Implement Median Blur processor

src/ImageSharp/Primitives/DenseMatrix{T}.cs

@@ -96,6 +96,27 @@ public DenseMatrix(T[,] data)
             }
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref=" DenseMatrix{T}"/> struct.
+        /// </summary>
+        /// <param name="columns">The number of columns.</param>
+        /// <param name="rows">The number of rows.</param>
+        /// <param name="data">The array to provide access to.</param>
+        public DenseMatrix(int columns, int rows, Span<T> data)
+        {
+            Guard.MustBeGreaterThan(rows, 0, nameof(this.Rows));
+            Guard.MustBeGreaterThan(columns, 0, nameof(this.Columns));
+            Guard.IsTrue(rows * columns == data.Length, nameof(data), "Length should be equal to ros * columns");
+
+            this.Rows = rows;
+            this.Columns = columns;
+            this.Size = new Size(columns, rows);
+            this.Count = this.Columns * this.Rows;
+            this.Data = new T[this.Columns * this.Rows];
+
+            data.CopyTo(this.Data);
+        }
+
         /// <summary>
         /// Gets a span wrapping the <see cref="Data"/>.
         /// </summary>

src/ImageSharp/Processing/Extensions/Convolution/MedianBlurExtensions.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.Processing.Processors.Convolution;
+
+namespace SixLabors.ImageSharp.Processing
+{
+    /// <summary>
+    /// Defines extensions that allow the applying of the median blur on an <see cref="Image"/>
+    /// using Mutate/Clone.
+    /// </summary>
+    public static class MedianBlurExtensions
+    {
+        /// <summary>
+        /// Applies a median blur on the image.
+        /// </summary>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="radius">The radius of the area to find the median for.</param>
+        /// <param name="preserveAlpha">
+        /// Whether the filter is applied to alpha as well as the color channels.
+        /// </param>
+        /// <returns>The <see cref="IImageProcessingContext"/> to allow chaining of operations.</returns>
+        public static IImageProcessingContext MedianBlur(this IImageProcessingContext source, int radius, bool preserveAlpha)
+            => source.ApplyProcessor(new MedianBlurProcessor(radius, preserveAlpha));
+
+        /// <summary>
+        /// Applies a median blur on the image.
+        /// </summary>
+        /// <param name="source">The image this method extends.</param>
+        /// <param name="radius">The radius of the area to find the median for.</param>
+        /// <param name="preserveAlpha">
+        /// Whether the filter is applied to alpha as well as the color channels.
+        /// </param>
+        /// <param name="rectangle">
+        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to alter.
+        /// </param>
+        /// <returns>The <see cref="IImageProcessingContext"/> to allow chaining of operations.</returns>
+        public static IImageProcessingContext MedianBlur(this IImageProcessingContext source, int radius, bool preserveAlpha, Rectangle rectangle)
+            => source.ApplyProcessor(new MedianBlurProcessor(radius, preserveAlpha), rectangle);
+    }
+}

src/ImageSharp/Processing/Processors/Convolution/Kernel.cs

@@ -0,0 +1,92 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Processing.Processors.Convolution
+{
+    /// <summary>
+    /// A stack only, readonly, kernel matrix that can be indexed without
+    /// bounds checks when compiled in release mode.
+    /// </summary>
+    internal readonly ref struct Kernel<T>
+        where T : struct, IEquatable<T>
+    {
+        private readonly Span<T> values;
+
+        public Kernel(DenseMatrix<T> matrix)
+        {
+            this.Columns = matrix.Columns;
+            this.Rows = matrix.Rows;
+            this.values = matrix.Span;
+        }
+
+        public int Columns
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get;
+        }
+
+        public int Rows
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get;
+        }
+
+        public ReadOnlySpan<T> Span
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                return this.values;
+            }
+        }
+
+        public T this[int row, int column]
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                this.CheckCoordinates(row, column);
+                ref T vBase = ref MemoryMarshal.GetReference(this.values);
+                return Unsafe.Add(ref vBase, (row * this.Columns) + column);
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void SetValue(int row, int column, T value)
+        {
+            this.SetValue((row * this.Columns) + column, value);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void SetValue(int index, T value)
+        {
+            ref T vBase = ref MemoryMarshal.GetReference(this.values);
+            Unsafe.Add(ref vBase, index) = value;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Clear()
+        {
+            this.values.Clear();
+        }
+
+        [Conditional("DEBUG")]
+        private void CheckCoordinates(int row, int column)
+        {
+            if (row < 0 || row >= this.Rows)
+            {
+                throw new ArgumentOutOfRangeException(nameof(row), row, $"{row} is outside the matrix bounds.");
+            }
+
+            if (column < 0 || column >= this.Columns)
+            {
+                throw new ArgumentOutOfRangeException(nameof(column), column, $"{column} is outside the matrix bounds.");
+            }
+        }
+    }
+}

src/ImageSharp/Processing/Processors/Convolution/MedianBlurProcessor.cs

@@ -0,0 +1,53 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Processors.Convolution
+{
+    /// <summary>
+    /// Applies an median filter.
+    /// </summary>
+    public sealed class MedianBlurProcessor : IImageProcessor
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MedianBlurProcessor"/> class.
+        /// </summary>
+        /// <param name="radius">
+        /// The 'radius' value representing the size of the area to filter over.
+        /// </param>
+        /// <param name="preserveAlpha">
+        /// Whether the filter is applied to alpha as well as the color channels.
+        /// </param>
+        public MedianBlurProcessor(int radius, bool preserveAlpha)
+        {
+            this.Radius = radius;
+            this.PreserveAlpha = preserveAlpha;
+        }
+
+        /// <summary>
+        /// Gets the size of the area to find the median of.
+        /// </summary>
+        public int Radius { get; }
+
+        /// <summary>
+        /// Gets a value indicating whether the filter is applied to alpha as well as the color channels.
+        /// </summary>
+        public bool PreserveAlpha { get; }
+
+        /// <summary>
+        /// Gets the <see cref="BorderWrappingMode"/> to use when mapping the pixels outside of the border, in X direction.
+        /// </summary>
+        public BorderWrappingMode BorderWrapModeX { get; }
+
+        /// <summary>
+        /// Gets the <see cref="BorderWrappingMode"/> to use when mapping the pixels outside of the border, in Y direction.
+        /// </summary>
+        public BorderWrappingMode BorderWrapModeY { get; }
+
+        /// <inheritdoc />
+        public IImageProcessor<TPixel> CreatePixelSpecificProcessor<TPixel>(Configuration configuration, Image<TPixel> source, Rectangle sourceRectangle)
+            where TPixel : unmanaged, IPixel<TPixel>
+            => new MedianBlurProcessor<TPixel>(configuration, this, source, sourceRectangle);
+    }
+}

src/ImageSharp/Processing/Processors/Convolution/MedianBlurProcessor{TPixel}.cs

@@ -0,0 +1,60 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Numerics;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Processors.Convolution
+{
+    /// <summary>
+    /// Applies an median filter.
+    /// </summary>
+    internal sealed class MedianBlurProcessor<TPixel> : ImageProcessor<TPixel>
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        private readonly MedianBlurProcessor definition;
+
+        public MedianBlurProcessor(Configuration configuration, MedianBlurProcessor definition, Image<TPixel> source, Rectangle sourceRectangle)
+            : base(configuration, source, sourceRectangle)
+        {
+            this.definition = definition;
+        }
+
+        protected override void OnFrameApply(ImageFrame<TPixel> source)
+        {
+            int kernelSize = (2 * this.definition.Radius) + 1;
+
+            MemoryAllocator allocator = this.Configuration.MemoryAllocator;
+            using Buffer2D<TPixel> targetPixels = allocator.Allocate2D<TPixel>(source.Width, source.Height);
+
+            source.CopyTo(targetPixels);
+
+            Rectangle interest = Rectangle.Intersect(this.SourceRectangle, source.Bounds());
+
+            // We use a rectangle with width set to 2 * kernelSize^2 + width, to allocate a buffer big enough
+            // for kernel source and target bulk pixel conversion.
+            Rectangle operationBounds = new Rectangle(interest.X, interest.Y, (2 * (kernelSize * kernelSize)) + interest.Width, interest.Height);
+
+            using KernelSamplingMap map = new KernelSamplingMap(this.Configuration.MemoryAllocator);
+            map.BuildSamplingOffsetMap(kernelSize, kernelSize, interest, this.definition.BorderWrapModeX, this.definition.BorderWrapModeY);
+
+            var operation = new MedianRowOperation<TPixel>(
+                interest,
+                targetPixels,
+                source.PixelBuffer,
+                map,
+                kernelSize,
+                this.Configuration,
+                this.definition.PreserveAlpha);
+
+            ParallelRowIterator.IterateRows<MedianRowOperation<TPixel>, Vector4>(
+                this.Configuration,
+                operationBounds,
+                in operation);
+
+            Buffer2D<TPixel>.SwapOrCopyContent(source.PixelBuffer, targetPixels);
+        }
+    }
+}

src/ImageSharp/Processing/Processors/Convolution/MedianConvolutionState.cs

@@ -0,0 +1,46 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Processing.Processors.Convolution
+{
+    /// <summary>
+    /// A stack only struct used for reducing reference indirection during convolution operations.
+    /// </summary>
+    internal readonly ref struct MedianConvolutionState
+    {
+        private readonly Span<int> rowOffsetMap;
+        private readonly Span<int> columnOffsetMap;
+        private readonly int kernelHeight;
+        private readonly int kernelWidth;
+
+        public MedianConvolutionState(
+            in DenseMatrix<Vector4> kernel,
+            KernelSamplingMap map)
+        {
+            this.Kernel = new Kernel<Vector4>(kernel);
+            this.kernelHeight = kernel.Rows;
+            this.kernelWidth = kernel.Columns;
+            this.rowOffsetMap = map.GetRowOffsetSpan();
+            this.columnOffsetMap = map.GetColumnOffsetSpan();
+        }
+
+        public readonly Kernel<Vector4> Kernel
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public readonly ref int GetSampleRow(int row)
+            => ref Unsafe.Add(ref MemoryMarshal.GetReference(this.rowOffsetMap), row * this.kernelHeight);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public readonly ref int GetSampleColumn(int column)
+            => ref Unsafe.Add(ref MemoryMarshal.GetReference(this.columnOffsetMap), column * this.kernelWidth);
+    }
+}