Fix bug in ColorSpaceInverseTransform

src/ImageSharp/Formats/WebP/LosslessUtils.cs

@@ -89,25 +89,27 @@ namespace SixLabors.ImageSharp.Formats.WebP
             int remainingWidth = width - safeWidth;
             int tilesPerRow = SubSampleSize(width, transform.Bits);
             int y = 0;
-            uint predRow = transform.Data[(y >> transform.Bits) * tilesPerRow];
+            int predRowIdxStart = (y >> transform.Bits) * tilesPerRow;
 
             int pixelPos = 0;
             while (y < yEnd)
             {
-                uint pred = predRow;
+                int predRowIdx = predRowIdxStart;
                 Vp8LMultipliers m = default(Vp8LMultipliers);
                 int srcSafeEnd = pixelPos + safeWidth;
                 int srcEnd = pixelPos + width;
                 while (pixelPos < srcSafeEnd)
                 {
-                    ColorCodeToMultipliers(pred++, ref m);
+                    uint colorCode = transform.Data[predRowIdx++];
+                    ColorCodeToMultipliers(colorCode, ref m);
                     TransformColorInverse(m, pixelData, pixelPos, tileWidth);
                     pixelPos += tileWidth;
                 }
 
                 if (pixelPos < srcEnd)
                 {
-                    ColorCodeToMultipliers(pred++, ref m);
+                    uint colorCode = transform.Data[predRowIdx];
+                    ColorCodeToMultipliers(colorCode, ref m);
                     TransformColorInverse(m, pixelData, pixelPos, remainingWidth);
                     pixelPos += remainingWidth;
                 }
@@ -115,7 +117,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 ++y;
                 if ((y & mask) == 0)
                 {
-                    predRow += (uint)tilesPerRow;
+                    predRowIdxStart += tilesPerRow;
                 }
             }
         }
@@ -130,13 +132,13 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 uint red = argb >> 16;
                 int newRed = (int)(red & 0xff);
                 int newBlue = (int)argb & 0xff;
-                newRed += ColorTransformDelta(m.GreenToRed, (sbyte)green);
+                newRed += ColorTransformDelta((sbyte)m.GreenToRed, (sbyte)green);
                 newRed &= 0xff;
-                newBlue += ColorTransformDelta(m.GreenToBlue, (sbyte)green);
-                newBlue += ColorTransformDelta(m.RedToBlue, (sbyte)newRed);
+                newBlue += ColorTransformDelta((sbyte)m.GreenToBlue, (sbyte)green);
+                newBlue += ColorTransformDelta((sbyte)m.RedToBlue, (sbyte)newRed);
                 newBlue &= 0xff;
 
-                // uint pixelValue = (uint)((argb & 0xff00ff00u) | (newRed << 16) | newBlue);
+                uint pixelValue = (uint)((argb & 0xff00ff00u) | (newRed << 16) | newBlue);
                 pixelData[i] = (uint)((argb & 0xff00ff00u) | (newRed << 16) | newBlue);
             }
         }
@@ -612,23 +614,24 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         private static int ColorTransformDelta(sbyte colorPred, sbyte color)
         {
+            var delta = ((sbyte)colorPred * color) >> 5;
             return ((int)colorPred * color) >> 5;
         }
 
         private static void ColorCodeToMultipliers(uint colorCode, ref Vp8LMultipliers m)
         {
-            m.GreenToRed = (sbyte)(colorCode & 0xff);
-            m.GreenToBlue = (sbyte)((colorCode >> 8) & 0xff);
-            m.RedToBlue = (sbyte)((colorCode >> 16) & 0xff);
+            m.GreenToRed = (byte)(colorCode & 0xff);
+            m.GreenToBlue = (byte)((colorCode >> 8) & 0xff);
+            m.RedToBlue = (byte)((colorCode >> 16) & 0xff);
         }
 
         internal struct Vp8LMultipliers
         {
-            public sbyte GreenToRed;
+            public byte GreenToRed;
 
-            public sbyte GreenToBlue;
+            public byte GreenToBlue;
 
-            public sbyte RedToBlue;
+            public byte RedToBlue;
         }
     }
 }

src/ImageSharp/Formats/WebP/Vp8LTransform.cs

@@ -1,11 +1,14 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System.Diagnostics;
+
 namespace SixLabors.ImageSharp.Formats.WebP
 {
     /// <summary>
     /// Data associated with a VP8L transformation to reduce the entropy.
     /// </summary>
+    [DebuggerDisplay("Transformtype: {TransformType}")]
     internal class Vp8LTransform
     {
         public Vp8LTransform(Vp8LTransformType transformType, int xSize, int ySize)

src/ImageSharp/Formats/WebP/WebPLosslessDecoder.cs

@@ -306,7 +306,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 }
                 else
                 {
-                    // Error
+                    // TODO: throw appropriate error msg
                 }
             }
 
@@ -750,7 +750,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             int xOffset = 8 - (distCode & 0xf);
             int dist = (yOffset * xSize) + xOffset;
 
-            // dist < 1 can happen if xsize is very small.
+            // dist < 1 can happen if xSize is very small.
             return (dist >= 1) ? dist : 1;
         }