lower default tolerance TolerantImageComparer.DefaultImageThreshold for travis

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

@@ -15,8 +15,13 @@ namespace SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison
     {
         public static ImageComparer Exact { get; } = Tolerant(0, 0);
 
+        /// <summary>
+        /// Returns an instance of <see cref="TolerantImageComparer"/>.
+        /// Individual manhattan pixel difference is only added to total image difference when the individual difference is over 'perPixelManhattanThreshold'.
+        /// Negative value for 'imageThreshold' indicates default threshold (see <see cref="TolerantImageComparer.DefaultImageThreshold"/>).
+        /// </summary>
         public static ImageComparer Tolerant(
-            float imageThreshold = TolerantImageComparer.DefaultImageThreshold,
+            float imageThreshold = -1,
             int perPixelManhattanThreshold = 0)
         {
             return new TolerantImageComparer(imageThreshold, perPixelManhattanThreshold);

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

@@ -12,10 +12,19 @@
 
     public class TolerantImageComparer : ImageComparer
     {
-        public const float DefaultImageThreshold = 1.0f / (100 * 100 * 255);
+        public static readonly float DefaultImageThreshold = GetDefaultImageThreshold();
 
+        /// <summary>
+        /// Negative value for 'imageThreshold' indicates default threshold (see <see cref="DefaultImageThreshold"/>).
+        /// Individual manhattan pixel difference is only added to total image difference when the individual difference is over 'perPixelManhattanThreshold'.
+        /// </summary>
         public TolerantImageComparer(float imageThreshold, int perPixelManhattanThreshold = 0)
         {
+            if (imageThreshold < 0)
+            {
+                imageThreshold = DefaultImageThreshold;
+            }
+
             this.ImageThreshold = imageThreshold;
             this.PerPixelManhattanThreshold = perPixelManhattanThreshold;
         }
@@ -107,5 +116,15 @@
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static int Diff(byte a, byte b) => Math.Abs(a - b);
+
+        private static float GetDefaultImageThreshold()
+        {
+            // 1% of all pixels in a 100*100 pixel area are allowed to have a difference of 1 unit
+            float t = 1.0f / (100 * 100 * 255);
+
+            // but not with Mono System.Drawing!
+            // TODO: We may need a runtime-specific check here, instead of the OS specific one!
+            return TestEnvironment.IsWindows ? t : 4 * t;
+        }
     }
 }

tests/ImageSharp.Tests/TestUtilities/TestUtils.cs

@@ -10,6 +10,7 @@ using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Tests
 {
+    using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
     using SixLabors.Primitives;
 
     /// <summary>
@@ -154,7 +155,8 @@ namespace SixLabors.ImageSharp.Tests
         internal static void RunValidatingProcessorTest<TPixel>(
             this TestImageProvider<TPixel> provider,
             Action<IImageProcessingContext<TPixel>> process,
-            object testOutputDetails = null)
+            object testOutputDetails = null,
+            ImageComparer comparer = null)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> image = provider.GetImage())
@@ -171,7 +173,8 @@ namespace SixLabors.ImageSharp.Tests
         internal static void RunRectangleConstrainedValidatingProcessorTest<TPixel>(
             this TestImageProvider<TPixel> provider,
             Action<IImageProcessingContext<TPixel>, Rectangle> process,
-            object testOutputDetails = null)
+            object testOutputDetails = null,
+            ImageComparer comparer = null)
             where TPixel : struct, IPixel<TPixel>
         {
             using (Image<TPixel> image = provider.GetImage())

tests/ImageSharp.Tests/TestUtilities/Tests/ImageComparerTests.cs

@@ -68,7 +68,10 @@ namespace SixLabors.ImageSharp.Tests
             {
                 using (Image<TPixel> clone = image.Clone())
                 {
-                    ImagingTestCaseUtility.ModifyPixel(clone, 3, 1, 2);
+                    // Mono System.Drawing is trolling us! See TolerantImageComparer.GetDefaultImageThreshold()!
+                    byte tooBigToSucceed = TestEnvironment.IsWindows ? (byte)2 : (byte)6;
+
+                    ImagingTestCaseUtility.ModifyPixel(clone, 3, 1, tooBigToSucceed);
 
                     var comparer = ImageComparer.Tolerant();