1 files changed, 51 insertions, 0 deletions
diff --git a/gfx/cairo/libpixman/src/pixman-dither.h b/gfx/cairo/libpixman/src/pixman-dither.h
new file mode 100644
index 000000000..ead9f38d8
--- /dev/null
+++ b/gfx/cairo/libpixman/src/pixman-dither.h
@@ -0,0 +1,51 @@
+#define R16_BITS     5
+#define G16_BITS     6
+#define B16_BITS     5
+
+#define R16_SHIFT    (B16_BITS + G16_BITS)
+#define G16_SHIFT    (B16_BITS)
+#define B16_SHIFT    0
+
+#define MASK 0xff
+#define ONE_HALF 0x80
+
+#define A_SHIFT 8 * 3
+#define R_SHIFT 8 * 2
+#define G_SHIFT 8
+#define A_MASK 0xff000000
+#define R_MASK 0xff0000
+#define G_MASK 0xff00
+
+#define RB_MASK 0xff00ff
+#define AG_MASK 0xff00ff00
+#define RB_ONE_HALF 0x800080
+#define RB_MASK_PLUS_ONE 0x10000100
+
+#define ALPHA_8(x) ((x) >> A_SHIFT)
+#define RED_8(x) (((x) >> R_SHIFT) & MASK)
+#define GREEN_8(x) (((x) >> G_SHIFT) & MASK)
+#define BLUE_8(x) ((x) & MASK)
+
+// This uses the same dithering technique that Skia does.
+// It is essentially preturbing the lower bit based on the
+// high bit
+static inline uint16_t dither_32_to_16(uint32_t c)
+{
+    uint8_t b = BLUE_8(c);
+    uint8_t g = GREEN_8(c);
+    uint8_t r = RED_8(c);
+    r = ((r << 1) - ((r >> (8 - R16_BITS) << (8 - R16_BITS)) | (r >> R16_BITS))) >> (8 - R16_BITS);
+    g = ((g << 1) - ((g >> (8 - G16_BITS) << (8 - G16_BITS)) | (g >> G16_BITS))) >> (8 - G16_BITS);
+    b = ((b << 1) - ((b >> (8 - B16_BITS) << (8 - B16_BITS)) | (b >> B16_BITS))) >> (8 - B16_BITS);
+    return ((r << R16_SHIFT) | (g << G16_SHIFT) | (b << B16_SHIFT));
+}
+
+static inline uint16_t dither_8888_to_0565(uint32_t color, pixman_bool_t toggle)
+{
+    // alternate between a preturbed truncation and a regular truncation
+    if (toggle) {
+	return dither_32_to_16(color);
+    } else {
+	return convert_8888_to_0565(color);
+    }
+}