Fixed dithering for images with 8-bit alph-channel (closes #8), applied dithering on images loading step to generate previews using dithered image

Resources/Image/ColorError.cs

@@ -4,42 +4,50 @@ namespace Resources.Image
 {
     public class ColorError
     {
-        public const int LowLevel = 0x00;
+        public const byte LowLevel = 0x00;
-        public const int HighLevel = 0xff;
+        public const byte HighLevel = 0xff;
-        public const int Treshold = 0x80;
+        public const byte Treshold = 0x80;
 
         public ColorError(Color original)
         {
-            var r = original.R < Treshold ? LowLevel : HighLevel;
+            var a = NewValue(original.A);
-            var g = original.G < Treshold ? LowLevel : HighLevel;
+            var r = NewValue(original.R);
-            var b = original.B < Treshold ? LowLevel : HighLevel;
+            var g = NewValue(original.G);
+            var b = NewValue(original.B);
 
-            NewColor = Color.FromArgb(original.A, r, g, b);
+            NewColor = Color.FromArgb(a, r, g, b);
 
+            ErrorA = original.A - a;
             ErrorR = original.R - r;
             ErrorG = original.G - g;
             ErrorB = original.B - b;
         }
 
         public Color NewColor { get; }
+        public int ErrorA { get; }
         public int ErrorR { get; }
         public int ErrorG { get; }
         public int ErrorB { get; }
-        public bool IsZero => ErrorR == 0 && ErrorG == 0 && ErrorB == 0;
+        public bool IsZero => ErrorA == 0 && ErrorR == 0 && ErrorG == 0 && ErrorB == 0;
 
         public Color ApplyError(Color color, int part, int total)
         {
             return Color.FromArgb(
-                NewColor.A,
+                CheckBounds(color.A + ErrorA * part / total),
                 CheckBounds(color.R + ErrorR * part / total),
                 CheckBounds(color.G + ErrorG * part / total),
                 CheckBounds(color.B + ErrorB * part / total)
             );
         }
 
-        private byte CheckBounds(int value)
+        private static byte NewValue(byte value)
         {
-            return (byte) (value < 0 ? 0 : (value > 255 ? 255 : value));
+            return value < Treshold ? LowLevel : HighLevel;
+        }
+
+        private static byte CheckBounds(int value)
+        {
+            return (byte) (value < LowLevel ? LowLevel : (value > HighLevel ? HighLevel : value));
         }
     }
 }

Resources/Image/Writer.cs

@@ -37,7 +37,6 @@ namespace Resources.Image
             _width = (ushort) image.Width;
             _height = (ushort) image.Height;
 
-            ApplyDithering();
             ExtractPalette();
 
             if (_bitsPerPixel == 3) _bitsPerPixel = 4;
@@ -57,17 +56,6 @@ namespace Resources.Image
             WriteImage();
         }
 
-        private void ApplyDithering()
-        {
-            var clone = new Bitmap(_image.Width, _image.Height, PixelFormat.Format32bppArgb);
-            using (var gr = Graphics.FromImage(clone))
-            {
-                gr.DrawImage(_image, new Rectangle(0, 0, clone.Width, clone.Height));
-            }
-            FloydSteinbergDitherer.Process(clone);
-            _image = clone;
-        }
-
         private void ExtractPalette()
         {
             Logger.Trace("Extracting palette...");

Resources/ImageLoader.cs

@@ -1,7 +1,9 @@
 using System.Drawing;
+using System.Drawing.Imaging;
 using System.IO;
 using System.Linq;
 using NLog;
+using Resources.Image;
 
 namespace Resources
 {
@@ -27,11 +29,25 @@ namespace Resources
                     continue;
                 }
 
+                var image = (Bitmap) System.Drawing.Image.FromFile(fullFileName);
                 Logger.Trace("Image was loaded from file {0}", fullFileName);
-                return (Bitmap) System.Drawing.Image.FromFile(fullFileName);
+                return ApplyDithering(image);
             }
 
             throw new FileNotFoundException($"File referenced by index {index} not found.");
         }
+
+        private static Bitmap ApplyDithering(Bitmap image)
+        {
+            var clone = new Bitmap(image.Width, image.Height, PixelFormat.Format32bppArgb);
+            using (var gr = Graphics.FromImage(clone))
+            {
+                gr.DrawImage(image, new Rectangle(0, 0, clone.Width, clone.Height));
+            }
+
+            FloydSteinbergDitherer.Process(clone);
+            clone.Save("tmp.png");
+            return clone;
+        }
     }
 }