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69964e10f4
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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|---|---|---|
| .. | ||
| 12-bit-progressive.jpg | ||
| 12-bit.jpg | ||
| big_image.jpg | ||
| buggie-cmyk.jpg | ||
| gradient_empty_icc.jpg | ||
| grayscale_app14.jpg | ||
| grayscale_mcu.jpg | ||
| odd-restart.jpg | ||
| oss-fuzz-testcase-59785.jpg | ||
| oss-fuzz-testcase-62584.jpg | ||
| oss-fuzz-testcase-63815.jpg | ||
| rgb24.jpg | ||
| rgb_components.jpg | ||
| several_scans.jpg | ||
| several_scans_odd_number_mcu.jpg | ||
| spectral_selection.jpg | ||
| successive_approximation.jpg | ||
| ycck-1111.jpg | ||
| ycck-2111.jpg | ||
| ycck-2112.jpg | ||
