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LibGfx/JPEG: Still iterate over AC coefficients of a EOB targeted block

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This commit is nonsense for anything else than SOF2 images with spectral
approximation. For this particular case, skips like EOB or ZRL only
apply to coefficients with a zero-history. This commit prepares the code
to handle this behavior.
This commit is contained in:
Lucas CHOLLET 2023-02-26 16:47:24 -05:00 committed by Linus Groh
parent ef98b06dff
commit 902d0ab58e
Notes: sideshowbarker 2024-07-17 00:16:31 +09:00 
Author: https://github.com/LucasChollet
Commit: https://github.com/SerenityOS/serenity/commit/902d0ab58e
Pull-request: https://github.com/SerenityOS/serenity/pull/17907
Issue: https://github.com/SerenityOS/serenity/issues/4039
Reviewed-by: https://github.com/gmta
Reviewed-by: https://github.com/nico ✅
Reviewed-by: https://github.com/trflynn89 ✅
1 changed files with 15 additions and 6 deletions
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21
Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp
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@ -365,6 +365,12 @@ static ErrorOr<bool> read_eob(Scan& scan, u32 symbol)
scan.end_of_bands_run_count = additional_value + (1 << eob_base) - 1;
if (scan.end_of_bands_run_count >= 1) {
// end_of_bands_run_count is decremented at the end of `build_macroblocks`.
// This line compensate this effect.
++scan.end_of_bands_run_count;
}
return true;
}
@ -390,6 +396,9 @@ static ErrorOr<void> add_ac(JPEGLoadingContext& context, Macroblock& macroblock,
auto first_coefficient = max(1, scan.spectral_selection_start);
for (int j = first_coefficient; j <= scan.spectral_selection_end;) {
if (scan.end_of_bands_run_count > 0)
continue;
// AC symbols encode 2 pieces of information, the high 4 bits represent
// number of zeroes to be stuffed before reading the coefficient. Low 4
// bits represent the magnitude of the coefficient.
@ -462,18 +471,18 @@ static ErrorOr<void> build_macroblocks(JPEGLoadingContext& context, Vector<Macro
continue;
}
// G.1.2.2 - Progressive encoding of AC coefficients with Huffman coding
if (context.current_scan.end_of_bands_run_count > 0) {
--context.current_scan.end_of_bands_run_count;
continue;
}
Macroblock& block = macroblocks[macroblock_index];
if (context.current_scan.spectral_selection_start == 0)
TRY(add_dc(context, block, scan_component));
if (context.current_scan.spectral_selection_end != 0)
TRY(add_ac(context, block, scan_component));
// G.1.2.2 - Progressive encoding of AC coefficients with Huffman coding
if (context.current_scan.end_of_bands_run_count > 0) {
--context.current_scan.end_of_bands_run_count;
continue;
}
}
}
}