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LibGfx+Tests: Improve calculation of restart interval
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.
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3 changed files with 14 additions and 3 deletions
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@ -324,6 +324,15 @@ TEST_CASE(test_jpeg_sof0_several_scans)
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TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 592, 800 }));
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}
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TEST_CASE(test_odd_mcu_restart_interval)
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{
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auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/odd-restart.jpg"sv)));
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EXPECT(Gfx::JPEGImageDecoderPlugin::sniff(file->bytes()));
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auto plugin_decoder = TRY_OR_FAIL(Gfx::JPEGImageDecoderPlugin::create(file->bytes()));
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TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 102, 77 }));
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}
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TEST_CASE(test_jpeg_rgb_components)
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{
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auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("jpg/rgb_components.jpg"sv)));
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