using ImageSharp PNG codecs as reference codecs on linux

tests/ImageSharp.Tests/TestUtilities/TestEnvironment.cs

@@ -57,10 +57,17 @@ namespace SixLabors.ImageSharp.Tests
                 new BmpConfigurationModule()
                 );
 
-            configuration.SetDecoder(ImageFormats.Png, SystemDrawingReferenceDecoder.Instance);
-            configuration.SetEncoder(ImageFormats.Png, SystemDrawingReferenceEncoder.Png);
-            configuration.AddImageFormatDetector(new PngImageFormatDetector());
-
+            if (!IsLinux)
+            {
+                configuration.SetDecoder(ImageFormats.Png, SystemDrawingReferenceDecoder.Instance);
+                configuration.SetEncoder(ImageFormats.Png, SystemDrawingReferenceEncoder.Png);
+                configuration.AddImageFormatDetector(new PngImageFormatDetector());
+            }
+            else
+            {
+                new PngConfigurationModule().Configure(configuration);
+            }
+            
             return configuration;
         }
 

tests/ImageSharp.Tests/TestUtilities/TestImageExtensions.cs

@@ -90,7 +90,6 @@ namespace SixLabors.ImageSharp.Tests
         /// <param name="extension">The extension</param>
         /// <param name="grayscale">A boolean indicating whether we should debug save + compare against a grayscale image, smaller in size.</param>
         /// <param name="appendPixelTypeToFileName">A boolean indicating whether to append the pixel type to the  output file name.</param>
-        /// <param name="decoder">Custom decoder</param>
         /// <returns></returns>
         public static Image<TPixel> CompareToReferenceOutput<TPixel>(
             this Image<TPixel> image,
@@ -99,16 +98,14 @@ namespace SixLabors.ImageSharp.Tests
             object testOutputDetails = null,
             string extension = "png",
             bool grayscale = false,
-            bool appendPixelTypeToFileName = true,
-            IImageDecoder decoder = null)
+            bool appendPixelTypeToFileName = true)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> referenceImage = GetReferenceOutputImage<TPixel>(
                 provider,
                 testOutputDetails,
                 extension,
-                appendPixelTypeToFileName,
-                decoder)) 
+                appendPixelTypeToFileName)) 
             {
                 comparer.VerifySimilarity(referenceImage, image);
             }
@@ -119,8 +116,7 @@ namespace SixLabors.ImageSharp.Tests
         public static Image<TPixel> GetReferenceOutputImage<TPixel>(this ITestImageProvider provider,
                                                                     object testOutputDetails = null,
                                                                     string extension = "png",
-                                                                    bool appendPixelTypeToFileName = true,
-                                                                    IImageDecoder decoder = null)
+                                                                    bool appendPixelTypeToFileName = true)
             where TPixel : struct, IPixel<TPixel>
         {
             string referenceOutputFile = provider.Utility.GetReferenceOutputFileName(extension, testOutputDetails, appendPixelTypeToFileName);
@@ -130,7 +126,7 @@ namespace SixLabors.ImageSharp.Tests
                 throw new Exception("Reference output file missing: " + referenceOutputFile);
             }
 
-            decoder = decoder ?? TestEnvironment.GetReferenceDecoder(referenceOutputFile);
+            IImageDecoder decoder = TestEnvironment.GetReferenceDecoder(referenceOutputFile);
 
             return Image.Load<TPixel>(referenceOutputFile, decoder);
         }
@@ -141,15 +137,13 @@ namespace SixLabors.ImageSharp.Tests
             ImageComparer comparer,
             object testOutputDetails = null,
             string extension = "png",
-            bool appendPixelTypeToFileName = true,
-            IImageDecoder decoder = null)
+            bool appendPixelTypeToFileName = true)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> referenceImage = provider.GetReferenceOutputImage<TPixel>(
                 testOutputDetails,
                 extension,
-                appendPixelTypeToFileName,
-                decoder))
+                appendPixelTypeToFileName))
             {
                 return comparer.CompareImages(referenceImage, image);
             }

tests/ImageSharp.Tests/TestUtilities/Tests/TestEnvironmentTests.cs

@@ -14,6 +14,8 @@ using Xunit.Abstractions;
 // ReSharper disable InconsistentNaming
 namespace SixLabors.ImageSharp.Tests
 {
+    using SixLabors.ImageSharp.Formats.Png;
+
     public class TestEnvironmentTests
     {
         public TestEnvironmentTests(ITestOutputHelper output)
@@ -67,8 +69,10 @@ namespace SixLabors.ImageSharp.Tests
         [InlineData("lol/foo.png", typeof(SystemDrawingReferenceEncoder))]
         [InlineData("lol/Baz.JPG", typeof(JpegEncoder))]
         [InlineData("lol/Baz.gif", typeof(GifEncoder))]
-        public void GetReferenceEncoder_ReturnsCorrectEncoders(string fileName, Type expectedEncoderType)
+        public void GetReferenceEncoder_ReturnsCorrectEncoders_Windows(string fileName, Type expectedEncoderType)
         {
+            if (TestEnvironment.IsLinux) return;
+
             IImageEncoder encoder = TestEnvironment.GetReferenceEncoder(fileName);
             Assert.IsType(expectedEncoderType, encoder);
         }
@@ -77,8 +81,34 @@ namespace SixLabors.ImageSharp.Tests
         [InlineData("lol/foo.png", typeof(SystemDrawingReferenceDecoder))]
         [InlineData("lol/Baz.JPG", typeof(JpegDecoder))]
         [InlineData("lol/Baz.gif", typeof(GifDecoder))]
-        public void GetReferenceDecoder_ReturnsCorrectEncoders(string fileName, Type expectedDecoderType)
+        public void GetReferenceDecoder_ReturnsCorrectEncoders_Windows(string fileName, Type expectedDecoderType)
+        {
+            if (TestEnvironment.IsLinux) return;
+
+            IImageDecoder decoder = TestEnvironment.GetReferenceDecoder(fileName);
+            Assert.IsType(expectedDecoderType, decoder);
+        }
+
+        [Theory]
+        [InlineData("lol/foo.png", typeof(PngEncoder))]
+        [InlineData("lol/Baz.JPG", typeof(JpegEncoder))]
+        [InlineData("lol/Baz.gif", typeof(GifEncoder))]
+        public void GetReferenceEncoder_ReturnsCorrectEncoders_Linux(string fileName, Type expectedEncoderType)
         {
+            if (!TestEnvironment.IsLinux) return;
+
+            IImageEncoder encoder = TestEnvironment.GetReferenceEncoder(fileName);
+            Assert.IsType(expectedEncoderType, encoder);
+        }
+
+        [Theory]
+        [InlineData("lol/foo.png", typeof(PngDecoder))]
+        [InlineData("lol/Baz.JPG", typeof(JpegDecoder))]
+        [InlineData("lol/Baz.gif", typeof(GifDecoder))]
+        public void GetReferenceDecoder_ReturnsCorrectEncoders_Linux(string fileName, Type expectedDecoderType)
+        {
+            if (!TestEnvironment.IsLinux) return;
+
             IImageDecoder decoder = TestEnvironment.GetReferenceDecoder(fileName);
             Assert.IsType(expectedDecoderType, decoder);
         }