Refactor and fix dithering API + algorithm.

src/ImageSharp/Dithering/ErrorDiffusion/ErrorDiffuserBase.cs

@@ -70,22 +70,10 @@ namespace SixLabors.ImageSharp.Dithering.Base
 
         /// <inheritdoc />
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Dither<TPixel>(ImageFrame<TPixel> pixels, TPixel source, TPixel transformed, int x, int y, int minX, int minY, int maxX, int maxY)
+        public void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel transformed, int x, int y, int minX, int minY, int maxX, int maxY)
             where TPixel : struct, IPixel<TPixel>
         {
-            this.Dither(pixels, source, transformed, x, y, minX, minY, maxX, maxY, true);
+            image[x, y] = transformed;
-        }
-
-        /// <inheritdoc />
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel transformed, int x, int y, int minX, int minY, int maxX, int maxY, bool replacePixel)
-            where TPixel : struct, IPixel<TPixel>
-        {
-            if (replacePixel)
-            {
-                // Assign the transformed pixel to the array.
-                image[x, y] = transformed;
-            }
 
             // Calculate the error
             Vector4 error = source.ToVector4() - transformed.ToVector4();
@@ -117,6 +105,8 @@ namespace SixLabors.ImageSharp.Dithering.Base
                             var offsetColor = pixel.ToVector4();
 
                             Vector4 result = ((error * coefficient) / this.divisorVector) + offsetColor;
+
+                            // result.W = offsetColor.W;
                             pixel.PackFromVector4(result);
                         }
                     }

src/ImageSharp/Dithering/ErrorDiffusion/IErrorDiffuser.cs

@@ -6,7 +6,7 @@ using SixLabors.ImageSharp.PixelFormats;
 namespace SixLabors.ImageSharp.Dithering
 {
     /// <summary>
-    /// Encapsulates properties and methods required to perfom diffused error dithering on an image.
+    /// Encapsulates properties and methods required to perform diffused error dithering on an image.
     /// </summary>
     public interface IErrorDiffuser
     {
@@ -25,25 +25,5 @@ namespace SixLabors.ImageSharp.Dithering
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel transformed, int x, int y, int minX, int minY, int maxX, int maxY)
             where TPixel : struct, IPixel<TPixel>;
-
-        /// <summary>
-        /// Transforms the image applying the dither matrix. This method alters the input pixels array
-        /// </summary>
-        /// <param name="image">The image</param>
-        /// <param name="source">The source pixel</param>
-        /// <param name="transformed">The transformed pixel</param>
-        /// <param name="x">The column index.</param>
-        /// <param name="y">The row index.</param>
-        /// <param name="minX">The minimum column value.</param>
-        /// <param name="minY">The minimum row value.</param>
-        /// <param name="maxX">The maximum column value.</param>
-        /// <param name="maxY">The maximum row value.</param>
-        /// <param name="replacePixel">
-        /// Whether to replace the pixel at the given coordinates with the transformed value.
-        /// Generally this would be true for standard two-color dithering but when used in conjunction with color quantization this should be false.
-        /// </param>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
-        void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel transformed, int x, int y, int minX, int minY, int maxX, int maxY, bool replacePixel)
-            where TPixel : struct, IPixel<TPixel>;
     }
-}
+}

src/ImageSharp/Dithering/Ordered/Bayer2x2Dither.cs

@@ -0,0 +1,19 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Dithering
+{
+    /// <summary>
+    /// Applies order dithering using the 2x2 Bayer dithering matrix.
+    /// </summary>
+    public sealed class Bayer2x2Dither : BayerDither
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Bayer2x2Dither"/> class.
+        /// </summary>
+        public Bayer2x2Dither()
+            : base(1)
+        {
+        }
+    }
+}

src/ImageSharp/Dithering/Ordered/Bayer4x4Dither.cs

@@ -0,0 +1,19 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Dithering
+{
+    /// <summary>
+    /// Applies order dithering using the 4x4 Bayer dithering matrix.
+    /// </summary>
+    public sealed class Bayer4x4Dither : BayerDither
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Bayer4x4Dither"/> class.
+        /// </summary>
+        public Bayer4x4Dither()
+            : base(2)
+        {
+        }
+    }
+}

src/ImageSharp/Dithering/Ordered/Bayer8x8Dither.cs

@@ -0,0 +1,19 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Dithering
+{
+    /// <summary>
+    /// Applies order dithering using the 8x8 Bayer dithering matrix.
+    /// </summary>
+    public sealed class Bayer8x8Dither : BayerDither
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Bayer8x8Dither"/> class.
+        /// </summary>
+        public Bayer8x8Dither()
+            : base(3)
+        {
+        }
+    }
+}

src/ImageSharp/Dithering/Ordered/BayerDither.cs

@@ -1,36 +1,60 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using SixLabors.ImageSharp.Dithering.Base;
+using System.Runtime.CompilerServices;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Dithering
 {
     /// <summary>
-    /// Applies error diffusion based dithering using the 4x4 Bayer dithering matrix.
+    /// Applies order dithering using a Bayer dithering matrix of arbitrary length.
-    /// <see href="http://www.efg2.com/Lab/Library/ImageProcessing/DHALF.TXT"/>
+    /// <see href="https://en.wikipedia.org/wiki/Ordered_dithering"/>
     /// </summary>
-    public sealed class BayerDither : OrderedDitherBase
+    public class BayerDither : OrderedDitherBase
     {
         /// <summary>
-        /// The threshold matrix.
+        /// Initializes a new instance of the <see cref="BayerDither"/> class.
-        /// This is calculated by multiplying each value in the original matrix by 16 and subtracting 1
         /// </summary>
-        private static readonly Fast2DArray<byte> ThresholdMatrix =
+        /// <param name="exponent">
-            new byte[,]
+        /// The exponent used to raise the base value (2).
+        /// The value given determines the dimensions of the matrix with each dimension a power of 2. e.g 2^2 = 4, 2^3 = 8
+        /// </param>
+        public BayerDither(uint exponent)
+            : base(ComputeBayer(exponent))
+        {
+        }
+
+        private static Fast2DArray<uint> ComputeBayer(uint order)
+        {
+            uint dimension = (uint)(1 << (int)order);
+            var matrix = new Fast2DArray<uint>((int)dimension);
+            uint i = 0;
+            for (int y = 0; y < dimension; y++)
             {
-                { 15, 143, 47, 175 },
+                for (int x = 0; x < dimension; x++)
-                { 207, 79, 239, 111 },
+                {
-                { 63, 191, 31, 159 },
+                    matrix[y, x] = Bayer(i / dimension, i % dimension, order);
-                { 255, 127, 223, 95 }
+                    i++;
-            };
+                }
+            }
 
-        /// <summary>
+            return matrix;
-        /// Initializes a new instance of the <see cref="BayerDither"/> class.
+        }
-        /// </summary>
+
-        public BayerDither()
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-            : base(ThresholdMatrix)
+        private static uint Bayer(uint x, uint y, uint order)
         {
+            uint res = 0;
+            for (uint i = 0; i < order; ++i)
+            {
+                uint xOdd_XOR_yOdd = (x & 1) ^ (y & 1);
+                uint xOdd = x & 1;
+                res = ((res << 1 | xOdd_XOR_yOdd) << 1) | xOdd;
+                x >>= 1;
+                y >>= 1;
+            }
+
+            return res;
         }
     }
 }

src/ImageSharp/Dithering/Ordered/IOrderedDither.cs

@@ -6,7 +6,7 @@ using SixLabors.ImageSharp.PixelFormats;
 namespace SixLabors.ImageSharp.Dithering
 {
     /// <summary>
-    /// Encapsulates properties and methods required to perfom ordered dithering on an image.
+    /// Encapsulates properties and methods required to perform ordered dithering on an image.
     /// </summary>
     public interface IOrderedDither
     {
@@ -17,12 +17,11 @@ namespace SixLabors.ImageSharp.Dithering
         /// <param name="source">The source pixel</param>
         /// <param name="upper">The color to apply to the pixels above the threshold.</param>
         /// <param name="lower">The color to apply to the pixels below the threshold.</param>
-        /// <param name="rgba">The <see cref="Rgba32"/> to pack/unpack to.</param>
+        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
-        /// <param name="index">The component index to test the threshold against. Must range from 0 to 3.</param>
         /// <param name="x">The column index.</param>
         /// <param name="y">The row index.</param>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
-        void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, ref Rgba32 rgba, int index, int x, int y)
+        void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, byte threshold, int x, int y)
             where TPixel : struct, IPixel<TPixel>;
     }
 }

src/ImageSharp/Dithering/Ordered/OrderedDither.cs

@@ -1,36 +0,0 @@
-// Copyright (c) Six Labors and contributors.
-// Licensed under the Apache License, Version 2.0.
-
-using SixLabors.ImageSharp.Dithering.Base;
-using SixLabors.ImageSharp.Memory;
-
-namespace SixLabors.ImageSharp.Dithering
-{
-    /// <summary>
-    /// Applies error diffusion based dithering using the 4x4 ordered dithering matrix.
-    /// <see href="https://en.wikipedia.org/wiki/Ordered_dithering"/>
-    /// </summary>
-    public sealed class OrderedDither : OrderedDitherBase
-    {
-        /// <summary>
-        /// The threshold matrix.
-        /// This is calculated by multiplying each value in the original matrix by 16
-        /// </summary>
-        private static readonly Fast2DArray<byte> ThresholdMatrix =
-            new byte[,]
-            {
-               { 0, 128, 32, 160 },
-               { 192, 64, 224, 96 },
-               { 48, 176, 16, 144 },
-               { 240, 112, 208, 80 }
-            };
-
-        /// <summary>
-        /// Initializes a new instance of the <see cref="OrderedDither"/> class.
-        /// </summary>
-        public OrderedDither()
-            : base(ThresholdMatrix)
-        {
-        }
-    }
-}

src/ImageSharp/Dithering/Ordered/OrderedDitherBase.cs

@@ -1,53 +1,48 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 
-namespace SixLabors.ImageSharp.Dithering.Base
+namespace SixLabors.ImageSharp.Dithering
 {
     /// <summary>
-    /// The base class for performing ordered dithering using a 4x4 matrix.
+    /// The base class for performing ordered dithering using a dither matrix.
     /// </summary>
     public abstract class OrderedDitherBase : IOrderedDither
     {
-        /// <summary>
+        private readonly Fast2DArray<uint> matrix;
-        /// The dithering matrix
+        private readonly Fast2DArray<uint> thresholdMatrix;
-        /// </summary>
+        private readonly int modulusX;
-        private Fast2DArray<byte> matrix;
+        private readonly int modulusY;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="OrderedDitherBase"/> class.
         /// </summary>
         /// <param name="matrix">The thresholding matrix. </param>
-        internal OrderedDitherBase(Fast2DArray<byte> matrix)
+        internal OrderedDitherBase(Fast2DArray<uint> matrix)
         {
             this.matrix = matrix;
+            this.modulusX = matrix.Width;
+            this.modulusY = matrix.Height;
+            this.thresholdMatrix = new Fast2DArray<uint>(matrix.Width, matrix.Height);
+
+            // Adjust the matrix range for 0-255
+            int multiplier = 256 / (this.modulusX * this.modulusY);
+            for (int y = 0; y < matrix.Height; y++)
+            {
+                for (int x = 0; x < matrix.Width; x++)
+                {
+                    this.thresholdMatrix[y, x] = (uint)((matrix[y, x] + 1) * multiplier) - 1;
+                }
+            }
         }
 
         /// <inheritdoc />
-        public void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, ref Rgba32 rgba, int index, int x, int y)
+        public void Dither<TPixel>(ImageFrame<TPixel> image, TPixel source, TPixel upper, TPixel lower, byte threshold, int x, int y)
             where TPixel : struct, IPixel<TPixel>
         {
-            source.ToRgba32(ref rgba);
+            image[x, y] = this.thresholdMatrix[y % this.modulusY, x % this.modulusX] >= threshold ? lower : upper;
-            switch (index)
-            {
-                case 0:
-                    image[x, y] = this.matrix[y % 3, x % 3] >= rgba.R ? lower : upper;
-                    return;
-                case 1:
-                    image[x, y] = this.matrix[y % 3, x % 3] >= rgba.G ? lower : upper;
-                    return;
-                case 2:
-                    image[x, y] = this.matrix[y % 3, x % 3] >= rgba.B ? lower : upper;
-                    return;
-                case 3:
-                    image[x, y] = this.matrix[y % 3, x % 3] >= rgba.A ? lower : upper;
-                    return;
-            }
-
-            throw new ArgumentOutOfRangeException(nameof(index), "Index should be between 0 and 3 inclusive.");
         }
     }
 }

src/ImageSharp/Memory/Fast2DArray{T}.cs

@@ -28,6 +28,15 @@ namespace SixLabors.ImageSharp.Memory
         /// </summary>
         public int Height;
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Fast2DArray{T}" /> struct.
+        /// </summary>
+        /// <param name="length">The length of each dimension.</param>
+        public Fast2DArray(int length)
+            : this(length, length)
+        {
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="Fast2DArray{T}" /> struct.
         /// </summary>
@@ -96,7 +105,7 @@ namespace SixLabors.ImageSharp.Memory
         /// </summary>
         /// <param name="data">The source array.</param>
         /// <returns>
-        /// The <see cref="Fast2DArray{T}"/> represenation on the source data.
+        /// The <see cref="Fast2DArray{T}"/> representation on the source data.
         /// </returns>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static implicit operator Fast2DArray<T>(T[,] data)

src/ImageSharp/PixelFormats/ColorConstants.cs

@@ -1,9 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
-using System.Collections.Generic;
-
 namespace SixLabors.ImageSharp.PixelFormats
 {
     /// <summary>
@@ -11,23 +8,17 @@ namespace SixLabors.ImageSharp.PixelFormats
     /// </summary>
     public static class ColorConstants
     {
-        /// <summary>
-        /// Provides a lazy, one time method of returning the colors.
-        /// </summary>
-        private static readonly Lazy<Rgba32[]> SafeColors = new Lazy<Rgba32[]>(GetWebSafeColors);
-
         /// <summary>
         /// Gets a collection of named, web safe, colors as defined in the CSS Color Module Level 4.
         /// </summary>
-        public static Rgba32[] WebSafeColors => SafeColors.Value;
+        public static readonly Rgba32[] WebSafeColors = GetWebSafeColors();
 
         /// <summary>
         /// Returns an array of web safe colors.
         /// </summary>
         /// <returns>The <see cref="T:Color[]"/></returns>
         private static Rgba32[] GetWebSafeColors()
-        {
+            => new Rgba32[]
-            return new List<Rgba32>
             {
                 Rgba32.AliceBlue,
                 Rgba32.AntiqueWhite,
@@ -171,7 +162,6 @@ namespace SixLabors.ImageSharp.PixelFormats
                 Rgba32.WhiteSmoke,
                 Rgba32.Yellow,
                 Rgba32.YellowGreen
-            }.ToArray();
+            };
-        }
     }
-}
+}

src/ImageSharp/PixelFormats/NamedColors{TPixel}.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
+
 namespace SixLabors.ImageSharp.PixelFormats
 {
     /// <summary>
@@ -719,5 +721,20 @@ namespace SixLabors.ImageSharp.PixelFormats
         /// Represents a <see paramref="TPixel"/> matching the W3C definition that has an hex value of #9ACD32.
         /// </summary>
         public static readonly TPixel YellowGreen = ColorBuilder<TPixel>.FromRGBA(154, 205, 50, 255);
+
+        /// <summary>
+        /// Represents a <see cref="T:TPixel[]"/> matching the W3C definition of web safe colors.
+        /// </summary>
+        public static readonly TPixel[] WebSafePalette = GetWebSafePalette();
+
+        private static TPixel[] GetWebSafePalette()
+        {
+            Rgba32[] constants = ColorConstants.WebSafeColors;
+            TPixel[] safe = new TPixel[constants.Length + 1];
+
+            Span<byte> constantsBytes = constants.AsSpan().NonPortableCast<Rgba32, byte>();
+            PixelOperations<TPixel>.Instance.PackFromRgba32Bytes(constantsBytes, safe, constants.Length);
+            return safe;
+        }
     }
 }

src/ImageSharp/Processing/Binarization/BinaryDiffuse.cs

@@ -0,0 +1,86 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// </summary>
+    public static partial class ImageExtensions
+    {
+        /// <summary>
+        /// Dithers the image reducing it to two colors using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDiffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryErrorDiffusionProcessor<TPixel>(diffuser, threshold));
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to two colors using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDiffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryErrorDiffusionProcessor<TPixel>(diffuser, threshold), rectangle);
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to two colors using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDiffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, TPixel upperColor, TPixel lowerColor)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryErrorDiffusionProcessor<TPixel>(diffuser, threshold, upperColor, lowerColor));
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to two colors using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDiffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, TPixel upperColor, TPixel lowerColor, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryErrorDiffusionProcessor<TPixel>(diffuser, threshold, upperColor, lowerColor), rectangle);
+            return source;
+        }
+    }
+}

src/ImageSharp/Processing/Binarization/BinaryDither.cs

@@ -0,0 +1,82 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// </summary>
+    public static partial class ImageExtensions
+    {
+        /// <summary>
+        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="dither">The ordered ditherer.</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryOrderedDitherProcessor<TPixel>(dither));
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="dither">The ordered ditherer.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, TPixel upperColor, TPixel lowerColor)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryOrderedDitherProcessor<TPixel>(dither, upperColor, lowerColor));
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="dither">The ordered ditherer.</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryOrderedDitherProcessor<TPixel>(dither), rectangle);
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="dither">The ordered ditherer.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryDither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, TPixel upperColor, TPixel lowerColor, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryOrderedDitherProcessor<TPixel>(dither, upperColor, lowerColor), rectangle);
+            return source;
+        }
+    }
+}

src/ImageSharp/Processing/Binarization/BinaryThreshold.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Processors;
 using SixLabors.Primitives;
@@ -43,5 +42,40 @@ namespace SixLabors.ImageSharp
             source.ApplyProcessor(new BinaryThresholdProcessor<TPixel>(threshold), rectangle);
             return source;
         }
+
+        /// <summary>
+        /// Applies binarization to the image splitting the pixels at the given threshold.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryThreshold<TPixel>(this IImageProcessingContext<TPixel> source, float threshold, TPixel upperColor, TPixel lowerColor)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryThresholdProcessor<TPixel>(threshold, upperColor, lowerColor));
+            return source;
+        }
+
+        /// <summary>
+        /// Applies binarization to the image splitting the pixels at the given threshold.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> BinaryThreshold<TPixel>(this IImageProcessingContext<TPixel> source, float threshold, TPixel upperColor, TPixel lowerColor, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new BinaryThresholdProcessor<TPixel>(threshold, upperColor, lowerColor), rectangle);
+            return source;
+        }
     }
 }

src/ImageSharp/Processing/Dithering/Diffuse.cs

@@ -0,0 +1,84 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp
+{
+    /// <summary>
+    /// Extension methods for the <see cref="Image{TPixel}"/> type.
+    /// </summary>
+    public static partial class ImageExtensions
+    {
+        /// <summary>
+        /// Dithers the image reducing it to a web-safe palette using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> Diffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new ErrorDiffusionPaletteProcessor<TPixel>(diffuser, threshold));
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to a web-safe palette using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> Diffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new ErrorDiffusionPaletteProcessor<TPixel>(diffuser, threshold), rectangle);
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to the given palette using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="palette">The palette to select substitute colors from.</param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> Diffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, TPixel[] palette)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new ErrorDiffusionPaletteProcessor<TPixel>(diffuser, threshold, palette));
+            return source;
+        }
+
+        /// <summary>
+        /// Dithers the image reducing it to the given palette using error diffusion.
+        /// </summary>
+        /// <typeparam name="TPixel">The pixel format.</typeparam>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="diffuser">The diffusion algorithm to apply.</param>
+        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
+        /// <param name="palette">The palette to select substitute colors from.</param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
+        public static IImageProcessingContext<TPixel> Diffuse<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, TPixel[] palette, Rectangle rectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            source.ApplyProcessor(new ErrorDiffusionPaletteProcessor<TPixel>(diffuser, threshold, palette), rectangle);
+            return source;
+        }
+    }
+}

src/ImageSharp/Processing/Binarization/Dither.cs → src/ImageSharp/Processing/Dithering/Dither.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using SixLabors.ImageSharp.Dithering;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Processors;
@@ -15,7 +14,7 @@ namespace SixLabors.ImageSharp
     public static partial class ImageExtensions
     {
         /// <summary>
-        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// Dithers the image reducing it to a web-safe palette using ordered dithering.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
@@ -24,27 +23,27 @@ namespace SixLabors.ImageSharp
         public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither)
             where TPixel : struct, IPixel<TPixel>
         {
-            source.ApplyProcessor(new OrderedDitherProcessor<TPixel>(dither, 0));
+            source.ApplyProcessor(new OrderedDitherPaletteProcessor<TPixel>(dither));
             return source;
         }
 
         /// <summary>
-        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// Dithers the image reducing it to the given palette using ordered dithering.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="dither">The ordered ditherer.</param>
-        /// <param name="index">The component index to test the threshold against. Must range from 0 to 3.</param>
+        /// <param name="palette">The palette to select substitute colors from.</param>
         /// <returns>The <see cref="Image{TPixel}"/>.</returns>
-        public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, int index)
+        public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, TPixel[] palette)
             where TPixel : struct, IPixel<TPixel>
         {
-            source.ApplyProcessor(new OrderedDitherProcessor<TPixel>(dither, index));
+            source.ApplyProcessor(new OrderedDitherPaletteProcessor<TPixel>(dither, palette));
             return source;
         }
 
         /// <summary>
-        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// Dithers the image reducing it to a web-safe palette using ordered dithering.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
@@ -56,58 +55,25 @@ namespace SixLabors.ImageSharp
         public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, Rectangle rectangle)
             where TPixel : struct, IPixel<TPixel>
         {
-            source.ApplyProcessor(new OrderedDitherProcessor<TPixel>(dither, 0), rectangle);
+            source.ApplyProcessor(new OrderedDitherPaletteProcessor<TPixel>(dither), rectangle);
             return source;
         }
 
         /// <summary>
-        /// Dithers the image reducing it to two colors using ordered dithering.
+        /// Dithers the image reducing it to the given palette using ordered dithering.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <param name="source">The image this method extends.</param>
         /// <param name="dither">The ordered ditherer.</param>
+        /// <param name="palette">The palette to select substitute colors from.</param>
         /// <param name="rectangle">
         /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
         /// </param>
-        /// <param name="index">The component index to test the threshold against. Must range from 0 to 3.</param>
         /// <returns>The <see cref="Image{TPixel}"/>.</returns>
-        public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, Rectangle rectangle, int index)
+        public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IOrderedDither dither, TPixel[] palette, Rectangle rectangle)
             where TPixel : struct, IPixel<TPixel>
         {
-            source.ApplyProcessor(new OrderedDitherProcessor<TPixel>(dither, index), rectangle);
+            source.ApplyProcessor(new OrderedDitherPaletteProcessor<TPixel>(dither, palette), rectangle);
-            return source;
-        }
-
-        /// <summary>
-        /// Dithers the image reducing it to two colors using error diffusion.
-        /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="source">The image this method extends.</param>
-        /// <param name="diffuser">The diffusion algorithm to apply.</param>
-        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
-        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
-        public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold)
-            where TPixel : struct, IPixel<TPixel>
-        {
-            source.ApplyProcessor(new ErrorDiffusionDitherProcessor<TPixel>(diffuser, threshold));
-            return source;
-        }
-
-        /// <summary>
-        /// Dithers the image reducing it to two colors using error diffusion.
-        /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="source">The image this method extends.</param>
-        /// <param name="diffuser">The diffusion algorithm to apply.</param>
-        /// <param name="threshold">The threshold to apply binarization of the image. Must be between 0 and 1.</param>
-        /// <param name="rectangle">
-        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
-        /// </param>
-        /// <returns>The <see cref="Image{TPixel}"/>.</returns>
-        public static IImageProcessingContext<TPixel> Dither<TPixel>(this IImageProcessingContext<TPixel> source, IErrorDiffuser diffuser, float threshold, Rectangle rectangle)
-            where TPixel : struct, IPixel<TPixel>
-        {
-            source.ApplyProcessor(new ErrorDiffusionDitherProcessor<TPixel>(diffuser, threshold), rectangle);
             return source;
         }
     }

src/ImageSharp/Processing/Processors/Binarization/BinaryErrorDiffusionProcessor.cs

@@ -0,0 +1,123 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.Helpers;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// Performs binary threshold filtering against an image using error diffusion.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    internal class BinaryErrorDiffusionProcessor<TPixel> : ImageProcessor<TPixel>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryErrorDiffusionProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="diffuser">The error diffuser</param>
+        public BinaryErrorDiffusionProcessor(IErrorDiffuser diffuser)
+            : this(diffuser, .5F)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryErrorDiffusionProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="diffuser">The error diffuser</param>
+        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
+        public BinaryErrorDiffusionProcessor(IErrorDiffuser diffuser, float threshold)
+            : this(diffuser, threshold, NamedColors<TPixel>.White, NamedColors<TPixel>.Black)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryErrorDiffusionProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="diffuser">The error diffuser</param>
+        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold.</param>
+        public BinaryErrorDiffusionProcessor(IErrorDiffuser diffuser, float threshold, TPixel upperColor, TPixel lowerColor)
+        {
+            Guard.NotNull(diffuser, nameof(diffuser));
+            Guard.MustBeBetweenOrEqualTo(threshold, 0, 1, nameof(threshold));
+
+            this.Diffuser = diffuser;
+            this.Threshold = threshold;
+            this.UpperColor = upperColor;
+            this.LowerColor = lowerColor;
+        }
+
+        /// <summary>
+        /// Gets the error diffuser.
+        /// </summary>
+        public IErrorDiffuser Diffuser { get; }
+
+        /// <summary>
+        /// Gets the threshold value.
+        /// </summary>
+        public float Threshold { get; }
+
+        /// <summary>
+        /// Gets the color to use for pixels that are above the threshold.
+        /// </summary>
+        public TPixel UpperColor { get; }
+
+        /// <summary>
+        /// Gets the color to use for pixels that fall below the threshold.
+        /// </summary>
+        public TPixel LowerColor { get; }
+
+        /// <inheritdoc/>
+        protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
+        {
+            float threshold = this.Threshold * 255F;
+            var rgba = default(Rgba32);
+            bool isAlphaOnly = typeof(TPixel) == typeof(Alpha8);
+
+            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
+            int startY = interest.Y;
+            int endY = interest.Bottom;
+            int startX = interest.X;
+            int endX = interest.Right;
+
+            // Collect the values before looping so we can reduce our calculation count for identical sibling pixels
+            TPixel sourcePixel = source[startX, startY];
+            TPixel previousPixel = sourcePixel;
+            sourcePixel.ToRgba32(ref rgba);
+
+            // Convert to grayscale using ITU-R Recommendation BT.709 if required
+            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+            for (int y = startY; y < endY; y++)
+            {
+                Span<TPixel> row = source.GetPixelRowSpan(y);
+
+                for (int x = startX; x < endX; x++)
+                {
+                    sourcePixel = row[x];
+
+                    // Check if this is the same as the last pixel. If so use that value
+                    // rather than calculating it again. This is an inexpensive optimization.
+                    if (!previousPixel.Equals(sourcePixel))
+                    {
+                        sourcePixel.ToRgba32(ref rgba);
+                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+                        // Setup the previous pointer
+                        previousPixel = sourcePixel;
+                    }
+
+                    TPixel transformedPixel = luminance >= threshold ? this.UpperColor : this.LowerColor;
+                    this.Diffuser.Dither(source, sourcePixel, transformedPixel, x, y, startX, startY, endX, endY);
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Processing/Processors/Binarization/BinaryOrderedDitherProcessor.cs

@@ -0,0 +1,103 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.Helpers;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// Performs binary threshold filtering against an image using ordered dithering.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    internal class BinaryOrderedDitherProcessor<TPixel> : ImageProcessor<TPixel>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryOrderedDitherProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="dither">The ordered ditherer.</param>
+        public BinaryOrderedDitherProcessor(IOrderedDither dither)
+            : this(dither, NamedColors<TPixel>.White, NamedColors<TPixel>.Black)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryOrderedDitherProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="dither">The ordered ditherer.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold.</param>
+        public BinaryOrderedDitherProcessor(IOrderedDither dither, TPixel upperColor, TPixel lowerColor)
+        {
+            Guard.NotNull(dither, nameof(dither));
+
+            this.Dither = dither;
+            this.UpperColor = upperColor;
+            this.LowerColor = lowerColor;
+        }
+
+        /// <summary>
+        /// Gets the ditherer.
+        /// </summary>
+        public IOrderedDither Dither { get; }
+
+        /// <summary>
+        /// Gets the color to use for pixels that are above the threshold.
+        /// </summary>
+        public TPixel UpperColor { get; }
+
+        /// <summary>
+        /// Gets the color to use for pixels that fall below the threshold.
+        /// </summary>
+        public TPixel LowerColor { get; }
+
+        /// <inheritdoc/>
+        protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
+        {
+            var rgba = default(Rgba32);
+            bool isAlphaOnly = typeof(TPixel) == typeof(Alpha8);
+
+            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
+            int startY = interest.Y;
+            int endY = interest.Bottom;
+            int startX = interest.X;
+            int endX = interest.Right;
+
+            // Collect the values before looping so we can reduce our calculation count for identical sibling pixels
+            TPixel sourcePixel = source[startX, startY];
+            TPixel previousPixel = sourcePixel;
+            sourcePixel.ToRgba32(ref rgba);
+
+            // Convert to grayscale using ITU-R Recommendation BT.709 if required
+            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+            for (int y = startY; y < endY; y++)
+            {
+                Span<TPixel> row = source.GetPixelRowSpan(y);
+
+                for (int x = startX; x < endX; x++)
+                {
+                    sourcePixel = row[x];
+
+                    // Check if this is the same as the last pixel. If so use that value
+                    // rather than calculating it again. This is an inexpensive optimization.
+                    if (!previousPixel.Equals(sourcePixel))
+                    {
+                        sourcePixel.ToRgba32(ref rgba);
+                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+                        // Setup the previous pointer
+                        previousPixel = sourcePixel;
+                    }
+
+                    this.Dither.Dither(source, sourcePixel, this.UpperColor, this.LowerColor, luminance, x, y);
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Processing/Processors/Binarization/BinaryThresholdProcessor.cs

@@ -4,14 +4,14 @@
 using System;
 using System.Threading.Tasks;
 using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Helpers;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.Primitives;
 
 namespace SixLabors.ImageSharp.Processing.Processors
 {
     /// <summary>
-    /// An <see cref="IImageProcessor{TPixel}"/> to perform binary threshold filtering against an
+    /// Performs simple binary threshold filtering against an image.
-    /// <see cref="Image{TPixel}"/>. The image will be converted to grayscale before thresholding occurs.
     /// </summary>
     /// <typeparam name="TPixel">The pixel format.</typeparam>
     internal class BinaryThresholdProcessor<TPixel> : ImageProcessor<TPixel>
@@ -22,14 +22,22 @@ namespace SixLabors.ImageSharp.Processing.Processors
         /// </summary>
         /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
         public BinaryThresholdProcessor(float threshold)
+            : this(threshold, NamedColors<TPixel>.White, NamedColors<TPixel>.Black)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BinaryThresholdProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
+        /// <param name="upperColor">The color to use for pixels that are above the threshold.</param>
+        /// <param name="lowerColor">The color to use for pixels that are below the threshold.</param>
+        public BinaryThresholdProcessor(float threshold, TPixel upperColor, TPixel lowerColor)
         {
-            // TODO: Check thresholding limit. Colors should probably have Max/Min/Middle properties.
             Guard.MustBeBetweenOrEqualTo(threshold, 0, 1, nameof(threshold));
             this.Threshold = threshold;
-
+            this.UpperColor = upperColor;
-            // Default to white/black for upper/lower.
+            this.LowerColor = lowerColor;
-            this.UpperColor = NamedColors<TPixel>.White;
-            this.LowerColor = NamedColors<TPixel>.Black;
         }
 
         /// <summary>
@@ -47,55 +55,38 @@ namespace SixLabors.ImageSharp.Processing.Processors
         /// </summary>
         public TPixel LowerColor { get; set; }
 
-        /// <inheritdoc/>
-        protected override void BeforeApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
-        {
-            new GrayscaleBt709Processor<TPixel>(1F).Apply(source, sourceRectangle, configuration);
-        }
-
         /// <inheritdoc/>
         protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
         {
-            float threshold = this.Threshold;
+            float threshold = this.Threshold * 255F;
             TPixel upper = this.UpperColor;
             TPixel lower = this.LowerColor;
 
-            int startY = sourceRectangle.Y;
+            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
-            int endY = sourceRectangle.Bottom;
+            int startY = interest.Y;
-            int startX = sourceRectangle.X;
+            int endY = interest.Bottom;
-            int endX = sourceRectangle.Right;
+            int startX = interest.X;
-
+            int endX = interest.Right;
-            // Align start/end positions.
-            int minX = Math.Max(0, startX);
-            int maxX = Math.Min(source.Width, endX);
-            int minY = Math.Max(0, startY);
-            int maxY = Math.Min(source.Height, endY);
-
-            // Reset offset if necessary.
-            if (minX > 0)
-            {
-                startX = 0;
-            }
 
-            if (minY > 0)
+            bool isAlphaOnly = typeof(TPixel) == typeof(Alpha8);
-            {
-                startY = 0;
-            }
 
             Parallel.For(
-                minY,
+                startY,
-                maxY,
+                endY,
                 configuration.ParallelOptions,
                 y =>
                 {
-                    Span<TPixel> row = source.GetPixelRowSpan(y - startY);
+                    Span<TPixel> row = source.GetPixelRowSpan(y);
+                    var rgba = default(Rgba32);
 
-                    for (int x = minX; x < maxX; x++)
+                    for (int x = startX; x < endX; x++)
                     {
-                        ref TPixel color = ref row[x - startX];
+                        ref TPixel color = ref row[x];
+                        color.ToRgba32(ref rgba);
 
-                        // Any channel will do since it's Grayscale.
+                        // Convert to grayscale using ITU-R Recommendation BT.709 if required
-                        color = color.ToVector4().X >= threshold ? upper : lower;
+                        float luminance = isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B);
+                        color = luminance >= threshold ? upper : lower;
                     }
                 });
         }

src/ImageSharp/Processing/Processors/Binarization/ErrorDiffusionDitherProcessor.cs

@@ -1,85 +0,0 @@
-// Copyright (c) Six Labors and contributors.
-// Licensed under the Apache License, Version 2.0.
-
-using System;
-using SixLabors.ImageSharp.Advanced;
-using SixLabors.ImageSharp.Dithering;
-using SixLabors.ImageSharp.Helpers;
-using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.Primitives;
-
-namespace SixLabors.ImageSharp.Processing.Processors
-{
-    /// <summary>
-    /// An <see cref="IImageProcessor{TPixel}"/> that dithers an image using error diffusion.
-    /// </summary>
-    /// <typeparam name="TPixel">The pixel format.</typeparam>
-    internal class ErrorDiffusionDitherProcessor<TPixel> : ImageProcessor<TPixel>
-        where TPixel : struct, IPixel<TPixel>
-    {
-        /// <summary>
-        /// Initializes a new instance of the <see cref="ErrorDiffusionDitherProcessor{TPixel}"/> class.
-        /// </summary>
-        /// <param name="diffuser">The error diffuser</param>
-        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
-        public ErrorDiffusionDitherProcessor(IErrorDiffuser diffuser, float threshold)
-        {
-            Guard.NotNull(diffuser, nameof(diffuser));
-
-            this.Diffuser = diffuser;
-            this.Threshold = threshold;
-
-            // Default to white/black for upper/lower.
-            this.UpperColor = NamedColors<TPixel>.White;
-            this.LowerColor = NamedColors<TPixel>.Black;
-        }
-
-        /// <summary>
-        /// Gets the error diffuser.
-        /// </summary>
-        public IErrorDiffuser Diffuser { get; }
-
-        /// <summary>
-        /// Gets the threshold value.
-        /// </summary>
-        public float Threshold { get; }
-
-        /// <summary>
-        /// Gets or sets the color to use for pixels that are above the threshold.
-        /// </summary>
-        public TPixel UpperColor { get; set; }
-
-        /// <summary>
-        /// Gets or sets the color to use for pixels that fall below the threshold.
-        /// </summary>
-        public TPixel LowerColor { get; set; }
-
-        /// <inheritdoc/>
-        protected override void BeforeApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
-        {
-            new GrayscaleBt709Processor<TPixel>(1F).Apply(source, sourceRectangle, configuration);
-        }
-
-        /// <inheritdoc/>
-        protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
-        {
-            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
-            int startY = interest.Y;
-            int endY = interest.Bottom;
-            int startX = interest.X;
-            int endX = interest.Right;
-
-            for (int y = startY; y < endY; y++)
-            {
-                Span<TPixel> row = source.GetPixelRowSpan(y);
-
-                for (int x = startX; x < endX; x++)
-                {
-                    TPixel sourceColor = row[x];
-                    TPixel transformedColor = sourceColor.ToVector4().X >= this.Threshold ? this.UpperColor : this.LowerColor;
-                    this.Diffuser.Dither(source, sourceColor, transformedColor, x, y, startX, startY, endX, endY);
-                }
-            }
-        }
-    }
-}

src/ImageSharp/Processing/Processors/Binarization/OrderedDitherProcessor.cs

@@ -1,93 +0,0 @@
-// Copyright (c) Six Labors and contributors.
-// Licensed under the Apache License, Version 2.0.
-
-using System;
-using System.Buffers;
-using SixLabors.ImageSharp.Advanced;
-using SixLabors.ImageSharp.Dithering;
-using SixLabors.ImageSharp.Helpers;
-using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.Primitives;
-
-namespace SixLabors.ImageSharp.Processing.Processors
-{
-    /// <summary>
-    /// An <see cref="IImageProcessor{TPixel}"/> that dithers an image using error diffusion.
-    /// </summary>
-    /// <typeparam name="TPixel">The pixel format.</typeparam>
-    internal class OrderedDitherProcessor<TPixel> : ImageProcessor<TPixel>
-        where TPixel : struct, IPixel<TPixel>
-    {
-        /// <summary>
-        /// Initializes a new instance of the <see cref="OrderedDitherProcessor{TPixel}"/> class.
-        /// </summary>
-        /// <param name="dither">The ordered ditherer.</param>
-        /// <param name="index">The component index to test the threshold against. Must range from 0 to 3.</param>
-        public OrderedDitherProcessor(IOrderedDither dither, int index)
-        {
-            Guard.NotNull(dither, nameof(dither));
-            Guard.MustBeBetweenOrEqualTo(index, 0, 3, nameof(index));
-
-            // Alpha8 only stores the pixel data in the alpha channel.
-            if (typeof(TPixel) == typeof(Alpha8))
-            {
-                index = 3;
-            }
-
-            this.Dither = dither;
-            this.Index = index;
-
-            // Default to white/black for upper/lower.
-            this.UpperColor = NamedColors<TPixel>.White;
-            this.LowerColor = NamedColors<TPixel>.Black;
-        }
-
-        /// <summary>
-        /// Gets the ditherer.
-        /// </summary>
-        public IOrderedDither Dither { get; }
-
-        /// <summary>
-        /// Gets the component index to test the threshold against.
-        /// </summary>
-        public int Index { get; }
-
-        /// <summary>
-        /// Gets or sets the color to use for pixels that are above the threshold.
-        /// </summary>
-        public TPixel UpperColor { get; set; }
-
-        /// <summary>
-        /// Gets or sets the color to use for pixels that fall below the threshold.
-        /// </summary>
-        public TPixel LowerColor { get; set; }
-
-        /// <inheritdoc/>
-        protected override void BeforeApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
-        {
-            new GrayscaleBt709Processor<TPixel>(1F).Apply(source, sourceRectangle, configuration);
-        }
-
-        /// <inheritdoc/>
-        protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
-        {
-            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
-            int startY = interest.Y;
-            int endY = interest.Bottom;
-            int startX = interest.X;
-            int endX = interest.Right;
-
-            var rgba = default(Rgba32);
-            for (int y = startY; y < endY; y++)
-            {
-                Span<TPixel> row = source.GetPixelRowSpan(y);
-
-                for (int x = startX; x < endX; x++)
-                {
-                    TPixel sourceColor = row[x];
-                    this.Dither.Dither(source, sourceColor, this.UpperColor, this.LowerColor, ref rgba, this.Index, x, y);
-                }
-            }
-        }
-    }
-}

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

@@ -0,0 +1,113 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.Helpers;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// An <see cref="IImageProcessor{TPixel}"/> that dithers an image using error diffusion.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    internal class ErrorDiffusionPaletteProcessor<TPixel> : PaletteDitherProcessorBase<TPixel>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ErrorDiffusionPaletteProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="diffuser">The error diffuser</param>
+        public ErrorDiffusionPaletteProcessor(IErrorDiffuser diffuser)
+            : this(diffuser, .5F)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ErrorDiffusionPaletteProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="diffuser">The error diffuser</param>
+        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
+        public ErrorDiffusionPaletteProcessor(IErrorDiffuser diffuser, float threshold)
+            : this(diffuser, threshold, NamedColors<TPixel>.WebSafePalette)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ErrorDiffusionPaletteProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="diffuser">The error diffuser</param>
+        /// <param name="threshold">The threshold to split the image. Must be between 0 and 1.</param>
+        /// <param name="palette">The palette to select substitute colors from.</param>
+        public ErrorDiffusionPaletteProcessor(IErrorDiffuser diffuser, float threshold, TPixel[] palette)
+            : base(palette)
+        {
+            Guard.NotNull(diffuser, nameof(diffuser));
+            Guard.MustBeBetweenOrEqualTo(threshold, 0, 1, nameof(threshold));
+
+            this.Diffuser = diffuser;
+            this.Threshold = threshold;
+        }
+
+        /// <summary>
+        /// Gets the error diffuser.
+        /// </summary>
+        public IErrorDiffuser Diffuser { get; }
+
+        /// <summary>
+        /// Gets the threshold value.
+        /// </summary>
+        public float Threshold { get; }
+
+        /// <inheritdoc/>
+        protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
+        {
+            float threshold = this.Threshold * 255F;
+            var rgba = default(Rgba32);
+            bool isAlphaOnly = typeof(TPixel) == typeof(Alpha8);
+
+            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
+            int startY = interest.Y;
+            int endY = interest.Bottom;
+            int startX = interest.X;
+            int endX = interest.Right;
+
+            // Collect the values before looping so we can reduce our calculation count for identical sibling pixels
+            TPixel sourcePixel = source[startX, startY];
+            TPixel previousPixel = sourcePixel;
+            PixelPair<TPixel> pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
+            sourcePixel.ToRgba32(ref rgba);
+
+            // Convert to grayscale using ITU-R Recommendation BT.709 if required
+            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+            for (int y = startY; y < endY; y++)
+            {
+                Span<TPixel> row = source.GetPixelRowSpan(y);
+
+                for (int x = startX; x < endX; x++)
+                {
+                    sourcePixel = row[x];
+
+                    // Check if this is the same as the last pixel. If so use that value
+                    // rather than calculating it again. This is an inexpensive optimization.
+                    if (!previousPixel.Equals(sourcePixel))
+                    {
+                        pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
+                        sourcePixel.ToRgba32(ref rgba);
+                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+                        // Setup the previous pointer
+                        previousPixel = sourcePixel;
+                    }
+
+                    TPixel transformedPixel = luminance >= threshold ? pair.First : pair.Second;
+                    this.Diffuser.Dither(source, sourcePixel, transformedPixel, x, y, startX, startY, endX, endY);
+                }
+            }
+        }
+    }
+}

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

@@ -0,0 +1,93 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Dithering;
+using SixLabors.ImageSharp.Helpers;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// An <see cref="IImageProcessor{TPixel}"/> that dithers an image using error diffusion.
+    /// If no palette is given this will default to the web safe colors defined in the CSS Color Module Level 4.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    internal class OrderedDitherPaletteProcessor<TPixel> : PaletteDitherProcessorBase<TPixel>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="OrderedDitherPaletteProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="dither">The ordered ditherer.</param>
+        public OrderedDitherPaletteProcessor(IOrderedDither dither)
+            : this(dither, NamedColors<TPixel>.WebSafePalette)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="OrderedDitherPaletteProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="dither">The ordered ditherer.</param>
+        /// <param name="palette">The palette to select substitute colors from.</param>
+        public OrderedDitherPaletteProcessor(IOrderedDither dither, TPixel[] palette)
+            : base(palette)
+        {
+            Guard.NotNull(dither, nameof(dither));
+            this.Dither = dither;
+        }
+
+        /// <summary>
+        /// Gets the ditherer.
+        /// </summary>
+        public IOrderedDither Dither { get; }
+
+        /// <inheritdoc/>
+        protected override void OnApply(ImageFrame<TPixel> source, Rectangle sourceRectangle, Configuration configuration)
+        {
+            var rgba = default(Rgba32);
+            bool isAlphaOnly = typeof(TPixel) == typeof(Alpha8);
+
+            var interest = Rectangle.Intersect(sourceRectangle, source.Bounds());
+            int startY = interest.Y;
+            int endY = interest.Bottom;
+            int startX = interest.X;
+            int endX = interest.Right;
+
+            // Collect the values before looping so we can reduce our calculation count for identical sibling pixels
+            TPixel sourcePixel = source[startX, startY];
+            TPixel previousPixel = sourcePixel;
+            PixelPair<TPixel> pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
+            sourcePixel.ToRgba32(ref rgba);
+
+            // Convert to grayscale using ITU-R Recommendation BT.709 if required
+            byte luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+            for (int y = startY; y < endY; y++)
+            {
+                Span<TPixel> row = source.GetPixelRowSpan(y);
+
+                for (int x = startX; x < endX; x++)
+                {
+                    sourcePixel = row[x];
+
+                    // Check if this is the same as the last pixel. If so use that value
+                    // rather than calculating it again. This is an inexpensive optimization.
+                    if (!previousPixel.Equals(sourcePixel))
+                    {
+                        pair = this.GetClosestPixelPair(ref sourcePixel, this.Palette);
+                        sourcePixel.ToRgba32(ref rgba);
+                        luminance = (byte)(isAlphaOnly ? rgba.A : (.2126F * rgba.R) + (.7152F * rgba.G) + (.0722F * rgba.B)).Clamp(0, 255);
+
+                        // Setup the previous pointer
+                        previousPixel = sourcePixel;
+                    }
+
+                    this.Dither.Dither(source, sourcePixel, pair.First, pair.Second, luminance, x, y);
+                }
+            }
+        }
+    }
+}

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

@@ -0,0 +1,76 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Collections.Generic;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// The base class for dither and diffusion processors that consume a palette.
+    /// </summary>
+    internal abstract class PaletteDitherProcessorBase<TPixel> : ImageProcessor<TPixel>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        private readonly Dictionary<TPixel, PixelPair<TPixel>> cache = new Dictionary<TPixel, PixelPair<TPixel>>();
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PaletteDitherProcessorBase{TPixel}"/> class.
+        /// </summary>
+        /// <param name="palette">The palette to select substitute colors from.</param>
+        public PaletteDitherProcessorBase(TPixel[] palette)
+        {
+            Guard.NotNull(palette, nameof(palette));
+            this.Palette = palette;
+        }
+
+        /// <summary>
+        /// Gets the palette to select substitute colors from.
+        /// </summary>
+        public TPixel[] Palette { get; }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        protected PixelPair<TPixel> GetClosestPixelPair(ref TPixel pixel, TPixel[] colorPalette)
+        {
+            // Check if the color is in the lookup table
+            if (this.cache.ContainsKey(pixel))
+            {
+                return this.cache[pixel];
+            }
+
+            // Not found - loop through the palette and find the nearest match.
+            float leastDistance = int.MaxValue;
+            float secondLeastDistance = int.MaxValue;
+            var vector = pixel.ToVector4();
+
+            var closest = default(TPixel);
+            var secondClosest = default(TPixel);
+            for (int index = 0; index < colorPalette.Length; index++)
+            {
+                TPixel temp = colorPalette[index];
+                var tempVector = temp.ToVector4();
+                float distance = Vector4.Distance(vector, tempVector);
+
+                if (distance < leastDistance)
+                {
+                    leastDistance = distance;
+                    secondClosest = closest;
+                    closest = temp;
+                }
+                else if (distance < secondLeastDistance)
+                {
+                    secondLeastDistance = distance;
+                    secondClosest = temp;
+                }
+            }
+
+            // Pop it into the cache for next time
+            var pair = new PixelPair<TPixel>(closest, secondClosest);
+            this.cache.Add(pixel, pair);
+
+            return pair;
+        }
+    }
+}

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

@@ -0,0 +1,49 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// Represents a composite pair of pixels. Used for caching color distance lookups.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    internal struct PixelPair<TPixel> : IEquatable<PixelPair<TPixel>>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PixelPair{TPixel}"/> struct.
+        /// </summary>
+        /// <param name="first">The first pixel color</param>
+        /// <param name="second">The second pixel color</param>
+        public PixelPair(TPixel first, TPixel second)
+        {
+            this.First = first;
+            this.Second = second;
+        }
+
+        /// <summary>
+        /// Gets the first pixel color
+        /// </summary>
+        public TPixel First { get; }
+
+        /// <summary>
+        /// Gets the second pixel color
+        /// </summary>
+        public TPixel Second { get; }
+
+        /// <inheritdoc/>
+        public bool Equals(PixelPair<TPixel> other)
+            => this.First.Equals(other.First) && this.Second.Equals(other.Second);
+
+        /// <inheritdoc/>
+        public override bool Equals(object obj)
+            => obj is PixelPair<TPixel> other && this.First.Equals(other.First) && this.Second.Equals(other.Second);
+
+        /// <inheritdoc/>
+        public override int GetHashCode()
+            => HashHelpers.Combine(this.First.GetHashCode(), this.Second.GetHashCode());
+    }
+}

src/ImageSharp/Processing/Processors/Filters/GrayscaleBt709Processor.cs

@@ -6,7 +6,7 @@ using SixLabors.ImageSharp.PixelFormats;
 namespace SixLabors.ImageSharp.Processing.Processors
 {
     /// <summary>
-    /// Applies a greyscale filter matrix using the given amount and the formula as specified by ITU-R Recommendation BT.709
+    /// Applies a grayscale filter matrix using the given amount and the formula as specified by ITU-R Recommendation BT.709
     /// </summary>
     /// <typeparam name="TPixel">The pixel format.</typeparam>
     internal class GrayscaleBt709Processor<TPixel> : FilterProcessor<TPixel>

src/ImageSharp/Quantizers/OctreeQuantizer{TPixel}.cs

@@ -107,7 +107,7 @@ namespace SixLabors.ImageSharp.Quantizers
                     if (this.Dither)
                     {
                         // Apply the dithering matrix. We have to reapply the value now as the original has changed.
-                        this.DitherType.Dither(source, sourcePixel, transformedPixel, x, y, 0, 0, width, height, false);
+                        this.DitherType.Dither(source, sourcePixel, transformedPixel, x, y, 0, 0, width, height);
                     }
 
                     output[(y * source.Width) + x] = pixelValue;

src/ImageSharp/Quantizers/PaletteQuantizer{TPixel}.cs

@@ -12,6 +12,7 @@ namespace SixLabors.ImageSharp.Quantizers
 {
     /// <summary>
     /// Encapsulates methods to create a quantized image based upon the given palette.
+    /// If no palette is given this will default to the web safe colors defined in the CSS Color Module Level 4.
     /// <see href="http://msdn.microsoft.com/en-us/library/aa479306.aspx"/>
     /// </summary>
     /// <typeparam name="TPixel">The pixel format.</typeparam>
@@ -31,27 +32,20 @@ namespace SixLabors.ImageSharp.Quantizers
         /// <summary>
         /// Initializes a new instance of the <see cref="PaletteQuantizer{TPixel}"/> class.
         /// </summary>
-        /// <param name="palette">
+        public PaletteQuantizer()
-        /// The color palette. If none is given this will default to the web safe colors defined
+            : this(NamedColors<TPixel>.WebSafePalette)
-        /// in the CSS Color Module Level 4.
+        {
-        /// </param>
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PaletteQuantizer{TPixel}"/> class.
+        /// </summary>
+        /// <param name="palette">The palette to select substitute colors from.</param>
         public PaletteQuantizer(TPixel[] palette = null)
             : base(true)
         {
-            if (palette == null)
+            Guard.NotNull(palette, nameof(palette));
-            {
+            this.colors = palette;
-                Rgba32[] constants = ColorConstants.WebSafeColors;
-                TPixel[] safe = new TPixel[constants.Length + 1];
-
-                Span<byte> constantsBytes = constants.AsSpan().NonPortableCast<Rgba32, byte>();
-
-                PixelOperations<TPixel>.Instance.PackFromRgba32Bytes(constantsBytes, safe, constants.Length);
-                this.colors = safe;
-            }
-            else
-            {
-                this.colors = palette;
-            }
         }
 
         /// <inheritdoc/>
@@ -102,7 +96,7 @@ namespace SixLabors.ImageSharp.Quantizers
                     if (this.Dither)
                     {
                         // Apply the dithering matrix. We have to reapply the value now as the original has changed.
-                        this.DitherType.Dither(source, sourcePixel, transformedPixel, x, y, 0, 0, width, height, false);
+                        this.DitherType.Dither(source, sourcePixel, transformedPixel, x, y, 0, 0, width, height);
                     }
 
                     output[(y * source.Width) + x] = pixelValue;

src/ImageSharp/Quantizers/WuQuantizer{TPixel}.cs

@@ -272,7 +272,7 @@ namespace SixLabors.ImageSharp.Quantizers
                     if (this.Dither)
                     {
                         // Apply the dithering matrix. We have to reapply the value now as the original has changed.
-                        this.DitherType.Dither(source, sourcePixel, transformedPixel, x, y, 0, 0, width, height, false);
+                        this.DitherType.Dither(source, sourcePixel, transformedPixel, x, y, 0, 0, width, height);
                     }
 
                     output[(y * source.Width) + x] = pixelValue;

tests/ImageSharp.Tests/Processing/Binarization/BinaryThresholdTest.cs

@@ -15,16 +15,40 @@ namespace SixLabors.ImageSharp.Tests.Processing.Binarization
         public void BinaryThreshold_CorrectProcessor()
         {
             this.operations.BinaryThreshold(.23f);
-            var p = this.Verify<BinaryThresholdProcessor<Rgba32>>();
+            BinaryThresholdProcessor<Rgba32> p = this.Verify<BinaryThresholdProcessor<Rgba32>>();
             Assert.Equal(.23f, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.White, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Black, p.LowerColor);
         }
 
         [Fact]
         public void BinaryThreshold_rect_CorrectProcessor()
         {
             this.operations.BinaryThreshold(.93f, this.rect);
-            var p = this.Verify<BinaryThresholdProcessor<Rgba32>>(this.rect);
+            BinaryThresholdProcessor<Rgba32> p = this.Verify<BinaryThresholdProcessor<Rgba32>>(this.rect);
             Assert.Equal(.93f, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.White, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Black, p.LowerColor);
+        }
+
+        [Fact]
+        public void BinaryThreshold_CorrectProcessorWithUpperLower()
+        {
+            this.operations.BinaryThreshold(.23f, NamedColors<Rgba32>.HotPink, NamedColors<Rgba32>.Yellow);
+            BinaryThresholdProcessor<Rgba32> p = this.Verify<BinaryThresholdProcessor<Rgba32>>();
+            Assert.Equal(.23f, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.HotPink, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Yellow, p.LowerColor);
+        }
+
+        [Fact]
+        public void BinaryThreshold_rect_CorrectProcessorWithUpperLower()
+        {
+            this.operations.BinaryThreshold(.93f, NamedColors<Rgba32>.HotPink, NamedColors<Rgba32>.Yellow, this.rect);
+            BinaryThresholdProcessor<Rgba32> p = this.Verify<BinaryThresholdProcessor<Rgba32>>(this.rect);
+            Assert.Equal(.93f, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.HotPink, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Yellow, p.LowerColor);
         }
     }
 }

tests/ImageSharp.Tests/Processing/Binarization/DitherTests.cs

@@ -5,7 +5,6 @@ using SixLabors.ImageSharp.Dithering;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing.Processors;
 using Moq;
-using SixLabors.Primitives;
 using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Processing.Binarization
@@ -23,55 +22,84 @@ namespace SixLabors.ImageSharp.Tests.Processing.Binarization
         [Fact]
         public void Dither_CorrectProcessor()
         {
-            this.operations.Dither(orderedDither);
+            this.operations.BinaryDither(this.orderedDither);
-            var p = this.Verify<OrderedDitherProcessor<Rgba32>>();
+            BinaryOrderedDitherProcessor<Rgba32> p = this.Verify<BinaryOrderedDitherProcessor<Rgba32>>();
             Assert.Equal(this.orderedDither, p.Dither);
-            Assert.Equal(0, p.Index);
+            Assert.Equal(NamedColors<Rgba32>.White, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Black, p.LowerColor);
         }
 
         [Fact]
         public void Dither_rect_CorrectProcessor()
         {
-            this.operations.Dither(orderedDither, this.rect);
+            this.operations.BinaryDither(this.orderedDither, this.rect);
-            var p = this.Verify<OrderedDitherProcessor<Rgba32>>(this.rect);
+            BinaryOrderedDitherProcessor<Rgba32> p = this.Verify<BinaryOrderedDitherProcessor<Rgba32>>(this.rect);
             Assert.Equal(this.orderedDither, p.Dither);
-            Assert.Equal(0, p.Index);
+            Assert.Equal(NamedColors<Rgba32>.White, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Black, p.LowerColor);
         }
         [Fact]
         public void Dither_index_CorrectProcessor()
         {
-            this.operations.Dither(orderedDither, 2);
+            this.operations.BinaryDither(this.orderedDither, NamedColors<Rgba32>.Yellow, NamedColors<Rgba32>.HotPink);
-            var p = this.Verify<OrderedDitherProcessor<Rgba32>>();
+            BinaryOrderedDitherProcessor<Rgba32> p = this.Verify<BinaryOrderedDitherProcessor<Rgba32>>();
             Assert.Equal(this.orderedDither, p.Dither);
-            Assert.Equal(2, p.Index);
+            Assert.Equal(NamedColors<Rgba32>.Yellow, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.HotPink, p.LowerColor);
         }
 
         [Fact]
         public void Dither_index_rect_CorrectProcessor()
         {
-            this.operations.Dither(orderedDither, this.rect, 2);
+            this.operations.BinaryDither(this.orderedDither, NamedColors<Rgba32>.Yellow, NamedColors<Rgba32>.HotPink, this.rect);
-            var p = this.Verify<OrderedDitherProcessor<Rgba32>>(this.rect);
+            BinaryOrderedDitherProcessor<Rgba32> p = this.Verify<BinaryOrderedDitherProcessor<Rgba32>>(this.rect);
             Assert.Equal(this.orderedDither, p.Dither);
-            Assert.Equal(2, p.Index);
+            Assert.Equal(NamedColors<Rgba32>.HotPink, p.LowerColor);
         }
 
 
         [Fact]
-        public void Dither_ErrorDifuser_CorrectProcessor()
+        public void Dither_ErrorDiffuser_CorrectProcessor()
         {
-            this.operations.Dither(errorDiffuser, 4);
+            this.operations.BinaryDiffuse(this.errorDiffuser, .4F);
-            var p = this.Verify<ErrorDiffusionDitherProcessor<Rgba32>>();
+            BinaryErrorDiffusionProcessor<Rgba32> p = this.Verify<BinaryErrorDiffusionProcessor<Rgba32>>();
             Assert.Equal(this.errorDiffuser, p.Diffuser);
-            Assert.Equal(4, p.Threshold);
+            Assert.Equal(.4F, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.White, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Black, p.LowerColor);
         }
 
         [Fact]
-        public void Dither_ErrorDifuser_rect_CorrectProcessor()
+        public void Dither_ErrorDiffuser_rect_CorrectProcessor()
         {
-            this.operations.Dither(this.errorDiffuser, 3, this.rect);
+            this.operations.BinaryDiffuse(this.errorDiffuser, .3F, this.rect);
-            var p = this.Verify<ErrorDiffusionDitherProcessor<Rgba32>>(this.rect);
+            BinaryErrorDiffusionProcessor<Rgba32> p = this.Verify<BinaryErrorDiffusionProcessor<Rgba32>>(this.rect);
             Assert.Equal(this.errorDiffuser, p.Diffuser);
-            Assert.Equal(3, p.Threshold);
+            Assert.Equal(.3F, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.White, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Black, p.LowerColor);
+        }
+
+        [Fact]
+        public void Dither_ErrorDiffuser_CorrectProcessorWithColors()
+        {
+            this.operations.BinaryDiffuse(this.errorDiffuser, .5F, NamedColors<Rgba32>.HotPink, NamedColors<Rgba32>.Yellow);
+            BinaryErrorDiffusionProcessor<Rgba32> p = this.Verify<BinaryErrorDiffusionProcessor<Rgba32>>();
+            Assert.Equal(this.errorDiffuser, p.Diffuser);
+            Assert.Equal(.5F, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.HotPink, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Yellow, p.LowerColor);
+        }
+
+        [Fact]
+        public void Dither_ErrorDiffuser_rect_CorrectProcessorWithColors()
+        {
+            this.operations.BinaryDiffuse(this.errorDiffuser, .5F, NamedColors<Rgba32>.HotPink, NamedColors<Rgba32>.Yellow, this.rect);
+            BinaryErrorDiffusionProcessor<Rgba32> p = this.Verify<BinaryErrorDiffusionProcessor<Rgba32>>(this.rect);
+            Assert.Equal(this.errorDiffuser, p.Diffuser);
+            Assert.Equal(.5F, p.Threshold);
+            Assert.Equal(NamedColors<Rgba32>.HotPink, p.UpperColor);
+            Assert.Equal(NamedColors<Rgba32>.Yellow, p.LowerColor);
         }
     }
 }

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

@@ -22,8 +22,9 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
 
         public static readonly TheoryData<string, IOrderedDither> Ditherers = new TheoryData<string, IOrderedDither>
         {
-            { "Ordered", new OrderedDither() },
+            { "Bayer8x8", new Bayer8x8Dither() },
-            { "Bayer", new BayerDither() }
+            { "Bayer4x4", new Bayer4x4Dither() },
+            { "Bayer2x2", new Bayer2x2Dither() }
         };
 
         public static readonly TheoryData<string, IErrorDiffuser> ErrorDiffusers = new TheoryData<string, IErrorDiffuser>
@@ -39,7 +40,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
         };
 
 
-        private static IOrderedDither DefaultDitherer => new OrderedDither();
+        private static IOrderedDither DefaultDitherer => new Bayer4x4Dither();
 
         private static IErrorDiffuser DefaultErrorDiffuser => new AtkinsonDiffuser();
 
@@ -51,7 +52,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                image.Mutate(x => x.Dither(ditherer));
+                image.Mutate(x => x.BinaryDither(ditherer));
                 image.DebugSave(provider, name);
             }
         }
@@ -64,7 +65,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                image.Mutate(x => x.Dither(diffuser, .5F));
+                image.Mutate(x => x.BinaryDiffuse(diffuser, .5F));
                 image.DebugSave(provider, name);
             }
         }
@@ -76,7 +77,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                image.Mutate(x => x.Dither(DefaultDitherer));
+                image.Mutate(x => x.BinaryDither(DefaultDitherer));
                 image.DebugSave(provider);
             }
         }
@@ -88,7 +89,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
         {
             using (Image<TPixel> image = provider.GetImage())
             {
-                image.Mutate(x => x.Dither(DefaultErrorDiffuser, 0.5f));
+                image.Mutate(x => x.BinaryDiffuse(DefaultErrorDiffuser, 0.5f));
                 image.DebugSave(provider);
             }
         }
@@ -103,7 +104,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
             {
                 var bounds = new Rectangle(10, 10, image.Width / 2, image.Height / 2);
 
-                image.Mutate(x => x.Dither(DefaultDitherer, bounds));
+                image.Mutate(x => x.BinaryDither(DefaultDitherer, bounds));
                 image.DebugSave(provider);
 
                 ImageComparer.Tolerant().VerifySimilarityIgnoreRegion(source, image, bounds);
@@ -120,7 +121,7 @@ namespace SixLabors.ImageSharp.Tests.Processing.Processors.Binarization
             {
                 var bounds = new Rectangle(10, 10, image.Width / 2, image.Height / 2);
 
-                image.Mutate(x => x.Dither(DefaultErrorDiffuser, .5F, bounds));
+                image.Mutate(x => x.BinaryDiffuse(DefaultErrorDiffuser, .5F, bounds));
                 image.DebugSave(provider);
 
                 ImageComparer.Tolerant().VerifySimilarityIgnoreRegion(source, image, bounds);