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

Problem

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

Error (redacted / normalized)

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

Confirm it is this problem

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

Not this when / boundary

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

Root cause

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

Fix

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.

How to verify

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

What did not work

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

Verified scope

gifsicle 1.9x, Oct 2026