Add global histogram equalization test which compares result to reference output

tests/ImageSharp.Tests/Processing/Normalization/HistogramEqualizationTests.cs

@@ -17,7 +17,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
         [Theory]
         [InlineData(256)]
         [InlineData(65536)]
-        public void HistogramEqualizationTest(int luminanceLevels)
+        public void GlobalHistogramEqualization_WithDifferentLumanceLevels(int luminanceLevels)
         {
             // Arrange
             var pixels = new byte[]
@@ -58,7 +58,8 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
                 // Act
                 image.Mutate(x => x.HistogramEqualization(new HistogramEqualizationOptions
                 {
-                    LuminanceLevels = luminanceLevels
+                    LuminanceLevels = luminanceLevels,
+                    Method = HistogramEqualizationMethod.Global
                 }));
 
                 // Assert
@@ -75,6 +76,24 @@ namespace SixLabors.ImageSharp.Tests.Processing.Normalization
             }
         }
 
+        [Theory]
+        [WithFile(TestImages.Jpeg.Baseline.HistogramEqImage, PixelTypes.Rgba32)]
+        public void GlobalHistogramEqualization_CompareToReferenceOutput<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                var options = new HistogramEqualizationOptions
+                {
+                    Method = HistogramEqualizationMethod.Global,
+                    LuminanceLevels = 256,
+                };
+                image.Mutate(x => x.HistogramEqualization(options));
+                image.DebugSave(provider);
+                image.CompareToReferenceOutput(ValidatorComparer, provider, extension: "png");
+            }
+        }
+
         [Theory]
         [WithFile(TestImages.Jpeg.Baseline.LowContrast, PixelTypes.Rgba32)]
         public void Adaptive_SlidingWindow_15Tiles_WithClipping<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -196,6 +196,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string YcckSubsample1222 = "Jpg/baseline/ycck-subsample-1222.jpg";
                 public const string Iptc = "Jpg/baseline/iptc.jpg";
                 public const string App13WithEmptyIptc = "Jpg/baseline/iptc-psAPP13-wIPTCempty.jpg";
+                public const string HistogramEqImage = "Jpg/baseline/640px-Unequalized_Hawkes_Bay_NZ.jpg";
 
                 public static readonly string[] All =
                 {

tests/Images/External

@@ -1 +1 @@
-Subproject commit cc6465910d092319ef9bf4e99698a0649996d3c5
+Subproject commit 8b43d14d21ce9b436af3d12a70d38402cdba176b

tests/Images/Input/Jpg/baseline/640px-Unequalized_Hawkes_Bay_NZ.jpg

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:d1fafc61231325c42d94fe163486a6c5144fb6211ccdceb902d5cb4ddebda9e1
+size 32428