Filter › Dither, as Compositor 1.3.3 (12.15)

kernels/DitherPixels.h

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+#ifndef DitherPixels_h
+#define DitherPixels_h
+#include <stdint.h>
+#include <stddef.h>
+
+// The dither styles, in the order the Filter panel lists them.
+enum {
+    DITHER_ATKINSON, DITHER_FLOYD_STEINBERG,
+    DITHER_BAYER_2, DITHER_BAYER_4, DITHER_BAYER_8,
+    DITHER_DOTS, DITHER_LINES, DITHER_DIAMONDS,
+    DITHER_PATTERNS, DITHER_GLYPHS
+};
+
+typedef struct {
+    int style;
+    // Tones per channel for diffusion and ordered styles, 2–8. Two is pure 1-bit.
+    int levels;
+    // How much of each pixel's error diffusion passes on, 0–1.
+    float diffusion;
+    // −1…1: darker (more ink) or lighter, and flatter or punchier, before dithering.
+    float density;
+    float contrast;
+    // Halftone and glyph cells, in pixels, and the halftone screen's angle in radians.
+    int cell;
+    float angle;
+    // Halftone dots, patterns and glyphs mark the light tones on the dark color instead of the dark on the light.
+    int lightOnDark;
+    // 0: the result is made of `dark` and `light` (straight sRGB). 1: it keeps the image's own colors.
+    int originalColors;
+    uint8_t dark[3];
+    uint8_t light[3];
+    // Glyphs: `glyphCount` coverage maps of `cell` × `cell` bytes (255 is fully inked), from least inked to most,
+    // with each map's mean coverage (0–1) in `glyphCoverage`.
+    const uint8_t *glyphs;
+    const float *glyphCoverage;
+    int glyphCount;
+} DitherParams;
+
+// Dithers premultiplied RGBA pixels (4 bytes per pixel, `stride` bytes per row) in place. Alpha is kept and fully
+// transparent pixels are left alone. Returns 0 if working memory couldn't be had.
+int dither_apply(uint8_t *rgba, size_t width, size_t height, size_t stride, const DitherParams *params);
+// Turns each `block` × `block` square of premultiplied RGBA pixels into a round dot in its own color on `gap` (straight
+// sRGB), like the lit pixels of a dot-matrix screen. The dot's edge is smoothed and alpha is kept.
+void dither_dots(uint8_t *rgba, size_t width, size_t height, size_t stride, int block, const uint8_t *gap);
+#endif