Merge pull request #293 from SolderedElectronics/new-color-dither-method

Add new reduced diffusion Floyd steinberg kernel for brighter colors when showing paintings

Author: JosipKuci

src/graphics/ImageColor/ImageColor.h

@@ -58,7 +58,8 @@ class ImageColor
         Atkinson,
         Burkes,
         Stucki,
-        SierraLite
+        SierraLite,
+        ReducedDiffusion // Floyd-Steinberg pattern at ~69% strength — more vibrant, less washed-out
     } DitherKernel;
 
     struct bitmapHeader

src/graphics/ImageColor/ImageDitherColor.cpp

@@ -46,8 +46,12 @@ uint8_t ImageColor::findClosestPalette(int16_t r, int16_t g, int16_t b)
         int32_t dg = g - pg;
         int32_t db = b - pb;
 
-        // Perceptual weighted RGB distance (Rec.601)
-        int32_t currentDistance = 30 * dr * dr + 59 * dg * dg + 11 * db * db;
+        // Perceptual weighted RGB distance.
+        // Rec.601 weights (30,59,11) give green a dominant 59% share, which causes
+        // yellow (high R+G, zero B) to win far too often on limited e-ink palettes.
+        // Using (30,40,30) reduces green dominance and raises the blue penalty,
+        // so yellow only wins when a pixel genuinely has negligible blue content.
+        int32_t currentDistance = 30 * dr * dr + 40 * dg * dg + 30 * db * db;
 
         if (currentDistance < minDistance)
         {

src/graphics/ImageColor/ImageDitherColorKernels.h

@@ -57,10 +57,15 @@ static const unsigned char _kernelSierraLite[] = {
     0, 0, 2, 1, 1, 0, 0, 0, 0,
 };
 
+static const unsigned char _kernelReducedDiffusion[] = {
+    0, 0, 5, 2, 3, 1,
+};
+
 static const DitherKernelDef DITHER_KERNELS[] = {
-    {3, 2, 1, 16, _kernelFloydSteinberg}, {5, 3, 2, 48, _kernelJarvisJudiceNinke},
-    {4, 3, 1, 8, _kernelAtkinson},        {5, 3, 2, 32, _kernelBurkes},
-    {5, 3, 2, 42, _kernelStucki},         {3, 3, 1, 4, _kernelSierraLite},
+    {3, 2, 1, 16, _kernelFloydSteinberg},   {5, 3, 2, 48, _kernelJarvisJudiceNinke},
+    {4, 3, 1, 8, _kernelAtkinson},          {5, 3, 2, 32, _kernelBurkes},
+    {5, 3, 2, 42, _kernelStucki},           {3, 3, 1, 4, _kernelSierraLite},
+    {3, 2, 1, 26, _kernelReducedDiffusion},
 };
 
 static const uint8_t DITHER_KERNEL_COUNT = sizeof DITHER_KERNELS / sizeof DITHER_KERNELS[0];