9, 15, 7, 13, 5 };
return ((float)m[(y & 3) * 4 + (x & 3)] + 0.5f) / 16;
}
default: return ((float)bayer8[(y & 7) * 8 + (x & 7)] + 0.5f) / 64;
}
}
static inline float ordered(float v, float threshold, int levels) {
float steps = (float)(levels - 1);
float q = floorf(clamp01(v) * steps + threshold);
return (q > steps ? steps : q) / steps;
}
// How much of a halftone cell a point must be covered by before it's marked, for each screen shape. `u` and `v`
// run from −0.5 to 0.5 across the cell; the shapes grow from its middle as coverage rises.
static inline float spot(int style, float u, float v) {
float au = fabsf(u), av = fabsf(v);
switch (style) {
case DITHER_DOTS: return 3.14159265f * (u * u + v * v);
case DITHER_LINES: return av * 2;
default: return au + av;
}
}
// Old Mac fill patterns, 8 × 8, one byte per row with the leftmost pixel in the top bit, from sparsest to fullest.
static const uint8_t patterns[][8] = {
{ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 },
{ 0x80, 0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00 },
{ 0x88, 0x00, 0x22, 0x00, 0x88, 0x00, 0x22, 0x00 },
{ 0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01 },
{ 0x88, 0x22, 0x88, 0x22, 0x88, 0x22, 0x88, 0x22 },
{ 0x00, 0xFF, 0x00, 0x00, 0x00, 0xFF, 0x00, 0x00 },
{ 0x11, 0x22, 0x44, 0x88, 0x11, 0x22, 0x44, 0x88 },
{ 0xAA, 0x00, 0xAA, 0x00, 0xAA, 0x00, 0xAA, 0x00 },
{ 0x88, 0x55, 0x22, 0x55, 0x88, 0x55, 0x22, 0x55 },
{ 0xFF, 0x80, 0x80, 0x80, 0xFF, 0x08, 0x08, 0x08 },
{ 0xAA, 0x55, 0xAA, 0x55, 0xAA, 0x55, 0xAA, 0x55 },
{ 0x81, 0x42, 0x24, 0x18, 0x18, 0x24, 0x42, 0x81 },
{ 0x77, 0xAA, 0xDD, 0xAA, 0x77, 0xAA, 0xDD, 0xAA },
{ 0xEE, 0xDD, 0xBB, 0x77, 0xEE, 0xDD, 0xBB, 0x77 },
{ 0x77, 0xFF, 0xDD, 0xFF, 0x77, 0xFF, 0xDD, 0xFF },
{ 0x7F, 0xFF, 0xFF, 0xFF, 0xF7, 0xFF, 0xFF, 0xFF },
{ 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF },
};
static const int patternCount = (int)(sizeof patterns / sizeof patterns[0]);
static inline void write_pixel(uint8_t *px, float r, float g, float b) {
float a = (float)px[3] / 255.0f;
px[0] = (uint8_t)lroundf(clamp01(r) * a * 255.0f);
px[1] = (uint8_t)lroundf(clamp01(g) * a * 255.0f);
px[2] = (uint8_t)lroundf(clamp01(b) * a * 255.0f);
}
int dither_apply(uint8_t *rgba, size_t width, size_t height, size_t stride, const DitherParams *p) {
size_t count = width * height;
if (!count) return 1;
int planes = p->originalColors ? 3 : 1;
float *tone = malloc(count * sizeof(float) * (size_t)planes);
uint8_t *alpha = malloc(count);
// The image's own colors, unadjusted: halftone dots and glyphs take them in Original mode.
float *source = p->originalColors ? malloc(count * sizeof(float) * 3) : NULL;
if (!tone || !alpha || (p->originalColors && !source)) { free(tone); free(alpha); free(source); return 0; }
float gamma = exp2f(p->density * 1.5f);
float contrast = p->contrast >= 0 ? 1.0f / (1.0f - 0.95f * p->contrast) : 1.0f + p->contrast;
for (size_t y = 0; y < height; ++y) {
const uint8_t *row = rgba + y * stride;
for (size_t x = 0; x < width; ++x) {
const uint8_t *px = row + x * 4;
size_t at = y * width + x;
alpha[at] = px[3];
float r = 0, g = 0, b = 0;
if (px[3]) {
float scale = 1.0f / (float)px[3];
r = px[0] * scale; g = px[1] * scale; b = px[2] * scale;
}
if (p->originalColors) {
tone[at] = adjust_tone(r, gamma, contrast);
tone[count + at] = adjust_tone(g, gamma, contrast);
tone[2 * count + at] = adjust_tone(b, gamma, contrast);
source[at * 3] = r; source[at * 3 + 1] = g; source[at * 3 + 2] = b;
} else {
tone[at] = adjust_tone(0.2126f * r + 0.7152f * g + 0.0722f * b, gamma, contrast);
}
}
}
float dark[3] = { p->dark[0] / 255.0f, p->dark[1] / 255.0f, p->dark[2] / 255.0f };
float light[3] = { p->light[0] / 255.0f, p->light[1] / 255.0f, p->light[2] / 255.0f };
int style = p->style;
int levels = p->levels < 2 ? 2 : p->levels > 16 ? 16 : p->levels;
if (style <= DITHER_BAYER_8) {
// Diffusion and ordered dithering: each plane is quantized to `levels` tones, then mapped to colors.
if (style <= DITHER_FLOYD_STEINBERG) {
DitherParams local = *p;
local.levels = levels;
for (int c = 0; c < planes; ++c) diffuse(tone + (size_t)c * count, alpha, width, height, &local);
} else {
for (int c = 0; c < planes; ++c) {
float *plane = tone + (size_t)c * count;
for (size_t y = 0; y < height; ++y)
for (size_t x = 0; x < width; ++x) {
size_t at = y * width + x;
if (alpha[at]) plane[at] = ordered(plane[at], ordered_threshold(style, x, y), levels);
}
}
}
for (size_t y = 0; y < height; ++y) {
uint8_t *row = rgba + y * stride;
for (size_t x = 0; x < width; ++x) {
size_t at = y * width + x;
if (!alpha[at]) continue;
if (p->originalColors) {
write_pixel(row + x * 4, tone[at], tone[count + at], tone[2 * count + at]);
} else {
float t = tone[at];
write_pixel(row + x * 4, dark[0] + (light[0] - dark[0]) * t, dark[1] + (light[1] - dark[1]) * t,
dark[2] + (light[2] - dark[2]) * t);
}
}
}
} else {
// Marks (halftone shapes, patterns, glyphs) cover as much of each spot as the tone calls for. On light, they
// stand for darkness and are drawn in the dark color; light on dark, the reverse.
float *marks = p->originalColors ? malloc(count * sizeof(float)) : tone;
if (!marks) { free(tone); free(alpha); free(source); return 0; }
if (p->originalColors)
for (size_t i = 0; i < count; ++i)
marks[i] = 0.2126f * tone[i] + 0.7152f * tone[count + i] + 0.0722f * tone[2 * count + i];
int cell = p->cell < 2 ? 2 : p->cell;
float cosA = cosf(p->angle), sinA = sinf(p->angle);
float *ink = p->lightOnDark ? light : dark, *paper = p->lightOnDark ? dark : light;
// Glyphs: each cell shares one, picked from the cell's average tone, worked out once per cell.
size_t columns = (width + (size_t)cell - 1) / (size_t)cell, cellRows = (height + (size_t)cell - 1) / (size_t)cell;
int *picked = NULL;
if (style == DITHER_GLYPHS && p->glyphCount > 0) {
picked = malloc(columns * cellRows * sizeof(int));
if (!picked) { if (marks != tone) free(marks); free(tone); free(alpha); free(source); return 0; }
for (size_t row = 0; row < cellRows; ++row)
for (size_t column = 0; column < columns; ++column) {
float sum = 0; int n = 0;
for (size_t yy = row * (size_t)cell; yy < (row + 1) * (size_t)cell && yy < height; ++yy)
for (size_t xx = column * (size_t)cell; xx < (column + 1) * (size_t)cell && xx < width; ++xx) {
size_t i = yy * width + xx;
if (alpha[i]) { sum += marks[i]; ++n; }
}
float t = n ? sum / (float)n : 1;
float wanted = (p->lightOnDark ? t : 1 - t) * p->glyphCoverage[p->glyphCount - 1];
int best = 0;
float bestDistance = 2;
for (int g = 0; g < p->glyphCount; ++g) {
float d = fabsf(p->glyphCoverage[g] - wanted);
if (d < bestDistance) { bestDistance = d; best = g; }
}
picked[row * columns + column] = best;
}
}
// Original colors: marks take the pixel's own color, on black (light on dark) or white.
float paperOriginal = p->lightOnDark ? 0.0f : 1.0f;
for (size_t y = 0; y < height; ++y) {
uint8_t *row = rgba + y * stride;
for (size_t x = 0; x < width; ++x) {
size_t at = y * width + x;
if (!alpha[at]) continue;
float amount;
if (picked) {
int glyph = picked[(y / (size_t)cell) * columns + x / (size_t)cell];
amount = p->glyphs[(size_t)glyph * (size_t)(cell * cell) + (y % (size_t)cell) * (size_t)cell + x % (size_t)cell] / 255.0f;
} else if (style == DITHER_PATTERNS) {
float t = marks[at];
float coverage = p->lightOnDark ? t : 1 - t;
int index = (int)lroundf(coverage * (float)(patternCount - 1));
amount = (patterns[index][y & 7] >> (7 - (x & 7))) & 1;
} else {
float fx = (float)x + 0.5f, fy = (float)y + 0.5f;
float u = (fx * cosA + fy * sinA) / (float)cell, v = (-fx * sinA + fy * cosA) / (float)cell;
u -= floorf(u) + 0.5f; v -= floorf(v) + 0.5f;
float t = marks[at];
amount = (p->lightOnDark ? t : 1 - t) > spot(style, u, v) ? 1 : 0;
}
if (p->originalColors) {
const float *s = source + at * 3;
write_pixel(row + x * 4, paperOriginal + (s[0] - paperOriginal) * amount,
paperOriginal + (s[1] - paperOriginal) * amount, paperOriginal + (s[2] - paperOriginal) * amount);
} else {
write_pixel(row + x * 4, paper[0] + (ink[0] - paper[0]) * amount, paper[1] + (ink[1] - paper[1]) * amount,
paper[2] + (ink[2] - paper[2]) * amount);
}
}
}
free(picked);
if (marks != tone) free(marks);
}
free(tone); free(alpha); free(source);
return 1;
}
void dither_dots(uint8_t *rgba, size_t width, size_t height, size_t stride, int block, const uint8_t *gap) {
if (block < 2) return;
float radius = (float)block * 0.42f, middle = (float)block / 2;
for (size_t y = 0; y < height; ++y) {
uint8_t *row = rgba + y * stride;
float dy = (float)(y % (size_t)block) + 0.5f - middle;
for (size_t x = 0; x < width; ++x) {
uint8_t *px = row + x * 4;
if (!px[3]) continue;
float dx = (float)(x % (size_t)block) + 0.5f - middle;
float cover = clamp01(radius - sqrtf(dx * dx + dy * dy) + 0.5f);
if (cover >= 1) continue;
for (int c = 0; c < 3; ++c)
px[c] = (uint8_t)lroundf((float)px[c] * cover + (float)gap[c] * (float)px[3] / 255.0f * (1 - cover));
}
}
}