PngDecoder is covered now, and proven to be buggy :P

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -12,6 +12,8 @@ using Xunit;
 
 namespace ImageSharp.Tests
 {
+    using ImageSharp.Tests.TestUtilities.ImageComparison;
+
     public class PngDecoderTests
     {
         private const PixelTypes PixelTypes = Tests.PixelTypes.Rgba32 | Tests.PixelTypes.RgbaVector | Tests.PixelTypes.Argb32;
@@ -19,17 +21,19 @@ namespace ImageSharp.Tests
         public static readonly string[] TestFiles =
             {
                 TestImages.Png.Splash, TestImages.Png.Indexed, TestImages.Png.Interlaced, TestImages.Png.FilterVar,
-                TestImages.Png.Bad.ChunkLength1, TestImages.Png.Bad.ChunkLength2, TestImages.Png.Rgb48Bpp, TestImages.Png.Rgb48BppInterlaced
+                TestImages.Png.Bad.ChunkLength1, TestImages.Png.Bad.ChunkLength2, TestImages.Png.Rgb48Bpp,
+                TestImages.Png.Rgb48BppInterlaced, TestImages.Png.SnakeGame
             };
 
         [Theory]
         [WithFileCollection(nameof(TestFiles), PixelTypes)]
-        public void Decode<TPixel>(TestImageProvider<TPixel> imageProvider)
+        public void Decode<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : struct, IPixel<TPixel>
         {
-            using (Image<TPixel> image = imageProvider.GetImage())
+            using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
             {
-                image.DebugSave(imageProvider);
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, ImageComparer.Exact);
             }
         }
 

tests/ImageSharp.Tests/TestImages.cs

@@ -31,6 +31,7 @@ namespace ImageSharp.Tests
             public const string Bike = "Png/Bike.png";
             public const string BikeGrayscale = "Png/BikeGrayscale.png";
             public const string Rgb48BppInterlaced = "Png/rgb-48bpp-interlaced.png";
+            public const string SnakeGame = "Png/SnakeGame.png";
 
             // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
             public const string Filter0 = "Png/filter0.png";

tests/ImageSharp.Tests/TestUtilities/ImageComparison/ImageComparer.cs

@@ -10,7 +10,7 @@ namespace ImageSharp.Tests.TestUtilities.ImageComparison
 
     public abstract class ImageComparer
     {
-        public static ImageComparer Exact { get; } = ExactImageComparer.Instance;
+        public static ImageComparer Exact { get; } = Tolerant(0, 0);
 
         public static ImageComparer Tolerant(
             float imageThreshold = TolerantImageComparer.DefaultImageThreshold,

tests/ImageSharp.Tests/TestUtilities/ImageComparison/TolerantImageComparer.cs

@@ -66,7 +66,7 @@
 
                 for (int x = 0; x < width; x++)
                 {
-                    int d = GetDifferenceInPixelByteSum(ref aBuffer[x], ref bBuffer[x]);
+                    int d = GetHammingDistanceInRgbaSpace(ref aBuffer[x], ref bBuffer[x]);
 
                     if (d > this.PixelThresholdInPixelByteSum)
                     {
@@ -92,7 +92,7 @@
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static int GetDifferenceInPixelByteSum(ref Rgba32 a, ref Rgba32 b)
+        private static int GetHammingDistanceInRgbaSpace(ref Rgba32 a, ref Rgba32 b)
         {
             return Diff(a.R, b.R) + Diff(a.G, b.G) + Diff(a.B, b.B) + Diff(a.A, b.A);
         }

tests/ImageSharp.Tests/TestUtilities/TestEnvironment.cs

@@ -119,7 +119,7 @@ namespace ImageSharp.Tests
             return Configuration.FindEncoder(format);
         }
 
-        private static IImageFormat GetImageFormat(string filePath)
+        internal static IImageFormat GetImageFormat(string filePath)
         {
             string extension = Path.GetExtension(filePath).ToLower();
             if (extension[0] == '.') extension = extension.Substring(1);

tests/ImageSharp.Tests/TestUtilities/TestImageExtensions.cs

@@ -7,7 +7,9 @@ namespace ImageSharp.Tests
 {
     using System;
     using System.IO;
+    using System.Linq;
 
+    using ImageSharp.Formats;
     using ImageSharp.PixelFormats;
     using ImageSharp.Tests.TestUtilities.ImageComparison;
     using ImageSharp.Tests.TestUtilities.ReferenceCodecs;
@@ -137,9 +139,17 @@ namespace ImageSharp.Tests
 
             var testFile = TestFile.Create(path);
 
-            using (var original = Image.Load<TPixel>(testFile.Bytes, SystemDrawingReferenceDecoder.Instance))
+            IImageDecoder referenceDecoder = TestEnvironment.GetReferenceDecoder(path);
+            IImageFormat format = TestEnvironment.GetImageFormat(path);
+            IImageDecoder defaultDecoder = Configuration.Default.FindDecoder(format);
+
+            if (referenceDecoder.GetType() == defaultDecoder.GetType())
+            {
+                throw new InvalidOperationException($"Can't use CompareToOriginal(): no actual reference decoder registered for {format.Name}");
+            }
+
+            using (var original = Image.Load<TPixel>(testFile.Bytes, referenceDecoder))
             {
-                //original.DebugSave(provider, "__SYSTEMDRAWING__");
                 comparer.VerifySimilarity(original, image);
             }