Spectral tests are now 100% accurate.

Error is now when collating blocks for color transform.

src/ImageSharp/Formats/Jpeg/Common/Decoder/ComponentUtils.cs

@@ -1,16 +0,0 @@
-namespace SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder
-{
-    /// <summary>
-    /// Various utilities for <see cref="IJpegComponent"/>.
-    /// </summary>
-    internal static class ComponentUtils
-    {
-        /// <summary>
-        /// Gets a reference to the <see cref="Block8x8"/> at the given row and column index from <see cref="IJpegComponent.SpectralBlocks"/>
-        /// </summary>
-        public static ref Block8x8 GetBlockReference(this IJpegComponent component, int bx, int by)
-        {
-            return ref component.SpectralBlocks[bx, by];
-        }
-    }
-}

src/ImageSharp/Formats/Jpeg/Common/Decoder/IJpegComponent.cs

@@ -42,5 +42,13 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder
         /// We need to apply IDCT and dequantiazition to transform them into color-space blocks.
         /// </summary>
         Buffer2D<Block8x8> SpectralBlocks { get; }
+
+        /// <summary>
+        /// Gets a reference to the <see cref="Block8x8"/> at the given row and column index from <see cref="SpectralBlocks"/>
+        /// </summary>
+        /// <param name="column">The column</param>
+        /// <param name="row">The row</param>
+        /// <returns>The <see cref="Block8x8"/></returns>
+        ref Block8x8 GetBlockReference(int column, int row);
     }
 }

src/ImageSharp/Formats/Jpeg/GolangPort/Components/Decoder/OrigComponent.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Runtime.CompilerServices;
 using SixLabors.ImageSharp.Formats.Jpeg.Common;
 using SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder;
 using SixLabors.ImageSharp.Memory;
@@ -237,6 +238,12 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.GolangPort.Components.Decoder
             this.SamplingFactors = new Size(h, v);
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public ref Block8x8 GetBlockReference(int column, int row)
+        {
+            return ref this.SpectralBlocks[column, row];
+        }
+
         public void Dispose()
         {
             this.SpectralBlocks.Dispose();

src/ImageSharp/Formats/Jpeg/PdfJsPort/Components/PdfJsFrameComponent.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Formats.Jpeg.Common;
 using SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder;
 using SixLabors.ImageSharp.Memory;
@@ -130,14 +131,20 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.PdfJsPort.Components
                 this.SubSamplingDivisors = c0.SamplingFactors.DivideBy(this.SamplingFactors);
             }
 
-            this.SpectralBlocks = this.memoryManager.Allocate2D<Block8x8>(this.SizeInBlocks.Width, this.SizeInBlocks.Height, true);
+            this.SpectralBlocks = this.memoryManager.Allocate2D<Block8x8>(this.BlocksPerColumnForMcu, this.BlocksPerLineForMcu + 1, true);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public ref Block8x8 GetBlockReference(int column, int row)
+        {
+            int offset = ((this.WidthInBlocks + 1) * row) + column;
+            return ref Unsafe.Add(ref MemoryMarshal.GetReference(this.SpectralBlocks.Span), offset);
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public int GetBlockBufferOffset(int row, int col)
         {
-            // return 64 * (((this.WidthInBlocks + 1) * row) + col);
+            return 64 * (((this.WidthInBlocks + 1) * row) + col);
-            return 64 * ((this.WidthInBlocks * row) + col);
         }
     }
 }

tests/ImageSharp.Tests/Formats/Jpg/Utils/LibJpegTools.ComponentData.cs

@@ -184,6 +184,11 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
                 }
             }
 
+            public ref Block8x8 GetBlockReference(int column, int row)
+            {
+                throw new NotImplementedException();
+            }
+
             public static bool operator ==(ComponentData left, ComponentData right)
             {
                 return Object.Equals(left, right);