Nico Weber 6971ba35d5 LibGfx+Tests: Support grayscale jpegs with 2x2 sampling and MCU reset ...

Non-interleaved files always have an MCU of one data unit.

(A "data unit" is an 8x8 tile of pixels, and an "MCU" is a
"minium coded unit", e.g. 2x2 data units for luminance and
1 data unit each for Cr and Cb for a YCrCb image with
4:2:0 subsampling.)

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 converted it to grayscale and saved it as a pgm in Photoshop.
Then I turned it into a weird jpeg like so:

    path/to/cjpeg \
        -outfile Tests/LibGfx/test-inputs/jpg/grayscale_mcu.jpg \
        -sample 2x2 -restart 3 out.pgm

Makes 3 of the 5 jpegs failing to decode at #22780 go.

2024-01-30 05:35:22 +01:00

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