Use premultiply for resize. Fix #428

src/ImageSharp/Processing/Processors/Transforms/AffineTransformProcessor.cs

@@ -204,7 +204,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                         var vector = source[i, j].ToVector4();
 
                                         // Values are first premultiplied to prevent darkening of edge pixels
-                                        var mupltiplied = new Vector4(new Vector3(vector.X, vector.Y, vector.Z) * vector.W, vector.W);
+                                        Vector4 mupltiplied = vector.Premultiply();
                                         sum += mupltiplied * xWeight * yWeight;
                                     }
                                 }
@@ -212,7 +212,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                 ref TPixel dest = ref destRow[x];
 
                                 // Reverse the premultiplication
-                                dest.PackFromVector4(new Vector4(new Vector3(sum.X, sum.Y, sum.Z) / sum.W, sum.W));
+                                dest.PackFromVector4(sum.UnPremultiply());
                             }
                         });
             }

src/ImageSharp/Processing/Processors/Transforms/ProjectiveTransformProcessor.cs

@@ -199,7 +199,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                                     var vector = source[i, j].ToVector4();
 
                                     // Values are first premultiplied to prevent darkening of edge pixels
-                                    var mupltiplied = new Vector4(new Vector3(vector.X, vector.Y, vector.Z) * vector.W, vector.W);
+                                    Vector4 mupltiplied = vector.Premultiply();
                                     sum += mupltiplied * xWeight * yWeight;
                                 }
                             }
@@ -207,7 +207,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                             ref TPixel dest = ref destRow[x];
 
                             // Reverse the premultiplication
-                            dest.PackFromVector4(new Vector4(new Vector3(sum.X, sum.Y, sum.Z) / sum.W, sum.W));
+                            dest.PackFromVector4(sum.UnPremultiply());
                         }
                     });
             }

src/ImageSharp/Processing/Processors/Transforms/WeightsWindow.cs

@@ -87,7 +87,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
             {
                 float weight = Unsafe.Add(ref horizontalValues, i);
                 Vector4 v = Unsafe.Add(ref vecPtr, i);
-                result += v * weight;
+                result += v.Premultiply() * weight;
             }
 
             return result;
@@ -114,10 +114,10 @@ namespace SixLabors.ImageSharp.Processing.Processors
             {
                 float weight = Unsafe.Add(ref horizontalValues, i);
                 Vector4 v = Unsafe.Add(ref vecPtr, i);
-                result += v.Expand() * weight;
+                result += v.Premultiply().Expand() * weight;
             }
 
-            return result;
+            return result.UnPremultiply();
         }
 
         /// <summary>
@@ -144,7 +144,7 @@ namespace SixLabors.ImageSharp.Processing.Processors
                 result += firstPassPixels[x, index] * yw;
             }
 
-            return result;
+            return result.UnPremultiply();
         }
     }
 }

tests/ImageSharp.Tests/Processing/Processors/Convolution/DetectEdgesTest.cs

@@ -14,7 +14,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Convolution
     public class DetectEdgesTest : FileTestBase
     {
         public static readonly string[] CommonTestImages = { TestImages.Png.Bike };
-        
+
         public static readonly TheoryData<EdgeDetection> DetectEdgesFilters = new TheoryData<EdgeDetection>
         {
             EdgeDetection.Kayyali,
@@ -54,9 +54,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Convolution
             {
                 image.Mutate(x => x.DetectEdges());
                 image.DebugSave(provider);
-
-                // TODO: Enable once we have updated the images
-                // image.CompareToReferenceOutput(provider);
+                image.CompareToReferenceOutput(provider);
             }
         }
 
@@ -85,10 +83,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Convolution
                 image.DebugSave(provider);
 
                 // TODO: Enable once we have updated the images
-                //image.CompareToReferenceOutput(provider);
-
-                // TODO: We don't need this any longer after switching to ReferenceImages
-                //ImageComparer.Tolerant().EnsureProcessorChangesAreConstrained(source, image, bounds);
+                image.CompareToReferenceOutput(provider);
             }
         }
     }

tests/ImageSharp.Tests/Processing/Processors/Transforms/ResizeTests.cs

@@ -89,7 +89,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Transforms
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                image.Mutate(x => x.Resize(image.Width / 2, image.Height / 2, true));
+                image.Mutate(x => x.Resize(image.Width / 2, image.Height / 2, KnownResamplers.NearestNeighbor));
 
                 // Comparer fights decoder with gif-s. Could not use CompareToReferenceOutput here :(
                 image.DebugSave(provider, extension: Extensions.Gif);