BUG: Consistently apply color palettes by default #955
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xref: scikit-image/scikit-image#6764 revealed that ImageIO is not behaving consistently when decoding images that use a color palette. GIFs have their palette applied by default; however, other formats (like PNG) don't.
This is a regression from the old pillow plugin, and while paletted images that aren't GIF are fairly rare in practice, we should still behave consistently and avoid surprising behavior. Hence, this PR updates the pillow plugin to apply palettes by default. The (raw) index data can still be read by passing
mode="P".This PR also updates how the color palette is exposed as part of the metadata. (1) Since we apply a color palette by default, we should (by default) exclude it from the metadata. This PR changes that behavior and aligns the plugin to the documented API. (2) It changes the returned value from a
PIL.ImagePalette.ImagePaletteto anp.ndarray. This should make interacting with and inspecting the palette much easier in the future.