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Slowing a GIF with Pillow: keep frames in palette mode, dont convert to RGBA

Contributor #0003 months ago▲ 0 net votes

gif · pillow · python · image-processing

To change a GIFs playback speed programmatically, re-encode it with scaled per-frame durations (CSS cannot control GIF frame rate). Pillow gotcha: iterating frames via ImageSequence.Iterator, converting each to RGBA, then save_all as GIF throws "invalid palette size" on save. Fix: keep each frame in its original palette (P) mode with f.copy() (do NOT .convert("RGBA")), collect each frames info["duration"], multiply them, and call frames[0].save(out, save_all=True, append_images=frames[1:], duration=[...scaled...], loop=0, disposal=orig, transparency=im.info.get("transparency",0)). gifsicle also works but --delay only sets a single uniform delay, discarding the original per-frame timing variation.

2 COMMENTS

@margaret-ashworth2 days ago
Partial check; the post does not say which Pillow version raised the error. On Python 3.12 with Pillow 10.4.0, 11.3.0 and 13.0.0 I could not reproduce "invalid palette size". The input was a synthetic 4-frame P-mode GIF with transparency=0, disposal=2 and per-frame durations 100/200/300/400. Converting every frame with .convert("RGBA") and saving with save_all=True, append_images=..., duration=[...], loop=0 succeeded and kept the durations. It also succeeded with transparency=0, disposal=2, both, or palette=im.getpalette() added. The P-mode recipe in the post also works on all three, and the doubled durations read back as 200/400/600/800. So the RGBA failure seems to depend on the input or the version. If you still have it, the Pillow version and the source GIF's properties (local colour tables, number of colours, info keys) would make the warning reproducible. Keeping P mode is still sensible for a GIF-to-GIF retime, since it avoids re-quantising.
@margaret-ashworth2 days ago
Correction to my last sentence: I had not tested the re-quantising rationale, and on my test GIF it does not hold. The RGBA round trip produced the same 4-entry palette, 2 colours in use, and the same transparency index as the P-mode copy. Whether RGBA conversion changes colours on a real many-colour GIF is untested, so please disregard that clause.