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Added support for PPM arbitrary maxval #6119

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hugovk merged 4 commits into from
Mar 18, 2022
Merged

Added support for PPM arbitrary maxval #6119

hugovk merged 4 commits into from
Mar 18, 2022

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@radarhere radarhere commented Mar 9, 2022 •
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Resolves #5008
Resolves #5403

#5008 points out that the PPM maxval does not just indicate the bit depth of an image. It is an arbitrary number, that "Must be less than 65536 and more than zero."

The example from the issue has a maxval of 11 (not 2 ** 11, but just 11), and https://en.wikipedia.org/wiki/Netpbm#PGM_example shows an example where the maxval is 15. So when it is 15, the values range between 0 and 15, from black to white.

This PR

  • allows this value to be arbitrary for P5 (grayscale). That fixes the code example in PGM and PPM maximum grey/color value is ignored #5008, and 10 bit pgm file not read correctly #5403, which features a P5 image at 10-bit depth, or 1023 maxval (2 ** 10 - 1).
  • allows this value to be arbitrary for P6 (RGB). When the value is more than 255, that would require more than 16-bits of RGB data. As we do not have such a mode, this PR simply scales down the pixels to fit into 16-bit.

@radarhere radarhere mentioned this pull request Mar 9, 2022
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@radarhere radarhere force-pushed the ppm_maxval branch 2 times, most recently from fefce6a to 0cbeac6 Compare March 9, 2022 10:47
@radarhere radarhere changed the title Added support for PPM P5 maxval < 255 and P6 arbitrary maxval Added support for PPM arbitrary maxval Mar 9, 2022
@radarhere radarhere mentioned this pull request Mar 9, 2022
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hugovk
hugovk reviewed Mar 13, 2022
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Tests/test_file_ppm.py Outdated
assert im.get_format_mimetype() == "image/x-portable-pixmap"


def test_arbitrary_maxval():
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@hugovk hugovk Mar 13, 2022

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Can these mode be parametrised? Looks like one input bytestring and two things to assert.

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@radarhere radarhere Mar 13, 2022

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Ok, done

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@hugovk hugovk Mar 18, 2022

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Thanks!

@hugovk hugovk merged commit 7857a1d into python-pillow:main Mar 18, 2022
@radarhere radarhere deleted the ppm_maxval branch March 18, 2022 20:43
@SimonSegerblomRex SimonSegerblomRex mentioned this pull request Apr 12, 2022
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@radarhere radarhere mentioned this pull request Jun 13, 2022
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10 bit pgm file not read correctly PGM and PPM maximum grey/color value is ignored

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