Revert to coarse cache default, additional feedback.

Author: JimBobSquarePants

src/ImageSharp/Advanced/AotCompilerTools.cs

@@ -138,6 +138,7 @@ private static void Seed<TPixel>()
         AotCompileResamplers<TPixel>();
         AotCompileQuantizers<TPixel>();
         AotCompilePixelSamplingStrategys<TPixel>();
+        AotCompilePixelMaps<TPixel>();
         AotCompileDithers<TPixel>();
         AotCompileMemoryManagers<TPixel>();
 
@@ -514,6 +515,20 @@ private static void AotCompilePixelSamplingStrategys<TPixel>()
         default(ExtensivePixelSamplingStrategy).EnumeratePixelRegions(default(ImageFrame<TPixel>));
     }
 
+    /// <summary>
+    /// This method pre-seeds the all <see cref="IColorIndexCache{T}" /> in the AoT compiler.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    [Preserve]
+    private static void AotCompilePixelMaps<TPixel>()
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        default(EuclideanPixelMap<TPixel, HybridCache>).GetClosestColor(default, out _);
+        default(EuclideanPixelMap<TPixel, AccurateCache>).GetClosestColor(default, out _);
+        default(EuclideanPixelMap<TPixel, CoarseCache>).GetClosestColor(default, out _);
+        default(EuclideanPixelMap<TPixel, NullCache>).GetClosestColor(default, out _);
+    }
+
     /// <summary>
     /// This method pre-seeds the all <see cref="IDither" /> in the AoT compiler.
     /// </summary>

src/ImageSharp/Processing/Processors/Dithering/PaletteDitherProcessor{TPixel}.cs

@@ -89,7 +89,7 @@ public DitherProcessor(
             float ditherScale)
         {
             this.Configuration = configuration;
-            this.pixelMap = PixelMapFactory.Create(configuration, palette, ColorMatchingMode.Hybrid);
+            this.pixelMap = PixelMapFactory.Create(configuration, palette, ColorMatchingMode.Coarse);
             this.Palette = palette;
             this.DitherScale = ditherScale;
         }

src/ImageSharp/Processing/Processors/Quantization/IColorIndexCache.cs

@@ -74,13 +74,13 @@ internal interface IColorIndexCache : IDisposable
 /// </remarks>
 internal unsafe struct HybridCache : IColorIndexCache<HybridCache>
 {
-    private ExactCache exactCache;
+    private AccurateCache accurateCache;
     private CoarseCache coarseCache;
 
     [RequiresPreviewFeatures]
     private HybridCache(MemoryAllocator allocator)
     {
-        this.exactCache = ExactCache.Create(allocator);
+        this.accurateCache = AccurateCache.Create(allocator);
         this.coarseCache = CoarseCache.Create(allocator);
     }
 
@@ -90,7 +90,7 @@ private HybridCache(MemoryAllocator allocator)
     [MethodImpl(InliningOptions.ShortMethod)]
     public bool TryAdd(Rgba32 color, short index)
     {
-        if (this.exactCache.TryAdd(color, index))
+        if (this.accurateCache.TryAdd(color, index))
         {
             return true;
         }
@@ -101,7 +101,7 @@ public bool TryAdd(Rgba32 color, short index)
     [MethodImpl(InliningOptions.ShortMethod)]
     public bool TryGetValue(Rgba32 color, out short value)
     {
-        if (this.exactCache.TryGetValue(color, out value))
+        if (this.accurateCache.TryGetValue(color, out value))
         {
             return true;
         }
@@ -111,13 +111,13 @@ public bool TryGetValue(Rgba32 color, out short value)
 
     public void Clear()
     {
-        this.exactCache.Clear();
+        this.accurateCache.Clear();
         this.coarseCache.Clear();
     }
 
     public void Dispose()
     {
-        this.exactCache.Dispose();
+        this.accurateCache.Dispose();
         this.coarseCache.Dispose();
     }
 }
@@ -209,7 +209,7 @@ public void Dispose()
 /// typically very short; in the worst-case, the number of iterations is bounded by 256.
 /// This guarantees highly efficient and predictable performance for small, fixed-size color palettes.
 /// </remarks>
-internal unsafe struct ExactCache : IColorIndexCache<ExactCache>
+internal unsafe struct AccurateCache : IColorIndexCache<AccurateCache>
 {
     // Buckets array: each bucket holds the index (0-based) into the entries array
     // of the first entry in the chain, or -1 if empty.
@@ -226,7 +226,7 @@ internal unsafe struct ExactCache : IColorIndexCache<ExactCache>
 
     public const int Capacity = 512;
 
-    private ExactCache(MemoryAllocator allocator)
+    private AccurateCache(MemoryAllocator allocator)
     {
         this.count = 0;
 
@@ -244,7 +244,7 @@ private ExactCache(MemoryAllocator allocator)
     }
 
     [RequiresPreviewFeatures]
-    public static ExactCache Create(MemoryAllocator allocator) => new(allocator);
+    public static AccurateCache Create(MemoryAllocator allocator) => new(allocator);
 
     public bool TryAdd(Rgba32 color, short value)
     {

src/ImageSharp/Processing/Processors/Quantization/QuantizerOptions.cs

@@ -41,6 +41,7 @@ public int MaxColors
 
     /// <summary>
     /// Gets or sets the color matching mode used for matching pixel values to palette colors.
+    /// Defaults to <see cref="ColorMatchingMode.Coarse"/>.
     /// </summary>
-    public ColorMatchingMode ColorMatchingMode { get; set; } = ColorMatchingMode.Hybrid;
+    public ColorMatchingMode ColorMatchingMode { get; set; } = ColorMatchingMode.Coarse;
 }

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

@@ -485,7 +485,7 @@ public void Encode_APng<TPixel>(TestImageProvider<TPixel> provider)
     }
 
     [Theory]
-    [WithFile(TestImages.Gif.Leo, PixelTypes.Rgba32, 0.7629F)]
+    [WithFile(TestImages.Gif.Leo, PixelTypes.Rgba32, 0.7921F)]
     [WithFile(TestImages.Gif.Issues.Issue2866, PixelTypes.Rgba32, 1.06F)]
     public void Encode_AnimatedFormatTransform_FromGif<TPixel>(TestImageProvider<TPixel> provider, float percentage)
         where TPixel : unmanaged, IPixel<TPixel>
@@ -718,11 +718,15 @@ public void Issue_2862()
         }
 
         using MemoryStream ms = new();
+        PaletteQuantizer quantizer = new(
+            palette.Select(Color.FromPixel).ToArray(),
+            new QuantizerOptions() { ColorMatchingMode = ColorMatchingMode.Hybrid});
+
         image.Save(ms, new PngEncoder
         {
             ColorType = PngColorType.Palette,
             BitDepth = PngBitDepth.Bit8,
-            Quantizer = new PaletteQuantizer(palette.Select(Color.FromPixel).ToArray())
+            Quantizer = quantizer
         });
 
         ms.Position = 0;

tests/Images/External/ReferenceOutput/BmpEncoderTests/Encode_8BitColor_WithOctreeQuantizer_rgb32.bmp

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:6785d108cab9e3b508e98ce10cef383037471beb2a6d14a69df6069b5fbf5f3d
+oid sha256:11375b15df083d98335f4a4baf0717e7fdd6b21ab2132a6815cadc787ac17e7d
 size 9270

tests/Images/External/ReferenceOutput/BmpEncoderTests/Encode_8BitColor_WithWuQuantizer_rgb32.bmp

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:5d9f2745de2b6e7fc3b1403fe651f3bbba835c67a6fb410fc8a9d91a15b44328
+oid sha256:e063e97cd8a000de6830adcc3961a7dc41785d40cd4d83af10ca38d96e071362
 size 9270