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Ensure Image and core palettes are in sync after PA conversion #9834

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7e91108
Add xfailing test for pa2p quirk
akx Jul 29, 2026
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Ensure Image and core palettes are in sync after PA conversion (alpha…
akx Jul 29, 2026
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12 changes: 12 additions & 0 deletions Tests/test_image_convert.py
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Original file line number Diff line number Diff line change
Expand Up @@ -290,6 +290,18 @@ def test_p2pa_palette() -> None:
assert im_pa.getpalette() == im.getpalette()


def test_pa2p_truly_drops_alpha() -> None:
im = Image.frombytes("P", (2, 1), bytes([0, 1])).convert("PA")
im.putpalette(bytes([255, 0, 0, 7, 0, 255, 0, 9]), "RGBA")
im.putalpha(Image.frombytes("L", (2, 1), bytes([240, 220])))
assert im.get_flattened_data() == ((0, 240), (1, 220)) # Matches the alpha band
Comment on lines +296 to +297

@radarhere radarhere Jul 30, 2026

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Suggested change
im.putalpha(Image.frombytes("L", (2, 1), bytes([240, 220])))
assert im.get_flattened_data() == ((0, 240), (1, 220)) # Matches the alpha band

These lines don't appear relevant.

@akx akx Jul 30, 2026

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They're meant to show the palette alpha isn't used.

@radarhere radarhere Jul 30, 2026

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I consider it pretty foundational that the palette doesn't affect pixel values (without conversion). I'm not sure it needs to be tested here? We already have

Pillow/Tests/test_image_putalpha.py

Lines 28 to 33 in abb8ba0

im = Image.new("P", (1, 1), 1)
assert im.getpixel((0, 0)) == 1
im.putalpha(2)
assert im.mode == "PA"
assert im.getpixel((0, 0)) == (1, 2)

@akx akx Jul 30, 2026

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That test_image_putalpha segment doesn't seem to really cover the contents of the palette.

I don't see how having this here really hurts?

im_p = im.convert("P")
assert im_p.palette is not None
assert im_p.im.getpalettemode() == im_p.palette.mode == "RGB"
rgba_data = im_p.convert("RGBA").get_flattened_data()
assert rgba_data == ((255, 0, 0, 255), (0, 255, 0, 255))


def test_matrix_illegal_conversion() -> None:
# Arrange
im = hopper("CMYK")
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5 changes: 2 additions & 3 deletions src/PIL/Image.py
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Expand Up @@ -1242,9 +1242,8 @@ def convert_transparency(

new_im = self._new(im)
if mode in ("P", "PA") and palette != Palette.ADAPTIVE:
from . import ImagePalette

new_im.palette = ImagePalette.ImagePalette("RGB", im.getpalette("RGB"))
# Install the original image's palette into the new copy
new_im.putpalette(im.getpalette("RGB"))
if delete_trns:
# crash fail if we leave a bytes transparency in an rgb/l mode.
del new_im.info["transparency"]
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