Slight improvement to indexed png

Premultiply partial alpha colors above threshold


Former-commit-id: e3180b0889327530c971611680ae34e71d24db79
Former-commit-id: 135d6264188a87e221d38e68e9c905782c4a5d7e
Former-commit-id: 546f847865693d9b0a72f73438cd9a1d83c3a669

src/ImageProcessorCore/Formats/Png/PngEncoderCore.cs

@@ -516,11 +516,19 @@ namespace ImageProcessorCore.Formats
                 {
                     int offset = i * 3;
                     Color color = new Color(palette[i].ToVector4());
+                    int alpha = color.A;
+
+                    // Premultiply the color. This helps prevent banding.
+                    if (alpha < 255 && alpha > this.Threshold)
+                    {
+                        color = Color.Multiply(color, new Color(alpha, alpha, alpha, 255));
+                    }
+
                     colorTable[offset] = color.R;
                     colorTable[offset + 1] = color.G;
                     colorTable[offset + 2] = color.B;
 
-                    if (color.A <= this.Threshold)
+                    if (alpha <= this.Threshold)
                     {
                         transparentPixels.Add((byte)offset);
                     }

src/ImageProcessorCore/Quantizers/Octree/OctreeQuantizer.cs

@@ -91,17 +91,6 @@ namespace ImageProcessorCore.Quantizers
         protected override List<TColor> GetPalette()
         {
             return this.octree.Palletize(Math.Max(this.colors, 1));
-
-            // First off convert the Octree to maxColors colors
-            //List<TColor> palette = this.octree.Palletize(Math.Max(this.colors, 1));
-
-            //int diff = this.colors - palette.Count;
-            //if (diff > 0)
-            //{
-            //    palette.AddRange(Enumerable.Repeat(default(TColor), diff));
-            //}
-
-            //return palette;
         }
 
         /// <summary>

src/ImageProcessorCore/Quantizers/Octree/Quantizer.cs

@@ -95,7 +95,7 @@ namespace ImageProcessorCore.Quantizers
         /// <param name="output">The output pixel array</param>
         /// <param name="width">The width in pixels of the image</param>
         /// <param name="height">The height in pixels of the image</param>
-        protected virtual void SecondPass(PixelAccessor<TColor,TPacked> source, byte[] output, int width, int height)
+        protected virtual void SecondPass(PixelAccessor<TColor, TPacked> source, byte[] output, int width, int height)
         {
             Parallel.For(
                 0,