wip - Add basic structure for the swizzler

src/ImageSharp/Processing/Extensions/Transforms/SwizzleExtensions.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Processing.Processors.Transforms;
+
+namespace SixLabors.ImageSharp.Processing.Extensions.Transforms
+{
+    /// <summary>
+    /// Defines extensions that allow the application of swizzle operations on an <see cref="Image"/>
+    /// </summary>
+    public static class SwizzleExtensions
+    {
+        /// <summary>
+        /// Swizzles an image.
+        /// </summary>
+        /// <param name="source">The image to swizzle.</param>
+        /// <param name="swizzler">The swizzler function.</param>
+        /// <typeparam name="TSwizzler">The swizzler function type.</typeparam>
+        /// <returns>The <see cref="IImageProcessingContext"/> to allow chaining of operations.</returns>
+        public static IImageProcessingContext Swizzle<TSwizzler>(this IImageProcessingContext source, TSwizzler swizzler)
+            where TSwizzler : struct, ISwizzler
+            => source.ApplyProcessor(new SwizzleProcessor<TSwizzler>(swizzler));
+    }
+}

src/ImageSharp/Processing/Processors/Transforms/ISwizzler.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Processing.Processors.Transforms
+{
+    /// <summary>
+    /// Encapsulate an algorithm to swizzle pixels in an image.
+    /// </summary>
+    public interface ISwizzler
+    {
+        /// <summary>
+        /// Applies the swizzle transformation to a given point.
+        /// </summary>
+        /// <param name="point">Point to transform.</param>
+        /// <returns>Transformed point.</returns>
+        Point Transform(Point point);
+    }
+}

src/ImageSharp/Processing/Processors/Transforms/SwizzleProcessor{TSwizzler,TPixel}.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Processors.Transforms
+{
+    internal class SwizzleProcessor<TSwizzler, TPixel> : ImageProcessor<TPixel>
+        where TSwizzler : struct, ISwizzler
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        private readonly TSwizzler swizzler;
+
+        public SwizzleProcessor(Configuration configuration, TSwizzler swizzler, Image<TPixel> source, Rectangle sourceRectangle)
+            : base(configuration, source, sourceRectangle)
+        {
+            this.swizzler = swizzler;
+        }
+
+        /// <inheritdoc/>
+        protected override void OnFrameApply(ImageFrame<TPixel> source)
+        {
+            for (int y = 0; y < source.Height; y++)
+            {
+                var pixelRowSpan = source.GetPixelRowSpan(y);
+                for (int x = 0; x < source.Width; x++)
+                {
+                    var newPoint = this.swizzler.Transform(new Point(x, y));
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Processing/Processors/Transforms/SwizzleProcessor{TSwizzler}.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Processing.Processors.Transforms
+{
+    /// <summary>
+    /// Defines a swizzle operation on an image.
+    /// </summary>
+    /// <typeparam name="TSwizzler">The swizzle function type.</typeparam>
+    public sealed class SwizzleProcessor<TSwizzler> : IImageProcessor
+        where TSwizzler : struct, ISwizzler
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SwizzleProcessor{TSwizzler}"/> class.
+        /// </summary>
+        /// <param name="swizzler">The swizzler operation.</param>
+        public SwizzleProcessor(TSwizzler swizzler)
+            => this.Swizzler = swizzler;
+
+        /// <summary>
+        /// Gets the swizzler operation.
+        /// </summary>
+        public TSwizzler Swizzler { get; }
+
+        /// <inheritdoc />
+        public IImageProcessor<TPixel> CreatePixelSpecificProcessor<TPixel>(Configuration configuration, Image<TPixel> source, Rectangle sourceRectangle)
+            where TPixel : unmanaged, IPixel<TPixel>
+            => new SwizzleProcessor<TSwizzler, TPixel>(configuration, this, source, sourceRectangle);
+    }
+}

tests/ImageSharp.Tests/Processing/Transforms/SwizzleTests.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Processing.Extensions.Transforms;
+using SixLabors.ImageSharp.Processing.Processors.Transforms;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Processing.Transforms
+{
+    public class SwizzleTests : BaseImageOperationsExtensionTest
+    {
+        private struct InvertXAndYSwizzler : ISwizzler
+        {
+            public Point Transform(Point point) => new Point(point.Y, point.X);
+        }
+
+        [Fact]
+        public void RotateDegreesFloatRotateProcessorWithAnglesSet()
+        {
+            this.operations.Swizzle(default(InvertXAndYSwizzler));
+            SwizzleProcessor<InvertXAndYSwizzler> processor = this.Verify<SwizzleProcessor<InvertXAndYSwizzler>>();
+
+            // assert that pixels have been changed
+
+            this.operations.Swizzle(default(InvertXAndYSwizzler));
+            SwizzleProcessor<InvertXAndYSwizzler> processor2 = this.Verify<SwizzleProcessor<InvertXAndYSwizzler>>();
+
+            // assert that pixels have been changed (i.e. back to original)
+        }
+    }
+}