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Correct transparency when converting from RGB to LA or La - #10390

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Correct transparency when converting from RGB to LA or La#10390
radarhere wants to merge 1 commit into
python-pillow:mainfrom
radarhere:convert_la

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@radarhere radarhere commented Nov 2, 2026

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Resolves #10389

When converting with transparency, Pillow usually converts to RGB first, and then applies transparency.

It compares all channels when detecting transparent pixels, presuming that the fourth channel is 255.

Pillow/src/libImaging/Convert.c

Lines 465 to 470 in c3fd401

rgbT2a(UINT8 *out, UINT8 *in, int xsize, int r, int g, int b, int premultiplied) {
#ifdef WORDS_BIGENDIAN
UINT32 trns = ((r & 0xff) << 24) | ((g & 0xff) << 16) | ((b & 0xff) << 8) | 0xff;
UINT32 repl = premultiplied ? 0 : (trns & 0xffffff00);
#else
UINT32 trns = (0xffU << 24) | ((b & 0xff) << 16) | ((g & 0xff) << 8) | (r & 0xff);

This is not necessarily the case though, as Image.new("RGB", (1, 1)) sets the fourth channel to 0.

Pillow/src/_imaging.c

Line 648 in c3fd401

a = (UINT8)(r >> 24);

So when checking the pixel value, let's treat the fourth channel as 255.

@radarhere radarhere mentioned this pull request Nov 2, 2026
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convert("LA") with an RGB info["transparency"] stores the red value as L of transparent pixels and misses pixels whose padding byte is not 255

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