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LibGfx/JPEG: Change the loop over AC coefficients

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We used to skip over zero coefficient by modifying the loop counter. It
is unfortunately impossible to perform this with SOF2 images as only
coefficients with a zero-history should be skipped.
This induces no behavior change for the user of the function.
This commit is contained in:
Lucas CHOLLET 2023-02-26 01:45:00 -05:00 • committed by Linus Groh
parent 902d0ab58e
commit 731c876ff7
1 changed files with 28 additions and 22 deletions
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50
Userland/Libraries/LibGfx/ImageFormats/JPEGLoader.cpp
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@ -365,11 +365,9 @@ 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;
}
// end_of_bands_run_count is decremented at the end of `build_macroblocks`.
// And we need to now that we reached End of Block in `add_ac`.
++scan.end_of_bands_run_count;
return true;
}
@ -395,28 +393,29 @@ static ErrorOr<void> add_ac(JPEGLoadingContext& context, Macroblock& macroblock,
// 0th coefficient is the dc, which is already handled
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;
u32 to_skip = 0;
Optional<u8> saved_symbol;
for (int j = first_coefficient; j <= scan.spectral_selection_end; ++j) {
// 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.
auto ac_symbol = TRY(get_next_symbol(scan.huffman_stream, ac_table));
if (scan.end_of_bands_run_count == 0 && !saved_symbol.has_value()) {
saved_symbol = TRY(get_next_symbol(scan.huffman_stream, ac_table));
if (TRY(read_eob(scan, ac_symbol)))
break;
// ac_symbol = JPEG_ZRL means we need to skip 16 zeroes.
u8 run_length = ac_symbol == JPEG_ZRL ? 16 : ac_symbol >> 4;
j += run_length;
if (j > scan.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");
if (!TRY(read_eob(scan, *saved_symbol))) {
to_skip = *saved_symbol >> 4;
}
}
u8 coeff_length = ac_symbol & 0x0F;
if (to_skip > 0) {
--to_skip;
continue;
}
if (scan.end_of_bands_run_count > 0)
continue;
u8 coeff_length = *saved_symbol & 0x0F;
if (coeff_length > 10) {
dbgln_if(JPEG_DEBUG, "AC coefficient too long: {}!", coeff_length);
return Error::from_string_literal("AC coefficient too long");
@ -427,8 +426,15 @@ static ErrorOr<void> add_ac(JPEGLoadingContext& context, Macroblock& macroblock,
if (ac_coefficient < (1 << (coeff_length - 1)))
ac_coefficient -= (1 << coeff_length) - 1;
select_component[zigzag_map[j++]] = ac_coefficient;
select_component[zigzag_map[j]] = ac_coefficient;
}
saved_symbol.clear();
}
if (to_skip > 0) {
dbgln_if(JPEG_DEBUG, "Run-length exceeded boundaries. Cursor: {}, Skipping: {}!", scan.spectral_selection_end + to_skip, to_skip);
return Error::from_string_literal("Run-length exceeded boundaries");
}
return {};