Test to compare behavior to ImageMagick

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

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+using SixLabors.ImageSharp.Processing.Processors.Normalization;
+using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
+
+using ImageMagick;
+
+namespace SixLabors.ImageSharp.Tests.Processing.Normalization;
+
+// ReSharper disable InconsistentNaming
+[Trait("Category", "Processors")]
+public class MagickCompareTests
+{
+    [Theory]
+    [WithFile(TestImages.Jpeg.Baseline.ForestBridgeDifferentComponentsQuality, PixelTypes.Rgba32)]
+    public void AutoLevel_CompareToMagick<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, ImageSharp.PixelFormats.IPixel<TPixel>
+    {
+        using Stream stream = LoadAsStream(provider);
+        var magickImage = new MagickImage(stream);
+
+        // Apply Auto Level using the Grey (BT.709) channel.
+        magickImage.AutoLevel(Channels.Gray);
+        Image<TPixel> imageFromMagick = ConvertImageFromMagick<TPixel>(magickImage);
+
+        using (Image<TPixel> image = provider.GetImage())
+        {
+            var options = new HistogramEqualizationOptions
+            {
+                Method = HistogramEqualizationMethod.AutoLevel,
+                LuminanceLevels = 256,
+                SyncChannels = true
+            };
+            image.Mutate(x => x.HistogramEqualization(options));
+            image.DebugSave(provider);
+            ExactImageComparer.Instance.CompareImages(imageFromMagick, image);
+        }
+    }
+
+    private Stream LoadAsStream<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, ImageSharp.PixelFormats.IPixel<TPixel>
+    {
+        string path = TestImageProvider<TPixel>.GetFilePathOrNull(provider);
+        if (path == null)
+        {
+            throw new InvalidOperationException("CompareToMagick() works only with file providers!");
+        }
+
+        var testFile = TestFile.Create(path);
+        return new FileStream(testFile.FullPath, FileMode.Open);
+    }
+
+    private Image<TPixel> ConvertImageFromMagick<TPixel>(MagickImage magickImage)
+        where TPixel : unmanaged, ImageSharp.PixelFormats.IPixel<TPixel>
+    {
+        Configuration configuration = Configuration.Default.Clone();
+        configuration.PreferContiguousImageBuffers = true;
+        var result = new Image<TPixel>(configuration, magickImage.Width, magickImage.Height);
+
+        Assert.True(result.DangerousTryGetSinglePixelMemory(out Memory<TPixel> resultPixels));
+
+        using (IUnsafePixelCollection<ushort> pixels = magickImage.GetPixelsUnsafe())
+        {
+            byte[] data = pixels.ToByteArray(PixelMapping.RGBA);
+
+            PixelOperations<TPixel>.Instance.FromRgba32Bytes(
+                configuration,
+                data,
+                resultPixels.Span,
+                resultPixels.Length);
+        }
+
+        return result;
+    }
+}