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Fix VP8X reserved bytes use Position += 3 instead of Write, causing 3-byte truncation in alpha WebP images #3036

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Merged
brianpopow merged 2 commits into from
Dec 28, 2025
+30 −1
Merged

Fix VP8X reserved bytes use Position += 3 instead of Write, causing 3-byte truncation in alpha WebP images #3036

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2edab61
Write into stream instead of simply just advancing it.
jrlost Dec 23, 2025
adf677a
Add test for writing reserved bytes in WebpVp8X
brianpopow Dec 28, 2025
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5 changes: 4 additions & 1 deletion src/ImageSharp/Formats/Webp/Chunks/WebpVp8X.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -123,7 +123,10 @@ public void WriteTo(Stream stream)
long pos = RiffHelper.BeginWriteChunk(stream, (uint)WebpChunkType.Vp8X);

stream.WriteByte(flags);
stream.Position += 3; // Reserved bytes

Span<byte> reserved = stackalloc byte[3];
stream.Write(reserved);

WebpChunkParsingUtils.WriteUInt24LittleEndian(stream, this.Width - 1);
WebpChunkParsingUtils.WriteUInt24LittleEndian(stream, this.Height - 1);

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26 changes: 26 additions & 0 deletions tests/ImageSharp.Tests/Formats/WebP/WebpVp8XTests.cs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,26 @@
// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

using SixLabors.ImageSharp.Formats.Webp.Chunks;

namespace SixLabors.ImageSharp.Tests.Formats.WebP;

[Trait("Format", "Webp")]
public class WebpVp8XTests
{
[Fact]
public void WebpVp8X_WriteTo_Writes_Reserved_Bytes()
{
// arrange
WebpVp8X header = new(false, false, false, false, false, 10, 40);
MemoryStream ms = new();
byte[] expected = [86, 80, 56, 88, 10, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 39, 0, 0];

// act
header.WriteTo(ms);

// assert
byte[] actual = ms.ToArray();
Assert.Equal(expected, actual);
}
}
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