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LibGfx+Tests: Improve calculation of restart interval

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JPEGs can store a `restart_interval`, which controls how many
minimum coded units (MCUs) apart the stream state resets.
This can be used for error correction, decoding parts of a jpeg
in parallel, etc.

We tried to use

    u32 i = vcursor * context.mblock_meta.hpadded_count + hcursor;
    i % (context.dc_restart_interval *
         context.sampling_factors.vertical *
         context.sampling_factors.horizontal) == 0

to check if we hit a multiple of an MCU.

`hcursor` is the horizontal offset into 8x8 blocks, vcursor the
vertical offset, and hpadded_count stores how many 8x8 blocks
we have per row, padded to a multiple of the sampling factor.

This isn't quite right if hcursor isn't divisible by both
the vertical and horizontal sampling factor. Tweak things so
that they work.

Also rename `i` to `number_of_mcus_decoded_so_far` since that
what it is, at least now.

For the test case, I converted an existing image to a ppm:

    Build/lagom/bin/image -o out.ppm \
        Tests/LibGfx/test-inputs/jpg/12-bit.jpg

Then I resized it to 102x77px in Photoshop and saved it again.
Then I turned it into a jpeg like so:

    path/to/cjpeg \
        -outfile Tests/LibGfx/test-inputs/jpg/odd-restart.jpg \
        -sample 2x2,1x1,1x1 -quality 5 -restart 3B out.ppm

The trick here is to:

a) Pick a size that's not divisible by the data size width (8),
   and that when rounded to a block size (13) still isn't divisible
   by the subsample factor -- done by picking a width of 102.
b) Pick a huffman table that doesn't happen to contain the bit
   pattern for a restart marker, so that reading a restart marker
   from the bitstream as data causes a failure (-quality 5 happens
   to do this)
c) Pick a restart interval where we fail to skip it if our calculation
   is off (-restart 3B)

Together with #22987, fixes #22780.
This commit is contained in:
Nico Weber 2024-01-29 12:49:22 -05:00 • committed by Andreas Kling
parent fb2166f19c
commit 69964e10f4
3 changed files with 14 additions and 3 deletions
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9
Tests/LibGfx/TestImageDecoder.cpp
View file

@ -324,6 +324,15 @@ TEST_CASE(test_jpeg_sof0_several_scans)
TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 592, 800 }));
}
TEST_CASE(test_odd_mcu_restart_interval)
{
auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/odd-restart.jpg"sv)));
EXPECT(Gfx::JPEGImageDecoderPlugin::sniff(file->bytes()));
auto plugin_decoder = TRY_OR_FAIL(Gfx::JPEGImageDecoderPlugin::create(file->bytes()));
TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 102, 77 }));
}
TEST_CASE(test_jpeg_rgb_components)
{
auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/rgb_components.jpg"sv)));