From e39adf85f6c7add528325f1d557411e9751e7bcb Mon Sep 17 00:00:00 2001
From: Dmitry Pentin <riotbr3aker@gmail.com>
Date: Sun, 11 Jul 2021 00:01:37 +0300
Subject: [PATCH] Fixed invalid baseline jpeg decoding

---
 .../Components/Decoder/HuffmanScanDecoder.cs     |  5 +++--
 .../Decoder/JpegComponentPostProcessor.cs        | 16 ++++++++++++++++
 .../Decoder/SpectralConverter{TPixel}.cs         | 10 ++++++++++
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/src/ImageSharp/Formats/Jpeg/Components/Decoder/HuffmanScanDecoder.cs b/src/ImageSharp/Formats/Jpeg/Components/Decoder/HuffmanScanDecoder.cs
index 914562831d..0e13f75e44 100644
--- a/src/ImageSharp/Formats/Jpeg/Components/Decoder/HuffmanScanDecoder.cs
+++ b/src/ImageSharp/Formats/Jpeg/Components/Decoder/HuffmanScanDecoder.cs
@@ -185,7 +185,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
                         for (int y = 0; y < v; y++)
                         {
                             int blockRow = (mcuRow * v) + y;
-                            Span<Block8x8> blockSpan = component.SpectralBlocks.GetRowSpan(blockRow);
+                            Span<Block8x8> blockSpan = component.SpectralBlocks.GetRowSpan(y);
                             ref Block8x8 blockRef = ref MemoryMarshal.GetReference(blockSpan);
 
                             for (int x = 0; x < h; x++)
@@ -213,7 +213,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
                 }
 
                 // convert from spectral to actual pixels via given converter
-                this.spectralConverter.ConvertStride(j, j);
+                this.spectralConverter.ConvertStride(j, 0);
+                this.spectralConverter.ClearStride(0);
             }
         }
 
diff --git a/src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponentPostProcessor.cs b/src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponentPostProcessor.cs
index 9eafaea0ae..6c25601c28 100644
--- a/src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponentPostProcessor.cs
+++ b/src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponentPostProcessor.cs
@@ -108,6 +108,22 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             }
         }
 
+        public void ClearSpectralStride(int step)
+        {
+            int yBlockStart = step * this.BlockRowsPerStep;
+            for (int y = 0; y < this.BlockRowsPerStep; y++)
+            {
+                int yBlock = yBlockStart + y;
+
+                if (yBlock >= this.SizeInBlocks.Height)
+                {
+                    break;
+                }
+
+                this.Component.SpectralBlocks.GetRowSpan(yBlock).Clear();
+            }
+        }
+
         public void CopyBlocksToColorBuffer()
         {
             this.CopyBlocksToColorBuffer(this.currentComponentRowInBlocks);
diff --git a/src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs b/src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs
index 580adb3ab7..3499704aed 100644
--- a/src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs
+++ b/src/ImageSharp/Formats/Jpeg/Components/Decoder/SpectralConverter{TPixel}.cs
@@ -21,6 +21,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
         public abstract void ConvertStride(int step, int spectralStep);
 
+        public abstract void ClearStride(int spectralStep);
+
         public abstract void Dispose();
     }
 
@@ -134,6 +136,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             }
         }
 
+        public override void ClearStride(int spectralStep)
+        {
+            foreach (JpegComponentPostProcessor cpp in this.componentProcessors)
+            {
+                cpp.ClearSpectralStride(spectralStep);
+            }
+        }
+
         public override void Dispose()
         {
             foreach (JpegComponentPostProcessor cpp in this.componentProcessors)