Resizing converter no longer depends on avx converter only

src/ImageSharp/Formats/Jpeg/Components/Decoder/ColorConverters/JpegColorConverterAvx.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 #if SUPPORTS_RUNTIME_INTRINSICS
+using System.Runtime.Intrinsics;
 using System.Runtime.Intrinsics.X86;
 
 namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters
@@ -25,7 +26,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters
             {
             }
 
-            public override bool IsAvailable => Avx.IsSupported;
+            public sealed override bool IsAvailable => Avx.IsSupported;
+
+            public sealed override int ElementsPerBatch => Vector256<float>.Count;
         }
     }
 }

src/ImageSharp/Formats/Jpeg/Components/Decoder/ColorConverters/JpegColorConverterBase.cs

@@ -35,6 +35,15 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters
         /// </summary>
         public abstract bool IsAvailable { get; }
 
+        /// <summary>
+        /// Gets a value indicating how many pixels are processed in a single batch.
+        /// </summary>
+        /// <remarks>
+        /// This generaly should be equal to register size,
+        /// e.g. 1 for scalar implementation, 8 for AVX implementation and so on.
+        /// </remarks>
+        public abstract int ElementsPerBatch { get; }
+
         /// <summary>
         /// Gets the <see cref="JpegColorSpace"/> of this converter.
         /// </summary>

src/ImageSharp/Formats/Jpeg/Components/Decoder/ColorConverters/JpegColorConverterScalar.cs

@@ -16,7 +16,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters
             {
             }
 
-            public override bool IsAvailable => true;
+            public sealed override bool IsAvailable => true;
+
+            public sealed override int ElementsPerBatch => 1;
         }
     }
 }

src/ImageSharp/Formats/Jpeg/Components/Decoder/ColorConverters/JpegColorConverterVector.cs

@@ -17,7 +17,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters
         /// Even though real life data is guaranteed to be of size
         /// divisible by 8 newer SIMD instructions like AVX512 won't work with
         /// such data out of the box. These converters have fallback code
-        /// for 'remainder' data.
+        /// for remainder data.
         /// </remarks>
         internal abstract class JpegColorConverterVector : JpegColorConverterBase
         {
@@ -28,7 +28,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters
 
             public sealed override bool IsAvailable => Vector.IsHardwareAccelerated && Vector<float>.Count % 4 == 0;
 
-            public override void ConvertToRgbInplace(in ComponentValues values)
+            public sealed override int ElementsPerBatch => Vector<float>.Count;
+
+            public sealed override void ConvertToRgbInplace(in ComponentValues values)
             {
                 DebugGuard.IsTrue(this.IsAvailable, $"{this.GetType().Name} converter is not supported on current hardware.");
 

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

@@ -91,10 +91,13 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             int blockPixelSize = 8 / scaleDenominator;
             this.pixelRowsPerStep = majorVerticalSamplingFactor * blockPixelSize;
 
-            // BUFFER SIZE MUST BE DIVISIBLE BY 8 ATM
+            // color converter
-            // TODO: fix this mess
+            JpegColorConverterBase converter = this.GetColorConverter(frame, jpegData);
+            this.colorConverter = converter;
+
             int bufferWidth = majorBlockWidth * blockPixelSize;
-            int correctedBufferWidth = bufferWidth + (8 - (bufferWidth % 8));
+            int batchSize = converter.ElementsPerBatch;
+            int correctedBufferWidth = bufferWidth + (batchSize - (bufferWidth % batchSize));
             var postProcessorBufferSize = new Size(correctedBufferWidth, this.pixelRowsPerStep);
 
             this.componentProcessors = new JpegComponentPostProcessor8[frame.Components.Length];
@@ -102,9 +105,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             {
                 this.componentProcessors[i] = new JpegComponentPostProcessor8(allocator, frame, jpegData, postProcessorBufferSize, frame.Components[i]);
             }
-
-            // color converter
-            this.colorConverter = this.GetColorConverter(frame, jpegData);
         }
 
         public override void ConvertStrideBaseline()