Introduced separate 420 converter

src/ImageSharp/Formats/Jpeg/Components/Encoder/YCbCrForwardConverter420{TPixel}.cs

@@ -0,0 +1,95 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
+{
+    /// <summary>
+    /// On-stack worker struct to efficiently encapsulate the TPixel -> Rgb24 -> YCbCr conversion chain of 8x8 pixel blocks.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel type to work on</typeparam>
+    internal ref struct YCbCrForwardConverter420<TPixel>
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        /// <summary>
+        /// The left Y component
+        /// </summary>
+        public Block8x8F YLeft;
+
+        /// <summary>
+        /// The left Y component
+        /// </summary>
+        public Block8x8F YRight;
+
+        /// <summary>
+        /// The Cb component
+        /// </summary>
+        public Block8x8F Cb;
+
+        /// <summary>
+        /// The Cr component
+        /// </summary>
+        public Block8x8F Cr;
+
+        /// <summary>
+        /// The color conversion tables
+        /// </summary>
+        private RgbToYCbCrConverterLut colorTables;
+
+        /// <summary>
+        /// Temporal 16x8 block to hold TPixel data
+        /// </summary>
+        private Span<TPixel> pixelSpan;
+
+        /// <summary>
+        /// Temporal RGB block
+        /// </summary>
+        private Span<Rgb24> rgbSpan;
+
+        public static YCbCrForwardConverter420<TPixel> Create()
+        {
+            var result = default(YCbCrForwardConverter420<TPixel>);
+
+            // TODO: this is subject to discuss
+            const int twoBlocksByteSizeWithPadding = 384 + 8; // converter.Convert comments for +8 padding
+            result.rgbSpan = MemoryMarshal.Cast<byte, Rgb24>(new byte[twoBlocksByteSizeWithPadding].AsSpan());
+
+            // TODO: this size should be configurable
+            result.pixelSpan = new TPixel[128].AsSpan();
+
+            // Avoid creating lookup tables, when vectorized converter is supported
+            if (!RgbToYCbCrConverterVectorized.IsSupported)
+            {
+                result.colorTables = RgbToYCbCrConverterLut.Create();
+            }
+
+            return result;
+        }
+
+        /// <summary>
+        /// Converts a 8x8 image area inside 'pixels' at position (x,y) placing the result members of the structure (<see cref="Y"/>, <see cref="Cb"/>, <see cref="Cr"/>)
+        /// </summary>
+        public void Convert(ImageFrame<TPixel> frame, int x, int y, ref RowOctet<TPixel> currentRows, int idx)
+        {
+            Memory.Buffer2D<TPixel> buffer = frame.PixelBuffer;
+            YCbCrForwardConverter<TPixel>.LoadAndStretchEdges(currentRows, this.pixelSpan, new Point(x, y), new Size(16, 8), new Size(buffer.Width, buffer.Height));
+
+            PixelOperations<TPixel>.Instance.ToRgb24(frame.GetConfiguration(), this.pixelSpan, this.rgbSpan);
+
+            if (RgbToYCbCrConverterVectorized.IsSupported)
+            {
+                RgbToYCbCrConverterVectorized.Convert420_16x8(this.rgbSpan, ref this.YLeft, ref this.YRight, ref this.Cb, ref this.Cr, idx);
+            }
+            else
+            {
+                throw new NotSupportedException("This is not yet implemented");
+                //this.colorTables.Convert(this.rgbSpan, ref yBlock, ref cbBlock, ref crBlock);
+            }
+        }
+    }
+}