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LibGfx: Fix JPG decoding bug on rare grayscale images

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Some grayscale JPG images might have the horizontal and vertical sample
factor set to 2. However, the macroblocks in these images are not
interleaved as they would be in color images.
This commit is contained in:
Michiel Visser 2022-02-19 21:05:05 +01:00 committed by Andreas Kling
parent 10581cfaeb
commit 7dcfb82e16
Notes: sideshowbarker 2024-07-17 18:31:17 +09:00 
Author: https://github.com/msvisser
Commit: https://github.com/SerenityOS/serenity/commit/7dcfb82e16
Pull-request: https://github.com/SerenityOS/serenity/pull/12664
1 changed files with 6 additions and 0 deletions
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6
Userland/Libraries/LibGfx/JPGLoader.cpp
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@ -742,6 +742,12 @@ static bool read_start_of_frame(InputMemoryStream& stream, JPGLoadingContext& co
component.vsample_factor = subsample_factors & 0x0F;
if (i == 0) {
// If there is only a single component, i.e. grayscale, the macroblocks will not be interleaved, even if
// the horizontal or vertical sample factor is larger than 1.
if (context.component_count == 1) {
component.hsample_factor = 1;
component.vsample_factor = 1;
}
// By convention, downsampling is applied only on chroma components. So we should
// hope to see the maximum sampling factor in the luma component.
if (!validate_luma_and_modify_context(component, context)) {