Add basic comparision tests

src/ImageSharp/PixelFormats/PixelBlenderMode.cs

@@ -26,7 +26,7 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// <summary>
         /// Blends the 2 values by subtraction.
         /// </summary>
-        Substract,
+        Subtract,
 
         /// <summary>
         /// Multiplies the complements of the backdrop and source values, then complements the result.

src/ImageSharp/PixelFormats/PixelOperations{TPixel}.PixelBenders.cs

@@ -22,7 +22,7 @@ namespace SixLabors.ImageSharp.PixelFormats
             {
                 case PixelBlenderMode.Multiply: return DefaultPixelBlenders<TPixel>.Multiply.Instance;
                 case PixelBlenderMode.Add: return DefaultPixelBlenders<TPixel>.Add.Instance;
-                case PixelBlenderMode.Substract: return DefaultPixelBlenders<TPixel>.Substract.Instance;
+                case PixelBlenderMode.Subtract: return DefaultPixelBlenders<TPixel>.Subtract.Instance;
                 case PixelBlenderMode.Screen: return DefaultPixelBlenders<TPixel>.Screen.Instance;
                 case PixelBlenderMode.Darken: return DefaultPixelBlenders<TPixel>.Darken.Instance;
                 case PixelBlenderMode.Lighten: return DefaultPixelBlenders<TPixel>.Lighten.Instance;

tests/ImageSharp.Tests/Drawing/DrawImageTest.cs

@@ -28,7 +28,7 @@ namespace SixLabors.ImageSharp.Tests
         [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Normal)]
         [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Multiply)]
         [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Add)]
-        [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Substract)]
+        [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Subtract)]
         [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Screen)]
         [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Darken)]
         [WithFileCollection(nameof(TestFiles), PixelTypes, PixelBlenderMode.Lighten)]

tests/ImageSharp.Tests/PixelFormats/PixelBlenders/PorterDuffCompositorTests.cs

@@ -0,0 +1,52 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Drawing;
+
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.PixelFormats.PixelBlenders
+{
+    public class PorterDuffCompositorTests
+    {
+        // TODO: Add other modes to compare.
+        private static PixelBlenderMode[] CompositingOperators =
+        {
+            PixelBlenderMode.Src,
+            PixelBlenderMode.Atop,
+            PixelBlenderMode.Over,
+            PixelBlenderMode.In,
+            PixelBlenderMode.Out,
+            PixelBlenderMode.Dest,
+            PixelBlenderMode.DestAtop,
+            PixelBlenderMode.DestOver,
+            PixelBlenderMode.DestIn,
+            PixelBlenderMode.DestOut,
+            PixelBlenderMode.Clear,
+            PixelBlenderMode.Xor
+        };
+
+        [Fact]
+        public void PorterDuffOutputIsCorrect()
+        {
+            string path = TestEnvironment.CreateOutputDirectory("PorterDuff");
+            var srcFile = TestFile.Create(TestImages.Png.PDSrc);
+            var destFile = TestFile.Create(TestImages.Png.PDDest);
+
+            using (Image<Rgba32> src = srcFile.CreateImage())
+            using (Image<Rgba32> dest = destFile.CreateImage())
+            {
+                foreach (PixelBlenderMode m in CompositingOperators)
+                {
+                    using (Image<Rgba32> res = dest.Clone(x => x.Blend(src, new GraphicsOptions { BlenderMode = m })))
+                    {
+                        // TODO: Generate reference files once this works.
+                        res.Save($"{path}/{m}.png");
+                    }
+                }
+            }
+        }
+    }
+}

tests/ImageSharp.Tests/PixelFormats/PixelBlenders/PorterDuffFunctionsTests_TPixel.cs

@@ -160,7 +160,7 @@ namespace SixLabors.ImageSharp.Tests.PixelFormats.PixelBlenders
         public void SubstractFunction_Blender<TPixel>(TestPixel<TPixel> back, TestPixel<TPixel> source, float amount, TestPixel<TPixel> expected)
             where TPixel : struct, IPixel<TPixel>
         {
-            TPixel actual = new DefaultPixelBlenders<TPixel>.Substract().Blend(back, source, amount);
+            TPixel actual = new DefaultPixelBlenders<TPixel>.Subtract().Blend(back, source, amount);
             VectorAssert.Equal(expected, actual, 2);
         }
 
@@ -170,7 +170,7 @@ namespace SixLabors.ImageSharp.Tests.PixelFormats.PixelBlenders
             where TPixel : struct, IPixel<TPixel>
         {
             Span<TPixel> dest = new Span<TPixel>(new TPixel[1]);
-            new DefaultPixelBlenders<TPixel>.Substract().Blend(this.MemoryManager, dest, back.AsSpan(), source.AsSpan(), AsSpan(amount));
+            new DefaultPixelBlenders<TPixel>.Subtract().Blend(this.MemoryManager, dest, back.AsSpan(), source.AsSpan(), AsSpan(amount));
             VectorAssert.Equal(expected, dest[0], 2);
         }
 

tests/ImageSharp.Tests/PixelFormats/PixelOperationsTests.Blender.cs

@@ -24,7 +24,7 @@ namespace SixLabors.ImageSharp.Tests.PixelFormats
             { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Darken), PixelBlenderMode.Darken },
             { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Lighten), PixelBlenderMode.Lighten },
             { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Add), PixelBlenderMode.Add },
-                                                                                           { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Substract), PixelBlenderMode.Substract },
+            { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Subtract), PixelBlenderMode.Subtract },
             { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Multiply), PixelBlenderMode.Multiply },
 
             { new TestPixel<Rgba32>(), typeof(DefaultPixelBlenders<Rgba32>.Src), PixelBlenderMode.Src },
@@ -47,7 +47,7 @@ namespace SixLabors.ImageSharp.Tests.PixelFormats
             { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Darken), PixelBlenderMode.Darken },
             { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Lighten), PixelBlenderMode.Lighten },
             { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Add), PixelBlenderMode.Add },
-                                                                                           { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Substract), PixelBlenderMode.Substract },
+            { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Subtract), PixelBlenderMode.Subtract },
             { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Multiply), PixelBlenderMode.Multiply },
             { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Src), PixelBlenderMode.Src },
             { new TestPixel<RgbaVector>(), typeof(DefaultPixelBlenders<RgbaVector>.Atop), PixelBlenderMode.Atop },

tests/ImageSharp.Tests/TestImages.cs

@@ -36,6 +36,8 @@ namespace SixLabors.ImageSharp.Tests
             public const string SnakeGame = "Png/SnakeGame.png";
             public const string Icon = "Png/icon.png";
             public const string Kaboom = "Png/kaboom.png";
+            public const string PDSrc = "Png/pd-source.png";
+            public const string PDDest = "Png/pd-dest.png";
 
             // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
             public const string Filter0 = "Png/filter0.png";

tests/Images/Input/Png/pd-dest.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:adb9f4448b7bb44d7b4c0ad8c2f3f71b17acb05148375663c7a203707f08660d
+size 2563

tests/Images/Input/Png/pd-source.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:dbf4a45b89eddeb727e0437846bcef5d875f532f94aaa441c75c8c3ddbac5150
+size 2393