Use premultiplication for convolution #428

src/ImageSharp/Common/Extensions/Vector4Extensions.cs

@@ -12,6 +12,32 @@ namespace SixLabors.ImageSharp
     /// </summary>
     internal static class Vector4Extensions
     {
+        /// <summary>
+        /// Premultiplies the "x", "y", "z" components of a vector by its "w" component leaving the "w" component intact.
+        /// </summary>
+        /// <param name="source">The <see cref="Vector4"/> to premultiply</param>
+        /// <returns>The <see cref="Vector4"/></returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Vector4 Premultiply(this Vector4 source)
+        {
+            float w = source.W;
+            Vector4 premultiplied = source * w;
+            return new Vector4(premultiplied.X, premultiplied.Y, premultiplied.Z, w);
+        }
+
+        /// <summary>
+        /// Reverses the result of premultiplying a vector via <see cref="Premultiply(Vector4)"/>.
+        /// </summary>
+        /// <param name="source">The <see cref="Vector4"/> to premultiply</param>
+        /// <returns>The <see cref="Vector4"/></returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static Vector4 UnPremultiply(this Vector4 source)
+        {
+            float w = source.W;
+            Vector4 unpremultiplied = source / w;
+            return new Vector4(unpremultiplied.X, unpremultiplied.Y, unpremultiplied.Z, w);
+        }
+
         /// <summary>
         /// Compresses a linear color signal to its sRGB equivalent.
         /// <see href="http://www.4p8.com/eric.brasseur/gamma.html#formulas"/>

src/ImageSharp/Processing/Processors/Convolution/Convolution2DProcessor.cs

@@ -93,7 +93,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                     int offsetX = x + fxr;
 
                                     offsetX = offsetX.Clamp(0, maxX);
-                                    var currentColor = sourceOffsetRow[offsetX].ToVector4();
+                                    Vector4 currentColor = sourceOffsetRow[offsetX].ToVector4().Premultiply();
 
                                     if (fy < kernelXHeight)
                                     {
@@ -118,7 +118,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                             float blue = MathF.Sqrt((bX * bX) + (bY * bY));
 
                             ref TPixel pixel = ref targetRow[x];
-                            pixel.PackFromVector4(new Vector4(red, green, blue, sourceRow[x].ToVector4().W));
+                            pixel.PackFromVector4(new Vector4(red, green, blue, sourceRow[x].ToVector4().W).UnPremultiply());
                         }
                     });
 

src/ImageSharp/Processing/Processors/Convolution/Convolution2PassProcessor.cs

@@ -113,13 +113,13 @@ namespace SixLabors.ImageSharp.Processing.Processors
 
                                 offsetX = offsetX.Clamp(0, maxX);
 
-                                var currentColor = row[offsetX].ToVector4();
+                                Vector4 currentColor = row[offsetX].ToVector4().Premultiply();
                                 destination += kernel[fy, fx] * currentColor;
                             }
                         }
 
                         ref TPixel pixel = ref targetRow[x];
-                        pixel.PackFromVector4(destination);
+                        pixel.PackFromVector4(destination.UnPremultiply());
                     }
                 });
         }

src/ImageSharp/Processing/Processors/Convolution/ConvolutionProcessor.cs

@@ -80,7 +80,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
 
                                  offsetX = offsetX.Clamp(0, maxX);
 
-                                 var currentColor = sourceOffsetRow[offsetX].ToVector4();
+                                 Vector4 currentColor = sourceOffsetRow[offsetX].ToVector4().Premultiply();
                                  currentColor *= this.KernelXY[fy, fx];
 
                                  red += currentColor.X;
@@ -90,7 +90,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                          }
 
                          ref TPixel pixel = ref targetRow[x];
-                         pixel.PackFromVector4(new Vector4(red, green, blue, sourceRow[x].ToVector4().W));
+                         pixel.PackFromVector4(new Vector4(red, green, blue, sourceRow[x].ToVector4().W).UnPremultiply());
                      }
                  });