Use Fast2DAarray for ordered dither.

src/ImageSharp/Common/Helpers/Fast2DArray{T}.cs

@@ -0,0 +1,102 @@
+// <copyright file="Fast2DArray{T}.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp
+{
+    using System;
+    using System.Diagnostics;
+    using System.Runtime.CompilerServices;
+
+    /// <summary>
+    /// Provides fast access to 2D arrays.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the array.</typeparam>
+    public struct Fast2DArray<T>
+    {
+        /// <summary>
+        /// The 1D representation of the 2D array.
+        /// </summary>
+        public T[] Data;
+
+        /// <summary>
+        /// Gets the width of the 2D array.
+        /// </summary>
+        public int Width;
+
+        /// <summary>
+        /// Gets the height of the 2D array.
+        /// </summary>
+        public int Height;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Fast2DArray{T}"/> struct.
+        /// </summary>
+        /// <param name="data">The 2D array to provide access to.</param>
+        public Fast2DArray(T[,] data)
+        {
+            Guard.NotNull(data, nameof(data));
+            this.Height = data.GetLength(0);
+            this.Width = data.GetLength(1);
+
+            Guard.MustBeGreaterThan(this.Width, 0, nameof(this.Width));
+            Guard.MustBeGreaterThan(this.Height, 0, nameof(this.Height));
+
+            this.Data = new T[this.Width * this.Height];
+
+            for (int y = 0; y < this.Height; y++)
+            {
+                for (int x = 0; x < this.Width; x++)
+                {
+                    this.Data[(y * this.Width) + x] = data[y, x];
+                }
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the item at the specified position.
+        /// </summary>
+        /// <param name="row">The row-coordinate of the item. Must be greater than or equal to zero and less than the height of the array.</param>
+        /// <param name="column">The column-coordinate of the item. Must be greater than or equal to zero and less than the width of the array.</param>
+        /// <returns>The <see typeparam="T"/> at the specified position.</returns>
+        public T this[int row, int column]
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                this.CheckCoordinates(row, column);
+                return this.Data[(row * this.Width) + column];
+            }
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            set
+            {
+                this.CheckCoordinates(row, column);
+                this.Data[(row * this.Width) + column] = value;
+            }
+        }
+
+        /// <summary>
+        /// Checks the coordinates to ensure they are within bounds.
+        /// </summary>
+        /// <param name="row">The row-coordinate of the item. Must be greater than zero and smaller than the height of the array.</param>
+        /// <param name="column">The column-coordinate of the item. Must be greater than zero and smaller than the width of the array.</param>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// Thrown if the coordinates are not within the bounds of the array.
+        /// </exception>
+        [Conditional("DEBUG")]
+        private void CheckCoordinates(int row, int column)
+        {
+            if (row < 0 || row >= this.Height)
+            {
+                throw new ArgumentOutOfRangeException(nameof(row), row, $"{row} is outwith the array bounds.");
+            }
+
+            if (column < 0 || column >= this.Width)
+            {
+                throw new ArgumentOutOfRangeException(nameof(column), column, $"{column} is outwith the array bounds.");
+            }
+        }
+    }
+}

src/ImageSharp/Dithering/Ordered/Bayer.cs

@@ -17,23 +17,24 @@ namespace ImageSharp.Dithering.Ordered
         /// The threshold matrix.
         /// This is calculated by multiplying each value in the original matrix by 16 and subtracting 1
         /// </summary>
-        private static readonly byte[][] ThresholdMatrix =
-        {
-            new byte[] { 15, 143, 47, 175 },
-            new byte[] { 207, 79, 239, 111 },
-            new byte[] { 63, 191, 31, 159 },
-            new byte[] { 255, 127, 223, 95 }
-        };
+        private static readonly Fast2DArray<byte> ThresholdMatrix =
+            new Fast2DArray<byte>(new byte[,]
+            {
+                { 15, 143, 47, 175 },
+                { 207, 79, 239, 111 },
+                { 63, 191, 31, 159 },
+                { 255, 127, 223, 95 }
+            });
 
         /// <inheritdoc />
-        public byte[][] Matrix { get; } = ThresholdMatrix;
+        public Fast2DArray<byte> Matrix { get; } = ThresholdMatrix;
 
         /// <inheritdoc />
         public void Dither<TColor>(PixelAccessor<TColor> pixels, TColor source, TColor upper, TColor lower, byte[] bytes, int index, int x, int y, int width, int height)
             where TColor : struct, IPackedPixel, IEquatable<TColor>
         {
             source.ToXyzwBytes(bytes, 0);
-            pixels[x, y] = ThresholdMatrix[x % 3][y % 3] >= bytes[index] ? lower : upper;
+            pixels[x, y] = ThresholdMatrix[y % 3, x % 3] >= bytes[index] ? lower : upper;
         }
     }
 }

src/ImageSharp/Dithering/Ordered/IOrderedDither.cs

@@ -15,7 +15,7 @@ namespace ImageSharp.Dithering
         /// <summary>
         /// Gets the dithering matrix
         /// </summary>
-        byte[][] Matrix { get; }
+        Fast2DArray<byte> Matrix { get; }
 
         /// <summary>
         /// Transforms the image applying the dither matrix. This method alters the input pixels array

src/ImageSharp/Dithering/Ordered/Ordered.cs

@@ -17,23 +17,24 @@ namespace ImageSharp.Dithering.Ordered
         /// The threshold matrix.
         /// This is calculated by multiplying each value in the original matrix by 16
         /// </summary>
-        private static readonly byte[][] ThresholdMatrix =
-        {
-           new byte[] { 0, 128, 32, 160 },
-           new byte[] { 192, 64, 224, 96 },
-           new byte[] { 48, 176, 16, 144 },
-           new byte[] { 240, 112, 208, 80 }
-        };
+        private static readonly Fast2DArray<byte> ThresholdMatrix =
+            new Fast2DArray<byte>(new byte[,]
+            {
+               { 0, 128, 32, 160 },
+               { 192, 64, 224, 96 },
+               { 48, 176, 16, 144 },
+               { 240, 112, 208, 80 }
+            });
 
         /// <inheritdoc />
-        public byte[][] Matrix { get; } = ThresholdMatrix;
+        public Fast2DArray<byte> Matrix { get; } = ThresholdMatrix;
 
         /// <inheritdoc />
         public void Dither<TColor>(PixelAccessor<TColor> pixels, TColor source, TColor upper, TColor lower, byte[] bytes, int index, int x, int y, int width, int height)
             where TColor : struct, IPackedPixel, IEquatable<TColor>
         {
             source.ToXyzwBytes(bytes, 0);
-            pixels[x, y] = ThresholdMatrix[x % 3][y % 3] >= bytes[index] ? lower : upper;
+            pixels[x, y] = ThresholdMatrix[y % 3, x % 3] >= bytes[index] ? lower : upper;
         }
     }
 }

src/ImageSharp/Image/PixelAccessor{TColor}.cs

@@ -153,8 +153,8 @@ namespace ImageSharp
         /// <summary>
         /// Gets or sets the pixel at the specified position.
         /// </summary>
-        /// <param name="x">The x-coordinate of the pixel. Must be greater than zero and smaller than the width of the pixel.</param>
-        /// <param name="y">The y-coordinate of the pixel. Must be greater than zero and smaller than the width of the pixel.</param>
+        /// <param name="x">The x-coordinate of the pixel. Must be greater than or equal to zero and less than the width of the image.</param>
+        /// <param name="y">The y-coordinate of the pixel. Must be greater than or equal to zero and less than the height of the image.</param>
         /// <returns>The <see typeparam="TColor"/> at the specified position.</returns>
         public TColor this[int x, int y]
         {
@@ -625,8 +625,8 @@ namespace ImageSharp
         /// Checks that the given area and offset are within the bounds of the image.
         /// </summary>
         /// <param name="area">The area.</param>
-        /// <param name="x">The x-coordinate of the pixel. Must be greater than zero and smaller than the width of the pixel.</param>
-        /// <param name="y">The y-coordinate of the pixel. Must be greater than zero and smaller than the width of the pixel.</param>
+        /// <param name="x">The x-coordinate of the pixel. Must be greater than zero and less than the width of the image.</param>
+        /// <param name="y">The y-coordinate of the pixel. Must be greater than zero and less than the height of the image.</param>
         /// <exception cref="ArgumentOutOfRangeException">
         /// Thrown if the dimensions are not within the bounds of the image.
         /// </exception>
@@ -649,8 +649,8 @@ namespace ImageSharp
         /// <summary>
         /// Checks the coordinates to ensure they are within bounds.
         /// </summary>
-        /// <param name="x">The x-coordinate of the pixel. Must be greater than zero and smaller than the width of the pixel.</param>
-        /// <param name="y">The y-coordinate of the pixel. Must be greater than zero and smaller than the width of the pixel.</param>
+        /// <param name="x">The x-coordinate of the pixel. Must be greater than zero and less than the width of the image.</param>
+        /// <param name="y">The y-coordinate of the pixel. Must be greater than zero and less than the height of the image.</param>
         /// <exception cref="ArgumentOutOfRangeException">
         /// Thrown if the coordinates are not within the bounds of the image.
         /// </exception>

tests/ImageSharp.Benchmarks/General/Array2D.cs

@@ -0,0 +1,57 @@
+// <copyright file="Array2D.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Benchmarks.General
+{
+    using BenchmarkDotNet.Attributes;
+
+    public class Array2D
+    {
+        private float[,] data;
+
+        private float[][] jaggedData;
+
+        private Fast2DArray<float> fastData;
+
+        [Params(10, 100, 1000, 10000)]
+        public int Count { get; set; }
+
+        public int Index { get; set; }
+
+        [Setup]
+        public void SetUp()
+        {
+            this.data = new float[this.Count, this.Count];
+            this.jaggedData = new float[this.Count][];
+
+            for (int i = 0; i < this.Count; i++)
+            {
+                this.jaggedData[i] = new float[this.Count];
+            }
+
+            this.fastData = new Fast2DArray<float>(this.data);
+
+            this.Index = this.Count / 2;
+        }
+
+        [Benchmark(Baseline = true, Description = "Array access using 2D array")]
+        public float ArrayIndex()
+        {
+            return this.data[this.Index, this.Index];
+        }
+
+        [Benchmark(Description = "Array access using a jagged array")]
+        public float ArrayJaggedIndex()
+        {
+            return this.jaggedData[this.Index][this.Index];
+        }
+
+        [Benchmark(Description = "Array access using Fast2DArray")]
+        public float ArrayFastIndex()
+        {
+            return this.fastData[this.Index, this.Index];
+        }
+    }
+}

tests/ImageSharp.Benchmarks/General/ArrayCopy.cs

@@ -2,6 +2,7 @@
 // Copyright (c) James Jackson-South and contributors.
 // Licensed under the Apache License, Version 2.0.
 // </copyright>
+
 namespace ImageSharp.Benchmarks.General
 {
     using System;
@@ -14,9 +15,9 @@ namespace ImageSharp.Benchmarks.General
         [Params(100, 1000, 10000)]
         public int Count { get; set; }
 
-        byte[] source;
+        private byte[] source;
 
-        byte[] destination;
+        private byte[] destination;
 
         [Setup]
         public void SetUp()

tests/ImageSharp.Tests/Common/Fast2DArrayTests.cs

@@ -0,0 +1,71 @@
+// <copyright file="Fast2DArrayTests.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Tests.Common
+{
+    using System;
+
+    using Xunit;
+
+    public class Fast2DArrayTests
+    {
+        private static readonly float[,] FloydSteinbergMatrix =
+        {
+            { 0, 0, 7 },
+            { 3, 5, 1 }
+        };
+
+        [Fact]
+        public void Fast2DArrayThrowsOnNullInitializer()
+        {
+            Assert.Throws<ArgumentNullException>(() =>
+                {
+                    Fast2DArray<float> fast = new Fast2DArray<float>(null);
+                });
+        }
+
+        [Fact]
+        public void Fast2DArrayThrowsOnEmptyInitializer()
+        {
+            Assert.Throws<ArgumentOutOfRangeException>(() =>
+            {
+                Fast2DArray<float> fast = new Fast2DArray<float>(new float[0, 0]);
+            });
+        }
+
+        [Fact]
+        public void Fast2DArrayReturnsCorrectDimensions()
+        {
+            Fast2DArray<float> fast = new Fast2DArray<float>(FloydSteinbergMatrix);
+            Assert.True(fast.Width == FloydSteinbergMatrix.GetLength(1));
+            Assert.True(fast.Height == FloydSteinbergMatrix.GetLength(0));
+        }
+
+        [Fact]
+        public void Fast2DArrayGetReturnsCorrectResults()
+        {
+            Fast2DArray<float> fast = new Fast2DArray<float>(FloydSteinbergMatrix);
+
+            for (int row = 0; row < fast.Height; row++)
+            {
+                for (int column = 0; column < fast.Width; column++)
+                {
+                    Assert.True(Math.Abs(fast[row, column] - FloydSteinbergMatrix[row, column]) < Constants.Epsilon);
+                }
+            }
+        }
+
+        [Fact]
+        public void Fast2DArrayGetSetReturnsCorrectResults()
+        {
+            Fast2DArray<float> fast = new Fast2DArray<float>(new float[4, 4]);
+            const float Val = 5F;
+
+            fast[3, 3] = Val;
+
+            Assert.True(Math.Abs(Val - fast[3, 3]) < Constants.Epsilon);
+        }
+    }
+}