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v3: Backport. Fix JPEG SIMD slicing and padding length handling #3106

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Merged
JimBobSquarePants merged 1 commit into from
Apr 7, 2026
+10 −8
Merged

v3: Backport. Fix JPEG SIMD slicing and padding length handling #3106

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12 changes: 7 additions & 5 deletions src/ImageSharp/Common/Helpers/SimdUtils.HwIntrinsics.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1077,8 +1077,10 @@ internal static void UnpackToRgbPlanesAvx2Reduce(
Vector256<byte> rgb, rg, bx;
Vector256<float> r, g, b;

// Each iteration consumes 8 Rgb24 pixels (24 bytes) but starts with a 32-byte load,
// so we need 3 extra pixels of addressable slack beyond the vectorized chunk.
const int bytesPerRgbStride = 24;
nuint count = (uint)source.Length / 8;
nuint count = source.Length > 3 ? (uint)(source.Length - 3) / 8 : 0;
for (nuint i = 0; i < count; i++)
{
rgb = Avx2.PermuteVar8x32(Unsafe.AddByteOffset(ref rgbByteSpan, (uint)(bytesPerRgbStride * i)).AsUInt32(), extractToLanesMask).AsByte();
Expand All @@ -1098,10 +1100,10 @@ internal static void UnpackToRgbPlanesAvx2Reduce(
}

int sliceCount = (int)(count * 8);
redChannel = redChannel.Slice(sliceCount);
greenChannel = greenChannel.Slice(sliceCount);
blueChannel = blueChannel.Slice(sliceCount);
source = source.Slice(sliceCount);
redChannel = redChannel[sliceCount..];
greenChannel = greenChannel[sliceCount..];
blueChannel = blueChannel[sliceCount..];
source = source[sliceCount..];
}
}
}
6 changes: 3 additions & 3 deletions src/ImageSharp/Formats/Jpeg/Components/Encoder/SpectralConverter{TPixel}.cs
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Expand Up @@ -113,9 +113,9 @@ private void ConvertStride(int spectralStep)
Span<TPixel> sourceRow = this.pixelBuffer.DangerousGetRowSpan(srcIndex);
PixelOperations<TPixel>.Instance.UnpackIntoRgbPlanes(rLane, gLane, bLane, sourceRow);

rLane.Slice(paddingStartIndex).Fill(rLane[paddingStartIndex - 1]);
gLane.Slice(paddingStartIndex).Fill(gLane[paddingStartIndex - 1]);
bLane.Slice(paddingStartIndex).Fill(bLane[paddingStartIndex - 1]);
rLane.Slice(paddingStartIndex, paddedPixelsCount).Fill(rLane[paddingStartIndex - 1]);
gLane.Slice(paddingStartIndex, paddedPixelsCount).Fill(gLane[paddingStartIndex - 1]);
bLane.Slice(paddingStartIndex, paddedPixelsCount).Fill(bLane[paddingStartIndex - 1]);

// Convert from rgb24 to target pixel type
var values = new JpegColorConverterBase.ComponentValues(this.componentProcessors, y);
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