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@radarhere radarhere commented Apr 13, 2025

Resolves #8888

https://www.ece.ualberta.ca/~elliott/ee552/studentAppNotes/2003_w/misc/bmp_file_format/bmp_file_format.htm states that the fourth channel in a BMP color table is 'unused (=0)'.

It seems reasonable to do so when saving 1 and L mode BMPs, and makes more intuitive sense than our current strategy of setting the fourth channel to the same value as the other three.

@hugovk hugovk merged commit ba37249 into python-pillow:main Jun 27, 2025
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@radarhere radarhere deleted the bmp branch June 27, 2025 15:17
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BmpImagePlugin save palette (in "1" mode) sets a "should probably be 0" value to 255...

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