Mirrored ping-pong GIF loop moves an asymmetric feature to the wrong side for half the loop

Problem

A back-and-forth loop was built by horizontally flipping half of the frames; for half of every loop the character's eye patch was on the other eye.

Confirm it is this problem

Second-half frames are horizontal mirrors of first-half frames.

Not this when / boundary

The character is symmetric or a flip is intended.

Root cause

'Reverse' was implemented as a spatial mirror instead of time reversal. Mirroring flips every asymmetric feature.

Fix

1. Take frames 0..N.
2. Append frames N-1..1 in reverse order with no image flip (ffmpeg: [0]split[a][b];[b]reverse[r];[a][r]concat=n=2:v=1).
3. Re-export keeping original per-frame delays. → expect: mark stays on one side

How to verify

Full-frame contact sheet shows the feature on the same side in every frame.

What did not work

Mirroring the whole clip; mirroring only the second half.

Verified scope

Oct 2026