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LibGfx: Don't assume that scans are always full when filling macroblocks

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In other words: only consider coefficient of the current scan when
adding coefficients to a macroblock. Information about which
coefficients are present in the stream are passed through the spectral
information in the context.
This commit is contained in:
Lucas CHOLLET 2023-02-21 01:16:56 -05:00 • committed by Andreas Kling
parent d421967394
commit cf6bceeb2c
1 changed files with 8 additions and 3 deletions
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11
Userland/Libraries/LibGfx/JPEGLoader.cpp
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@ -306,7 +306,11 @@ static ErrorOr<void> add_ac(JPEGLoadingContext& context, Macroblock& macroblock,
auto* select_component = get_component(macroblock, component_index);
// Compute the AC coefficients.
for (int j = 1; j < 64;) {
// 0th coefficient is the dc, which is already handled
auto first_coefficient = max(1, context.spectral_selection_start);
for (int j = first_coefficient; j <= context.spectral_selection_end;) {
// 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.
@ -318,7 +322,7 @@ static ErrorOr<void> add_ac(JPEGLoadingContext& context, Macroblock& macroblock,
u8 run_length = ac_symbol == 0xF0 ? 16 : ac_symbol >> 4;
j += run_length;
if (j >= 64) {
if (j > context.spectral_selection_end) {
dbgln_if(JPEG_DEBUG, "Run-length exceeded boundaries. Cursor: {}, Skipping: {}!", j, run_length);
return Error::from_string_literal("Run-length exceeded boundaries");
}
@ -371,7 +375,8 @@ static ErrorOr<void> build_macroblocks(JPEGLoadingContext& context, Vector<Macro
u32 mb_index = (vcursor + vfactor_i) * context.mblock_meta.hpadded_count + (hfactor_i + hcursor);
Macroblock& block = macroblocks[mb_index];
TRY(add_dc(context, block, component, component_i));
if (context.spectral_selection_start == 0)
TRY(add_dc(context, block, component, component_i));
TRY(add_ac(context, block, component, component_i));
}
}