Implement fix for alpha channel dithering (#15)

Author: booky10

plugin/src/main/java/de/pianoman911/mapengine/core/colors/ColorPalette.java

@@ -48,7 +48,10 @@ public final byte color(Color color) {
     }
 
     @Override
-    public final byte color(int rgb) {
+    public final byte color(final int rgb) {
+        if (((rgb >> 24) & 0xFF) < 128) {
+            return 0;
+        }
         return this.colors[rgb & 0xFFFFFF];
     }
 
@@ -64,7 +67,7 @@ public byte[] colors(int[] rgb, int threads) {
                 if (finalI == threads - 1) end = rgb.length;
                 for (int j = start; j < end; j++) {
                     int color = rgb[j];
-                    if (((color >> 24) & 0xFF) != 255) continue;
+                    if (((color >> 24) & 0xFF) < 128) continue;
                     result[j] = this.color(rgb[j]);
                 }
             });
@@ -247,8 +250,13 @@ public final int[] toRGBs(byte[] colors) {
         return rgb;
     }
 
-    public final int closestColor(int rgb) {
-        return this.reverseColors[rgb & 0xFFFFFF];
+    public final int closestColor(final int rgb) {
+        final int alpha = (rgb >> 24) & 0xFF;
+        if (alpha == 0) {
+            return 0;
+        }
+        final int ret = this.reverseColors[rgb & 0xFFFFFF];
+        return (ret & 0xFFFFFF) | (alpha << 24);
     }
 
     private void checkRetry() {

plugin/src/main/java/de/pianoman911/mapengine/core/colors/dithering/FloydSteinbergDithering.java

@@ -54,6 +54,7 @@ public static ColorBuffer dither(FullSpacedColorBuffer buffer, ColorPalette pale
 
                         int mc = palette.closestColor(rgb);
 
+                        int a;
                         int r = (mc >> 16) & 0xFF;
                         int g = (mc >> 8) & 0xFF;
                         int b = (mc) & 0xFF;
@@ -67,36 +68,40 @@ public static ColorBuffer dither(FullSpacedColorBuffer buffer, ColorPalette pale
                         if (!(x == w - 1)) {
                             index = x + 1 + y * w;
                             rgb = src[index];
+                            a = (rgb >> 24) & 0xFF;
                             r = Math.max(0, Math.min(255, (int) (((rgb >> 16) & 0xFF) + (errorR * FS_ERROR))));
                             g = Math.max(0, Math.min(255, (int) (((rgb >> 8) & 0xFF) + (errorG * FS_ERROR))));
                             b = Math.max(0, Math.min(255, (int) (((rgb) & 0xFF) + (errorB * FS_ERROR))));
-                            src[index] = (r << 16) | (g << 8) | b;
+                            src[index] = (a << 24) | (r << 16) | (g << 8) | b;
 
                             if (!(y == h - 1)) {
                                 index = x + 1 + (y + 1) * w;
                                 rgb = src[index];
+                                a = (rgb >> 24) & 0xFF;
                                 r = Math.max(0, Math.min(255, (int) (((rgb >> 16) & 0xFF) + (errorR * FS_ERROR2))));
                                 g = Math.max(0, Math.min(255, (int) (((rgb >> 8) & 0xFF) + (errorG * FS_ERROR2))));
                                 b = Math.max(0, Math.min(255, (int) (((rgb) & 0xFF) + (errorB * FS_ERROR2))));
-                                src[index] = (r << 16) | (g << 8) | b;
+                                src[index] = (a << 24) | (r << 16) | (g << 8) | b;
                             }
                         }
 
                         if (!(y == h - 1)) {
                             index = x + (y + 1) * w;
                             rgb = src[index];
+                            a = (rgb >> 24) & 0xFF;
                             r = Math.max(0, Math.min(255, (int) (((rgb >> 16) & 0xFF) + (errorR * FS_ERROR3))));
                             g = Math.max(0, Math.min(255, (int) (((rgb >> 8) & 0xFF) + (errorG * FS_ERROR3))));
                             b = Math.max(0, Math.min(255, (int) (((rgb) & 0xFF) + (errorB * FS_ERROR3))));
-                            src[index] = (r << 16) | (g << 8) | b;
+                            src[index] = (a << 24) | (r << 16) | (g << 8) | b;
 
                             if (!(x == 0)) {
                                 index = x - 1 + (y + 1) * w;
                                 rgb = src[index];
+                                a = (rgb >> 24) & 0xFF;
                                 r = Math.max(0, Math.min(255, (int) (((rgb >> 16) & 0xFF) + (errorR * FS_ERROR4))));
                                 g = Math.max(0, Math.min(255, (int) (((rgb >> 8) & 0xFF) + (errorG * FS_ERROR4))));
                                 b = Math.max(0, Math.min(255, (int) (((rgb) & 0xFF) + (errorB * FS_ERROR4))));
-                                src[index] = (r << 16) | (g << 8) | b;
+                                src[index] = (a << 24) | (r << 16) | (g << 8) | b;
                             }
                         }
                     }