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WebP/Lossy: Allow negative values from segment adjustment too

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The spec doesn't talk about this happening in the text, but
`dequant_init()` in 20.4 processes segment adjustment and quantization
index adjustment in the same variable `q` before clamping.
Since we had to adjust the latter step in the previous commit, do
it for the former step too.

I haven't seen this happen in the wild yet (and now, I hopefully
never will notice it if it happens).
This commit is contained in:
Nico Weber 2023-06-01 09:13:10 -04:00 committed by Andreas Kling
parent 661b2d394d
commit 2452cf6b55
Notes: sideshowbarker 2024-07-16 20:39:14 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/2452cf6b55
Pull-request: https://github.com/SerenityOS/serenity/pull/19203
1 changed files with 1 additions and 1 deletions
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Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
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@ -613,7 +613,7 @@ i16 dequantize_value(i16 value, bool is_dc, QuantizationIndices const& quantizat
// * For uv, the dc_qlookup index is clamped to 117 (instead of 127 for everything else)
// (or, alternatively, the value is clamped to 132 at most)
u8 y_ac_base = quantization_indices.y_ac;
int y_ac_base = quantization_indices.y_ac;
if (segmentation.update_macroblock_segmentation_map) {
if (segmentation.segment_feature_mode == SegmentFeatureMode::DeltaValueMode)
y_ac_base += segmentation.quantizer_update_value[segment_id];