Fix ordered dither output for small palette lengths.

src/ImageSharp/ImageSharp.csproj

@@ -31,8 +31,8 @@
     <None Include="..\..\shared-infrastructure\branding\icons\imagesharp\sixlabors.imagesharp.128.png" Pack="true" PackagePath="" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.1' ">
+  <ItemGroup>
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.1" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="5.0.0" />
   </ItemGroup>
 
   <ItemGroup Condition=" $(TargetFramework.StartsWith('netstandard')) OR '$(TargetFramework)' == 'net472'">

src/ImageSharp/Processing/KnownDitherings.cs

@@ -30,6 +30,11 @@ namespace SixLabors.ImageSharp.Processing
         /// </summary>
         public static IDither Bayer8x8 { get; } = OrderedDither.Bayer8x8;
 
+        /// <summary>
+        /// Gets the order ditherer using the 16x16 Bayer dithering matrix
+        /// </summary>
+        public static IDither Bayer16x16 { get; } = OrderedDither.Bayer16x16;
+
         /// <summary>
         /// Gets the error Dither that implements the Atkinson algorithm.
         /// </summary>

src/ImageSharp/Processing/Processors/Dithering/ErrorDither.cs

@@ -25,7 +25,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         /// </summary>
         /// <param name="matrix">The diffusion matrix.</param>
         /// <param name="offset">The starting offset within the matrix.</param>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public ErrorDither(in DenseMatrix<float> matrix, int offset)
         {
             this.matrix = matrix;
@@ -87,7 +87,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             => !(left == right);
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void ApplyQuantizationDither<TFrameQuantizer, TPixel>(
             ref TFrameQuantizer quantizer,
             ImageFrame<TPixel> source,
@@ -96,26 +96,25 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             where TFrameQuantizer : struct, IQuantizer<TPixel>
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            int offsetY = bounds.Top;
-            int offsetX = bounds.Left;
             float scale = quantizer.Options.DitherScale;
 
             for (int y = bounds.Top; y < bounds.Bottom; y++)
             {
-                ref TPixel sourceRowRef = ref MemoryMarshal.GetReference(source.GetPixelRowSpan(y));
+                ReadOnlySpan<TPixel> sourceRow = source.GetPixelRowSpan(y).Slice(bounds.X, bounds.Width);
-                ref byte destinationRowRef = ref MemoryMarshal.GetReference(destination.GetWritablePixelRowSpanUnsafe(y - offsetY));
+                Span<byte> destRow =
+                    destination.GetWritablePixelRowSpanUnsafe(y - bounds.Y).Slice(0, sourceRow.Length);
 
-                for (int x = bounds.Left; x < bounds.Right; x++)
+                for (int x = 0; x < sourceRow.Length; x++)
                 {
-                    TPixel sourcePixel = Unsafe.Add(ref sourceRowRef, x);
+                    TPixel sourcePixel = sourceRow[x];
-                    Unsafe.Add(ref destinationRowRef, x - offsetX) = quantizer.GetQuantizedColor(sourcePixel, out TPixel transformed);
+                    destRow[x] = quantizer.GetQuantizedColor(sourcePixel, out TPixel transformed);
                     this.Dither(source, bounds, sourcePixel, transformed, x, y, scale);
                 }
             }
         }
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void ApplyPaletteDither<TPaletteDitherImageProcessor, TPixel>(
             in TPaletteDitherImageProcessor processor,
             ImageFrame<TPixel> source,
@@ -124,13 +123,15 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             where TPixel : unmanaged, IPixel<TPixel>
         {
             float scale = processor.DitherScale;
+
             for (int y = bounds.Top; y < bounds.Bottom; y++)
             {
-                ref TPixel sourceRowRef = ref MemoryMarshal.GetReference(source.GetPixelRowSpan(y));
+                Span<TPixel> row = source.GetPixelRowSpan(y).Slice(bounds.X, bounds.Width);
-                for (int x = bounds.Left; x < bounds.Right; x++)
+
+                for (int x = 0; x < row.Length; x++)
                 {
-                    ref TPixel sourcePixel = ref Unsafe.Add(ref sourceRowRef, x);
+                    ref TPixel sourcePixel = ref row[x];
-                    TPixel transformed = Unsafe.AsRef(processor).GetPaletteColor(sourcePixel);
+                    TPixel transformed = processor.GetPaletteColor(sourcePixel);
                     this.Dither(source, bounds, sourcePixel, transformed, x, y, scale);
                     sourcePixel = transformed;
                 }
@@ -138,7 +139,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         }
 
         // Internal for AOT
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal TPixel Dither<TPixel>(
             ImageFrame<TPixel> image,
             Rectangle bounds,

src/ImageSharp/Processing/Processors/Dithering/OrderedDither.KnownTypes.cs

@@ -23,6 +23,11 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         /// </summary>
         public static OrderedDither Bayer8x8 = new OrderedDither(8);
 
+        /// <summary>
+        /// Applies order dithering using the 16x16 Bayer dithering matrix.
+        /// </summary>
+        public static OrderedDither Bayer16x16 = new OrderedDither(16);
+
         /// <summary>
         /// Applies order dithering using the 3x3 ordered dithering matrix.
         /// </summary>

src/ImageSharp/Processing/Processors/Dithering/OrderedDither.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Runtime.CompilerServices;
-using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Processors.Quantization;
@@ -23,7 +22,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         /// Initializes a new instance of the <see cref="OrderedDither"/> struct.
         /// </summary>
         /// <param name="length">The length of the matrix sides</param>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public OrderedDither(uint length)
         {
             DenseMatrix<uint> ditherMatrix = OrderedDitherFactory.CreateDitherMatrix(length);
@@ -102,7 +101,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             => !(left == right);
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void ApplyQuantizationDither<TFrameQuantizer, TPixel>(
             ref TFrameQuantizer quantizer,
             ImageFrame<TPixel> source,
@@ -125,7 +124,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
         }
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void ApplyPaletteDither<TPaletteDitherImageProcessor, TPixel>(
             in TPaletteDitherImageProcessor processor,
             ImageFrame<TPixel> source,
@@ -145,24 +144,25 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
                 in ditherOperation);
         }
 
-        [MethodImpl(InliningOptions.ShortMethod)]
+        // Spread assumes an even colorspace distribution and precision.
+        // Cubed root used because we always compare to Rgb.
+        // https://bisqwit.iki.fi/story/howto/dither/jy/
+        // https://en.wikipedia.org/wiki/Ordered_dithering#Algorithm
+        internal static int CalculatePaletteSpread(int colors) => (int)(255 / (Math.Pow(colors, 1.0 / 3) - 1));
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal TPixel Dither<TPixel>(
             TPixel source,
             int x,
             int y,
-            int bitDepth,
+            int spread,
             float scale)
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            Rgba32 rgba = default;
+            Unsafe.SkipInit(out Rgba32 rgba);
             source.ToRgba32(ref rgba);
-            Rgba32 attempt;
+            Unsafe.SkipInit(out Rgba32 attempt);
 
-            // Spread assumes an even colorspace distribution and precision.
-            // Calculated as 0-255/component count. 256 / bitDepth
-            // https://bisqwit.iki.fi/story/howto/dither/jy/
-            // https://en.wikipedia.org/wiki/Ordered_dithering#Algorithm
-            int spread = 256 / bitDepth;
             float factor = spread * this.thresholdMatrix[y % this.modulusY, x % this.modulusX] * scale;
 
             attempt.R = (byte)Numerics.Clamp(rgba.R + factor, byte.MinValue, byte.MaxValue);
@@ -181,7 +181,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             => obj is OrderedDither dither && this.Equals(dither);
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public bool Equals(OrderedDither other)
             => this.thresholdMatrix.Equals(other.thresholdMatrix) && this.modulusX == other.modulusX && this.modulusY == other.modulusY;
 
@@ -190,7 +190,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             => this.Equals((object)other);
 
         /// <inheritdoc/>
-        [MethodImpl(InliningOptions.ShortMethod)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public override int GetHashCode()
             => HashCode.Combine(this.thresholdMatrix, this.modulusX, this.modulusY);
 
@@ -203,9 +203,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             private readonly ImageFrame<TPixel> source;
             private readonly IndexedImageFrame<TPixel> destination;
             private readonly Rectangle bounds;
-            private readonly int bitDepth;
+            private readonly int spread;
 
-            [MethodImpl(InliningOptions.ShortMethod)]
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public QuantizeDitherRowOperation(
                 ref TFrameQuantizer quantizer,
                 in OrderedDither dither,
@@ -218,23 +218,24 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
                 this.source = source;
                 this.destination = destination;
                 this.bounds = bounds;
-                this.bitDepth = ColorNumerics.GetBitsNeededForColorDepth(destination.Palette.Length);
+                this.spread = CalculatePaletteSpread(destination.Palette.Length);
             }
 
-            [MethodImpl(InliningOptions.ShortMethod)]
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public void Invoke(int y)
             {
-                int offsetY = this.bounds.Top;
+                ref TFrameQuantizer quantizer = ref Unsafe.AsRef(this.quantizer);
-                int offsetX = this.bounds.Left;
+                int spread = this.spread;
                 float scale = this.quantizer.Options.DitherScale;
 
-                ref TPixel sourceRowRef = ref MemoryMarshal.GetReference(this.source.GetPixelRowSpan(y));
+                ReadOnlySpan<TPixel> sourceRow = this.source.GetPixelRowSpan(y).Slice(this.bounds.X, this.bounds.Width);
-                ref byte destinationRowRef = ref MemoryMarshal.GetReference(this.destination.GetWritablePixelRowSpanUnsafe(y - offsetY));
+                Span<byte> destRow =
+                    this.destination.GetWritablePixelRowSpanUnsafe(y - this.bounds.Y).Slice(0, sourceRow.Length);
 
-                for (int x = this.bounds.Left; x < this.bounds.Right; x++)
+                for (int x = 0; x < sourceRow.Length; x++)
                 {
-                    TPixel dithered = this.dither.Dither(Unsafe.Add(ref sourceRowRef, x), x, y, this.bitDepth, scale);
+                    TPixel dithered = this.dither.Dither(sourceRow[x], x, y, spread, scale);
-                    Unsafe.Add(ref destinationRowRef, x - offsetX) = Unsafe.AsRef(this.quantizer).GetQuantizedColor(dithered, out TPixel _);
+                    destRow[x] = quantizer.GetQuantizedColor(dithered, out TPixel _);
                 }
             }
         }
@@ -248,9 +249,9 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             private readonly ImageFrame<TPixel> source;
             private readonly Rectangle bounds;
             private readonly float scale;
-            private readonly int bitDepth;
+            private readonly int spread;
 
-            [MethodImpl(InliningOptions.ShortMethod)]
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public PaletteDitherRowOperation(
                 in TPaletteDitherImageProcessor processor,
                 in OrderedDither dither,
@@ -262,19 +263,23 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
                 this.source = source;
                 this.bounds = bounds;
                 this.scale = processor.DitherScale;
-                this.bitDepth = ColorNumerics.GetBitsNeededForColorDepth(processor.Palette.Length);
+                this.spread = CalculatePaletteSpread(processor.Palette.Length);
             }
 
-            [MethodImpl(InliningOptions.ShortMethod)]
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
             public void Invoke(int y)
             {
-                ref TPixel sourceRowRef = ref MemoryMarshal.GetReference(this.source.GetPixelRowSpan(y));
+                ref TPaletteDitherImageProcessor processor = ref Unsafe.AsRef(this.processor);
+                int spread = this.spread;
+                float scale = this.scale;
+
+                Span<TPixel> row = this.source.GetPixelRowSpan(y).Slice(this.bounds.X, this.bounds.Width);
 
-                for (int x = this.bounds.Left; x < this.bounds.Right; x++)
+                for (int x = 0; x < row.Length; x++)
                 {
-                    ref TPixel sourcePixel = ref Unsafe.Add(ref sourceRowRef, x);
+                    ref TPixel sourcePixel = ref row[x];
-                    TPixel dithered = this.dither.Dither(sourcePixel, x, y, this.bitDepth, this.scale);
+                    TPixel dithered = this.dither.Dither(sourcePixel, x, y, spread, scale);
-                    sourcePixel = Unsafe.AsRef(this.processor).GetPaletteColor(dithered);
+                    sourcePixel = processor.GetPaletteColor(dithered);
                 }
             }
         }

tests/ImageSharp.Tests/Processing/Processors/Dithering/DitherTests.cs

@@ -37,6 +37,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
                 { KnownDitherings.Bayer2x2, nameof(KnownDitherings.Bayer2x2) },
                 { KnownDitherings.Bayer4x4, nameof(KnownDitherings.Bayer4x4) },
                 { KnownDitherings.Bayer8x8, nameof(KnownDitherings.Bayer8x8) },
+                { KnownDitherings.Bayer16x16, nameof(KnownDitherings.Bayer16x16) },
                 { KnownDitherings.Ordered3x3, nameof(KnownDitherings.Ordered3x3) }
             };
 

tests/ImageSharp.Tests/Processing/Processors/Quantization/QuantizerTests.cs

@@ -169,8 +169,8 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Quantization
 
             provider.RunRectangleConstrainedValidatingProcessorTest(
                 (x, rect) => x.Quantize(quantizer, rect),
-                comparer: ValidatorComparer,
                 testOutputDetails: testOutputDetails,
+                comparer: ValidatorComparer,
                 appendPixelTypeToFileName: false);
         }
 

tests/ImageSharp.Tests/TestUtilities/TestUtils.cs

@@ -240,7 +240,7 @@ namespace SixLabors.ImageSharp.Tests
             using (Image<TPixel> image = provider.GetImage())
             {
                 FormattableString testOutputDetails = $"";
-                image.Mutate(ctx => { testOutputDetails = processAndGetTestOutputDetails(ctx); });
+                image.Mutate(ctx => testOutputDetails = processAndGetTestOutputDetails(ctx));
 
                 image.DebugSave(
                     provider,