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LibGfx/JBIG2: Add arithmetic coding decoder

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I think the context normally changes for every bit. But this here
is enough to correctly decode the test bitstream in Annex H.2 in
the spec, which seems like a good checkpoint.

The internals of the decoder use spec naming, to make the code
look virtually identical to what's in the spec. (Even so, I managed
to put in several typos that took a while to track down.)
This commit is contained in:
Nico Weber 2024-03-11 22:28:20 -04:00 • committed by Andrew Kaster
parent c4be9318a2
commit df9dd8ec69
3 changed files with 294 additions and 0 deletions
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33
Tests/LibGfx/TestImageDecoder.cpp
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@ -347,6 +347,39 @@ TEST_CASE(test_jbig2_white_47x23)
EXPECT_EQ(pixel, Gfx::Color(Gfx::Color::White).value());
}
TEST_CASE(test_jbig2_arithmetic_decoder)
{
// https://www.itu.int/rec/T-REC-T.88-201808-I
// H.2 Test sequence for arithmetic coder
// clang-format off
constexpr auto input = to_array<u8>({
0x84, 0xC7, 0x3B, 0xFC, 0xE1, 0xA1, 0x43, 0x04,
0x02, 0x20, 0x00, 0x00, 0x41, 0x0D, 0xBB, 0x86,
0xF4, 0x31, 0x7F, 0xFF, 0x88, 0xFF, 0x37, 0x47,
0x1A, 0xDB, 0x6A, 0xDF, 0xFF, 0xAC
});
constexpr auto output = to_array<u8>({
0x00, 0x02, 0x00, 0x51, 0x00, 0x00, 0x00, 0xC0,
0x03, 0x52, 0x87, 0x2A, 0xAA, 0xAA, 0xAA, 0xAA,
0x82, 0xC0, 0x20, 0x00, 0xFC, 0xD7, 0x9E, 0xF6,
0xBF, 0x7F, 0xED, 0x90, 0x4F, 0x46, 0xA3, 0xBF
});
// clang-format on
// "For this entire test, a single value of CX is used. I(CX) is initially 0 and MPS(CX) is initially 0."
Gfx::JBIG2::ArithmeticDecoder::Context context { 0, 0 };
auto decoder = MUST(Gfx::JBIG2::ArithmeticDecoder::initialize(input, context));
for (auto expected : output) {
u8 actual = 0;
for (size_t i = 0; i < 8; ++i) {
actual <<= 1;
actual |= static_cast<u8>(decoder.get_next_bit());
}
EXPECT_EQ(actual, expected);
}
}
TEST_CASE(test_jpeg_sof0_one_scan)
{
auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/rgb24.jpg"sv)));