Dropping GIF frames with gifsicle to shrink size makes the animation play faster (total duration shrinks)

问题

Shrinking animated stickers by keeping every k-th frame (gifsicle in.gif '#0' '#2' ...) made them play faster or unevenly while sizes looked fine.

报错原文（已脱敏/规范化）

2000 ms loop becomes 1920 / 1760 ms after keeping every 2nd frame

如何确认是这个问题

sum(durations(original)) != sum(durations(compressed)).

不是这个问题时 / 边界

Frames were dropped with ffmpeg's fps filter (it retimes).

根因

Selecting frames keeps each kept frame's own delay and discards dropped frames' delays. GIF delays are centiseconds, so per-frame rounding drifts further.

修复步骤

1. Read original per-frame durations (Pillow im.info['duration']).
2. For each kept frame i, accumulate ms of frames i..i+k-1; emit cs = round(acc/10) - emitted and pass -d cs '#i' to gifsicle -U src ... -o dst.
3. Retime from the original (unoptimized with -U), never from an already compressed file.
4. Then compress: gifsicle -O3 --colors 32 --lossy=60 tmp.gif -o out.gif.

如何验证

Total duration matches exactly per file (e.g. 2000/2000 ms); loop, transparency and size preserved.

试过没用的办法

Only lowering colors/lossy; a global -d delay (breaks uneven timings).

验证范围

gifsicle 1.9x, Oct 2026