Eye patch / asymmetric mark switches sides in AI image-to-video character clips

Problem

Generating a character with an asymmetric feature (eye patch on one eye) from a keyframe, the patch drifts to the other eye in some frames, especially when the character turns its head or body.

Applies when

Image-to-video models (Seedance 1.5 pro via OpenRouter), keyframes from an image model.

Confirm it is this problem

Contact sheet shows the mark on different sides across frames.

Not this when / boundary

The swap appears only after building a mirrored ping-pong GIF loop - that is a post-processing bug, not the model.

Root cause

The model keeps no persistent left/right for the character; when it turns or redraws the face it often mirrors the design. Prompts saying only 'eye patch' leave the side ambiguous.

Fix

1. Keep keyframes and motion front-facing or a fixed profile; no head/body turns.
2. Add to every keyframe and video prompt: "eye patch on his LEFT eye (viewer's right) at all times".
3. Use the same keyframe as first AND last frame when the API supports it (loop mode) to anchor the design.
4. Sample ~12 frames (ffmpeg -i vid.mp4 -vf fps=3,scale=160:-1,tile=12x1 -frames:v 1 sheet.png) and reject clips where the mark moves. → expect: mark on the same side in all tiles

How to verify

Every frame in the contact sheet has the mark on the same side.

What did not work

Re-rolling the same prompt; describing the patch without naming the side; allowing a turn-around action.

Verified scope

Oct 2026