As far as I know the file header of an MP4 video contains some information about the auto-rotation of a video file when played in video player (e.g.VLC).
So I guess users can change this header field (e.g. to 90deg clockwise) without re-encoding the full video.
How does this work in Avidemux?
Peter
This is not possible in Avidemux, but you can set the field using ffmpeg with the -metadata (https://www.ffmpeg.org/ffmpeg-all.html#Main-options) option (https://stackoverflow.com/questions/15335073/can-i-set-rotation-field-for-a-video-stream-with-ffmpeg (https://stackoverflow.com/questions/15335073/can-i-set-rotation-field-for-a-video-stream-with-ffmpeg)). First edit the video in Avidemux, then set the field with ffmpeg, otherwise it will be discarded.