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LibGfx/CCITT: Reimplement PassMode in a less naive way

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The old implementation of PassMode has only been tested with a single
image, and let's say that it didn't survive long in the wild. A few
cases were not considered:
 - We only supported VerticalMode right after PassMode.
 - It can happen that token need to be used but not consumed from the
 reference line.

 With that fix, we are able to decode every single PDF file from the
 1000-file zip "0000" (except 0000871.pdf, which uses byte alignment).
 This is massive progress compared to the hundred of errors that we were
 previously receiving.
This commit is contained in:
Lucas CHOLLET 2024-02-16 17:02:47 -05:00 • committed by Andreas Kling
parent 883b2268c0
commit 83f1775f15
1 changed files with 33 additions and 11 deletions
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44
Userland/Libraries/LibGfx/ImageFormats/CCITTDecoder.cpp
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@ -418,32 +418,42 @@ ErrorOr<ReferenceLine> decode_single_ccitt_2d_line(BigEndianInputBitStream& inpu
ReferenceLine current_line {};
Color current_color { ccitt_white };
u32 column {};
u32 remainder_from_pass_mode {};
auto const next_change_on_reference_line = [&](Search search = Search::B1) -> ErrorOr<Change> {
auto const next_change_on_reference_line = [&]() -> ErrorOr<Change> {
// 4.2.1.3.1 Definition of changing picture elements
Optional<Change> next_change {}; // This is referred to as b1 in the spec.
u32 offset {};
while (!next_change.has_value()) {
if (reference_line.is_empty())
if (reference_line.is_empty() || reference_line.size() <= offset)
return Error::from_string_literal("CCITTDecoder: Corrupted stream");
auto const change = reference_line.take_first();
if (change.column < column)
auto const change = reference_line[0 + offset];
// 4.2.1.3.4 Processing the first and last picture elements in a line
// "The first starting picture element a0 on each coding line is imaginarily set at a position just
// before the first picture element, and is regarded as a white picture element."
// To emulate this behavior we check for column == 0 here.
if (change.column <= column && column != 0) {
reference_line.take_first();
continue;
if ((search == Search::B1 && change.color != current_color)
|| (search == Search::B2 && change.color == current_color))
}
if (change.color != current_color)
next_change = change;
else
offset++;
}
return *next_change;
};
auto const encode_for = [&](Change change, i8 offset = 0) -> ErrorOr<void> {
auto const to_encode = change.column - column + offset;
auto const to_encode = remainder_from_pass_mode + change.column - column + offset;
for (u32 i {}; i < to_encode; ++i)
TRY(decoded_bits.write_bits(current_color == ccitt_white ? 0u : 1u, 1));
column += to_encode;
column = change.column + offset;
current_color = change.color;
remainder_from_pass_mode = 0;
TRY(current_line.try_empend(change.color, change.column + offset));
TRY(current_line.try_empend(current_color, column));
return {};
};
@ -452,9 +462,21 @@ ErrorOr<ReferenceLine> decode_single_ccitt_2d_line(BigEndianInputBitStream& inpu
// Behavior are described here 4.2.1.3.2 Coding modes.
switch (mode.mode) {
case Mode::Pass:
TRY(next_change_on_reference_line(Search::B2));
case Mode::Pass: {
auto const column_before = column;
// We search for b1.
auto change = TRY(next_change_on_reference_line());
current_color = change.color;
column = change.column;
// We search for b2, which is the same as searching for b1 after updating the state.
change = TRY(next_change_on_reference_line());
current_color = change.color;
column = change.column;
remainder_from_pass_mode += column - column_before;
break;
}
case Mode::Horizontal: {
// a0a1
auto run_length = TRY(read_run_length(input_bit_stream, OptionalNone {}, current_color, image_width, column));