Use less expensive update for RowOctet

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

@@ -59,9 +59,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         /// <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, in RowOctet<TPixel> currentRows)
+        public void Convert(ImageFrame<TPixel> frame, int x, int y, ref RowOctet<TPixel> currentRows)
         {
-            this.pixelBlock.LoadAndStretchEdges(frame.PixelBuffer, x, y, currentRows);
+            this.pixelBlock.LoadAndStretchEdges(frame.PixelBuffer, x, y, ref currentRows);
 
             Span<Rgb24> rgbSpan = this.rgbBlock.AsSpanUnsafe();
             PixelOperations<TPixel>.Instance.ToRgb24(frame.GetConfiguration(), this.pixelBlock.AsSpanUnsafe(), rgbSpan);
@@ -76,7 +76,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
             }
             else
             {
-                this.colorTables.Convert(rgbSpan,  ref yBlock, ref cbBlock, ref crBlock);
+                this.colorTables.Convert(rgbSpan, ref yBlock, ref cbBlock, ref crBlock);
             }
         }
     }

src/ImageSharp/Formats/Jpeg/Components/GenericBlock8x8.cs

@@ -57,7 +57,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
         /// Load a 8x8 region of an image into the block.
         /// The "outlying" area of the block will be stretched out with pixels on the right and bottom edge of the image.
         /// </summary>
-        public void LoadAndStretchEdges(Buffer2D<T> source, int sourceX, int sourceY, in RowOctet<T> currentRows)
+        public void LoadAndStretchEdges(Buffer2D<T> source, int sourceX, int sourceY, ref RowOctet<T> currentRows)
         {
             int width = Math.Min(8, source.Width - sourceX);
             int height = Math.Min(8, source.Height - sourceY);

src/ImageSharp/Formats/Jpeg/Components/RowOctet.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -12,18 +12,19 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
     /// Cache 8 pixel rows on the stack, which may originate from different buffers of a <see cref="MemoryGroup{T}"/>.
     /// </summary>
     [StructLayout(LayoutKind.Sequential)]
-    internal readonly ref struct RowOctet<T>
+    internal ref struct RowOctet<T>
         where T : struct
     {
-        private readonly Span<T> row0;
+        private Span<T> row0;
-        private readonly Span<T> row1;
+        private Span<T> row1;
-        private readonly Span<T> row2;
+        private Span<T> row2;
-        private readonly Span<T> row3;
+        private Span<T> row3;
-        private readonly Span<T> row4;
+        private Span<T> row4;
-        private readonly Span<T> row5;
+        private Span<T> row5;
-        private readonly Span<T> row6;
+        private Span<T> row6;
-        private readonly Span<T> row7;
+        private Span<T> row7;
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public RowOctet(Buffer2D<T> buffer, int startY)
         {
             int y = startY;
@@ -38,13 +39,12 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
             this.row7 = y < height ? buffer.GetRowSpan(y) : default;
         }
 
+        // No unsafe tricks, since Span<T> can't be used as a generic argument
         public Span<T> this[int y]
         {
-            [MethodImpl(InliningOptions.ShortMethod)]
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
-            get
+            get =>
-            {
+                y switch
-                // No unsafe tricks, since Span<T> can't be used as a generic argument
-                return y switch
                 {
                     0 => this.row0,
                     1 => this.row1,
@@ -56,13 +56,56 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
                     7 => this.row7,
                     _ => ThrowIndexOutOfRangeException()
                 };
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            private set
+            {
+                switch (y)
+                {
+                    case 0:
+                        this.row0 = value;
+                        break;
+                    case 1:
+                        this.row1 = value;
+                        break;
+                    case 2:
+                        this.row2 = value;
+                        break;
+                    case 3:
+                        this.row3 = value;
+                        break;
+                    case 4:
+                        this.row4 = value;
+                        break;
+                    case 5:
+                        this.row5 = value;
+                        break;
+                    case 6:
+                        this.row6 = value;
+                        break;
+                    default:
+                        this.row7 = value;
+                        break;
+                }
             }
         }
 
-        [MethodImpl(InliningOptions.ColdPath)]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        private static Span<T> ThrowIndexOutOfRangeException()
+        public void Update(Buffer2D<T> buffer, int startY)
         {
-            throw new IndexOutOfRangeException();
+            int y = startY;
+            int height = buffer.Height;
+
+            // We don't actually have to assign values outside of the
+            // frame pixel buffer since they are never requested.
+            for (int i = 0; i < 8 && y < height; i++)
+            {
+                this[i] = buffer.GetRowSpan(y++);
+            }
         }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private static Span<T> ThrowIndexOutOfRangeException()
+        => throw new IndexOutOfRangeException();
     }
 }

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -418,15 +418,16 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             var pixelConverter = YCbCrForwardConverter<TPixel>.Create();
             ImageFrame<TPixel> frame = pixels.Frames.RootFrame;
             Buffer2D<TPixel> pixelBuffer = frame.PixelBuffer;
+            RowOctet<TPixel> currentRows = default;
 
             for (int y = 0; y < pixels.Height; y += 8)
             {
                 cancellationToken.ThrowIfCancellationRequested();
-                var currentRows = new RowOctet<TPixel>(pixelBuffer, y);
+                currentRows.Update(pixelBuffer, y);
 
                 for (int x = 0; x < pixels.Width; x += 8)
                 {
-                    pixelConverter.Convert(frame, x, y, currentRows);
+                    pixelConverter.Convert(frame, x, y, ref currentRows);
 
                     prevDCY = this.WriteBlock(
                         QuantIndex.Luminance,
@@ -997,6 +998,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             int prevDCY = 0, prevDCCb = 0, prevDCCr = 0;
             ImageFrame<TPixel> frame = pixels.Frames.RootFrame;
             Buffer2D<TPixel> pixelBuffer = frame.PixelBuffer;
+            RowOctet<TPixel> currentRows = default;
 
             for (int y = 0; y < pixels.Height; y += 16)
             {
@@ -1008,10 +1010,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                         int xOff = (i & 1) * 8;
                         int yOff = (i & 2) * 4;
 
-                        // TODO: Try pushing this to the outer loop!
+                        currentRows.Update(pixelBuffer, y + yOff);
-                        var currentRows = new RowOctet<TPixel>(pixelBuffer, y + yOff);
+                        pixelConverter.Convert(frame, x + xOff, y + yOff, ref currentRows);
-
-                        pixelConverter.Convert(frame, x + xOff, y + yOff, currentRows);
 
                         cb[i] = pixelConverter.Cb;
                         cr[i] = pixelConverter.Cr;

tests/ImageSharp.Benchmarks/Codecs/Jpeg/EncodeJpeg.cs

@@ -22,7 +22,7 @@ namespace SixLabors.ImageSharp.Benchmarks.Codecs.Jpeg
         {
             if (this.bmpStream == null)
             {
-                const string TestImage = TestImages.Bmp.Car;
+                const string TestImage = TestImages.Bmp.NegHeight;
                 this.bmpStream = File.OpenRead(Path.Combine(TestEnvironment.InputImagesDirectoryFullPath, TestImage));
                 this.bmpCore = Image.Load<Rgba32>(this.bmpStream);
                 this.bmpStream.Position = 0;

tests/ImageSharp.Tests/Formats/Jpg/GenericBlock8x8Tests.cs

@@ -43,7 +43,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             {
                 var d = default(GenericBlock8x8<TPixel>);
                 var rowOctet = new RowOctet<TPixel>(s.GetRootFramePixelBuffer(), 0);
-                d.LoadAndStretchEdges(s.Frames.RootFrame.PixelBuffer, 0, 0, rowOctet);
+                d.LoadAndStretchEdges(s.Frames.RootFrame.PixelBuffer, 0, 0, ref rowOctet);
 
                 TPixel a = s.Frames.RootFrame[0, 0];
                 TPixel b = d[0, 0];
@@ -68,7 +68,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             {
                 var d = default(GenericBlock8x8<TPixel>);
                 var rowOctet = new RowOctet<TPixel>(s.GetRootFramePixelBuffer(), 7);
-                d.LoadAndStretchEdges(s.Frames.RootFrame.PixelBuffer, 6, 7, rowOctet);
+                d.LoadAndStretchEdges(s.Frames.RootFrame.PixelBuffer, 6, 7, ref rowOctet);
 
                 Assert.Equal(s[6, 7], d[0, 0]);
                 Assert.Equal(s[6, 8], d[0, 1]);