Fixed Wu Quantizer transparency

Former-commit-id: 79819485015944b40893f385643943b566fba85a
Former-commit-id: 253f7dd509357930ba5aacc2a5d40c1938161810

src/ImageProcessor.Playground/Program.cs

@@ -50,7 +50,7 @@ namespace ImageProcessor.PlayGround
             }
 
             // Image mask = Image.FromFile(Path.Combine(resolvedPath, "mask2.png"));
-            //FileInfo fileInfo = new FileInfo(Path.Combine(resolvedPath, "effect_24bit.png"));
+            //FileInfo fileInfo = new FileInfo(Path.Combine(resolvedPath, "circle.png"));
             IEnumerable<FileInfo> files = GetFilesByExtensions(di, ".png");
             //IEnumerable<FileInfo> files = GetFilesByExtensions(di, ".gif", ".webp", ".bmp", ".jpg", ".png", ".tif");
 

src/ImageProcessor.Playground/images/input/cow_PNG2140.png.REMOVED.git-id

@@ -0,0 +1 @@
+04b381ec37fffdbffa55d5b02478ad81375eaa4b

src/ImageProcessor.Playground/images/input/eye-162182_1280.png.REMOVED.git-id

@@ -0,0 +1 @@
+0693daaf5dc3451fcad6b9ad910fbdfcfa3f9882

src/ImageProcessor.Playground/images/input/grad.png.REMOVED.git-id

@@ -0,0 +1 @@
+c30305a6617a68a2f5671b6a69dbe832d9f73e08

src/ImageProcessor.Playground/images/input/rose_PNG658.png.REMOVED.git-id

@@ -0,0 +1 @@
+60379fd852541c3f736148862adf54691f39fdee

src/ImageProcessor/Imaging/Formats/PngFormat.cs

@@ -100,8 +100,8 @@ namespace ImageProcessor.Imaging.Formats
             if (this.IsIndexed)
             {
                 // The Wu Quantizer expects a 32bbp image.
-                if (Image.GetPixelFormatSize(image.PixelFormat) != 32)
-                {
+                //if (Image.GetPixelFormatSize(image.PixelFormat) != 32)
+                //{
                     Bitmap clone = new Bitmap(image.Width, image.Height, PixelFormat.Format32bppPArgb);
                     clone.SetResolution(image.HorizontalResolution, image.VerticalResolution);
 
@@ -116,11 +116,11 @@ namespace ImageProcessor.Imaging.Formats
                     image = new WuQuantizer().QuantizeImage(clone);
 
                     // image = new OctreeQuantizer(255, 8).Quantize(image);
-                }
-                else
-                {
-                    image = new WuQuantizer().QuantizeImage((Bitmap)image);
-                }
+                //}
+                //else
+                //{
+                //    image = new WuQuantizer().QuantizeImage((Bitmap)image);
+                //}
             }
 
             return base.Save(path, image);

src/ImageProcessor/Imaging/Quantizers/WuQuantizer/WuQuantizer.cs

@@ -92,12 +92,14 @@ namespace ImageProcessor.Imaging.Quantizers.WuQuantizer
             byte[] lineIndexes = new byte[image.Image.Width];
             PaletteLookup lookup = new PaletteLookup(lookups);
 
+            // Determine the correct fallback color.
+            byte fallback = (byte)(lookups.Length < AlphaColor ? 0 : AlphaColor);
             foreach (Pixel[] pixelLine in image.PixelLines)
             {
                 for (int pixelIndex = 0; pixelIndex < pixelLine.Length; pixelIndex++)
                 {
                     Pixel pixel = pixelLine[pixelIndex];
-                    byte bestMatch = 0;
+                    byte bestMatch = fallback;
                     if (pixel.Alpha > alphaThreshold)
                     {
                         bestMatch = lookup.GetPaletteIndex(pixel);

src/ImageProcessor/Imaging/Quantizers/WuQuantizer/WuQuantizerBase.cs

@@ -168,9 +168,10 @@ namespace ImageProcessor.Imaging.Quantizers.WuQuantizer
                         moments[pixelAlpha, pixelRed, pixelGreen, pixelBlue].Add(pixel);
                     }
                 }
-
-                moments[0, 0, 0, 0].Add(new Pixel(0, 255, 255, 255));
             }
+
+            // Set a default pixel for images with less than 256 colors.
+            moments[0, 0, 0, 0].Add(new Pixel(0, 0, 0, 0));
         }
 
         private static void CalculateMoments(ColorMoment[, , ,] moments)