Fix the 3D flag in an mp4
Your 3D video is fine — the player just doesn't know it. When a file is missing the stereoscopic flag, players show it as a flat widescreen frame with two copies squeezed side by side. This tool writes the flag into the video track, entirely in your browser. Nothing is uploaded.
What it writes
- st3d — the stereoscopic-mode box: "side by side" or "top & bottom", matching your choice. Players that auto-detect 3D (Skybox, DeoVR, 4XVR, VLC, Infuse…) read this.
- pasp 1:1 — square pixels, so players stop stretching a 3840×1080 file to 32:9.
- If a flag already exists and "overwrite" is ticked, it is corrected in place; otherwise it is added. Audio, video and every other byte stay untouched.
Limits, stated plainly
- Works on plain mp4/mov/m4v with H.264 or HEVC video. Fragmented streaming-style mp4 and MKV are not supported — re-mux first (for example
ffmpeg -i in.mkv -c copy out.mp4). - The file must fit in memory; up to a couple of GB is comfortable on a modern machine.
- It cannot make a 2D movie stereoscopic — for that, convert it with DepthSnap.
Related: Play SBS/TAB files in the browser · Why SBS looks stretched · Where to get 3D movies · All tools
DepthSnap — free browser-based 2D to 3D converter. No upload, no signup, no watermark.