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Fixed seeking to an L frame in a GIF #6576

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merged 3 commits into from Oct 18, 2022
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Resolves #6571

When pasting an L frame onto an RGB(A) GIF, convert the frame to RGB(A) first.

The test image was created using a modified Pillow, so that there is no global palette, and the image is comprised of a frame with a palette, and then two frames without. This means that the first frame is P mode, the second is L mode, combining to make RGB, and then there is an L mode frame pasted onto that RGB frame.

@radarhere radarhere added the GIF label Sep 13, 2022
@radarhere radarhere changed the title Fixed pasting an L frame onto an RGB(A) GIF Fixed seek to an L frame from an RGB(A) GIF Sep 13, 2022
@radarhere radarhere changed the title Fixed seek to an L frame from an RGB(A) GIF Fixed seeking to an L frame from an RGB(A) GIF Sep 13, 2022
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I've added a commit to address #6610 (comment)

That comment found a GIF where on a later frame, the palette is "not needed" because the frame is grayscale. The problem is that Pillow then concludes that since there isn't a palette, it should fallback to the global palette.

if self._is_palette_needed(p):
palette = ImagePalette.raw("RGB", p)
# image data
bits = self.fp.read(1)[0]
self.__offset = self.fp.tell()
break
else:
pass
# raise OSError, "illegal GIF tag `%x`" % s[0]
s = None
if interlace is None:
# self._fp = None
raise EOFError
self.__frame = frame
if not update_image:
return
self.tile = []
if self.dispose:
self.im.paste(self.dispose, self.dispose_extent)
self._frame_palette = palette or self.global_palette

This is incorrect, as the frame is specifying custom palette behaviour, it's just that custom behaviour is to have a grayscale palette.

@radarhere radarhere changed the title Fixed seeking to an L frame from an RGB(A) GIF Fixed seeking to an L frame in a GIF Oct 5, 2022
@hugovk hugovk merged commit 745dc33 into python-pillow:main Oct 18, 2022
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@radarhere radarhere deleted the gif branch October 18, 2022 21:13
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Exception using "seek" on GIF that contains frames with different modes
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