WIP spectral converter

src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Buffers;
+using System.Collections.Generic;
+using System.Numerics;
+using System.Text;
+using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
+{
+    internal abstract class SpectralConverter
+    {
+        public abstract void ConvertStride();
+    }
+
+    internal class SpectralConverter<TPixel> : SpectralConverter
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        private Configuration configuration;
+
+        private JpegComponentPostProcessor[] componentProcessors;
+
+        private JpegColorConverter colorConverter;
+
+        private IMemoryOwner<Vector4> rgbaBuffer;
+
+        private Buffer2D<TPixel> pixelBuffer;
+
+        public JpegFrame Frame
+        {
+            set => this.InjectFrame(value);
+        }
+
+        public SpectralConverter(Configuration configuration)
+        {
+            this.configuration = configuration;
+        }
+
+        private void InjectFrame(JpegFrame frame)
+        {
+            MemoryAllocator allocator = this.configuration.MemoryAllocator;
+
+            // pixel buffer for resulting image
+            this.pixelBuffer = allocator.Allocate2D<TPixel>(frame.PixelWidth, frame.PixelHeight, AllocationOptions.Clean);
+
+            // component processors from spectral to Rgba32
+            this.componentProcessors = new JpegComponentPostProcessor[frame.Components.Length];
+            for (int i = 0; i < this.componentProcessors.Length; i++)
+            {
+                this.componentProcessors[i] = new JpegComponentPostProcessor(allocator, this, frame.Components[i]);
+            }
+
+            // single 'stride' rgba32 buffer for conversion between spectral and TPixel
+            this.rgbaBuffer = allocator.Allocate<Vector4>(frame.PixelWidth);
+
+            // color converter from Rgba32 to TPixel
+            this.colorConverter = JpegColorConverter.GetConverter(rawJpeg.ColorSpace, frame.Precision);
+        }
+
+        public override void ConvertStride()
+        {
+            int maxY = Math.Min(this.pixelBuffer.Height, this.PixelRowCounter + PixelRowsPerStep);
+
+            var buffers = new Buffer2D<float>[this.componentProcessors.Length];
+            for (int i = 0; i < this.componentProcessors.Length; i++)
+            {
+                this.componentProcessors[i].CopyBlocksToColorBuffer();
+                buffers[i] = this.componentProcessors[i].ColorBuffer;
+            }
+
+            for (int yy = this.PixelRowCounter; yy < maxY; yy++)
+            {
+                int y = yy - this.PixelRowCounter;
+
+                var values = new JpegColorConverter.ComponentValues(buffers, y);
+                this.colorConverter.ConvertToRgba(values, this.rgbaBuffer.GetSpan());
+
+                Span<TPixel> destRow = destination.GetPixelRowSpan(yy);
+
+                // TODO: Investigate if slicing is actually necessary
+                PixelOperations<TPixel>.Instance.FromVector4Destructive(this.configuration, this.rgbaBuffer.GetSpan().Slice(0, destRow.Length), destRow);
+            }
+
+            this.PixelRowCounter += PixelRowsPerStep;
+        }
+    }
+}