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Fix 16-bit ImageFilter accuracy by akx · Pull Request #9786 · python-pillow/Pillow · GitHub

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Fix 16-bit ImageFilter accuracy - #9786

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radarhere merged 2 commits into
python-pillow:mainfrom
akx:just-bugfix-f16
Jul 17, 2026
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Fix 16-bit ImageFilter accuracy#9786
radarhere merged 2 commits into
python-pillow:mainfrom
akx:just-bugfix-f16

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akx commented Jul 9, 2026 •
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Follows up on #8438 (418ae7c)

Also included in #9760.

  1. The high byte of each 16-bit channel was being truncated due to an operator typo.
  2. 16-bit values were inadvertently rounded up by a pixel value per iteration.

radarhere mentioned this pull request Jul 10, 2026
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radarhere commented Jul 13, 2026

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I've created akx#9 with a suggestion.

akx commented Jul 13, 2026

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Sure, though then the relationship between the 1000 requested and the 1001 we're testing against is a little muddier.

radarhere commented Jul 16, 2026

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We can restore it if you would prefer, I just didn't see any need to not be precise.

Is that what 'Allow for kernel rounding errors' was meant to refer to?

akx commented Jul 16, 2026

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Is that what 'Allow for kernel rounding errors' was meant to refer to?

Yes... Now that I look into it again without just allowing "well it's probably floating point 🤷", the source of the bug is the "Add one time for rounding" from 847dd67 being accumulated into 1.0 throughout due to int ss_int = ROUND_UP(ss);.

That needs to be fixed, and then we can truly use exact values 😄

akx force-pushed the just-bugfix-f16 branch from b0f428d to 4b87863 Compare July 16, 2026 07:22
akx changed the title Fix high byte of I;16* images being truncated by ImageFilter Fix I;16* ImageFilter accuracy Jul 16, 2026
akx force-pushed the just-bugfix-f16 branch from 4b87863 to edd2d82 Compare July 16, 2026 07:25
akx changed the title Fix I;16* ImageFilter accuracy Fix 16-bit ImageFilter accuracy Jul 16, 2026
radarhere reviewed Jul 16, 2026
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Comment thread Tests/test_image_filter.py Outdated
radarhere reviewed Jul 16, 2026
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Comment thread Tests/test_image_filter.py Outdated
akx force-pushed the just-bugfix-f16 branch from edd2d82 to 4d4d54b Compare July 16, 2026 09:04
akx force-pushed the just-bugfix-f16 branch from 4d4d54b to 77bd329 Compare July 16, 2026 09:04
radarhere reviewed Jul 17, 2026
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Comment thread Tests/test_image_filter.py
akx mentioned this pull request Jul 17, 2026
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radarhere merged commit 78be7e8 into python-pillow:main Jul 17, 2026
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akx deleted the just-bugfix-f16 branch August 10, 2026 18:55
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