Rotate Video

A flag in the file, or pixels actually moved

MP4 and MOV files carry a small transformation matrix describing how the picture should be turned before it is shown. Your phone writes it every time you film: the sensor always records the same way round, and the note in the file says which way you were holding it. Setting that matrix rewrites a handful of bytes — nothing is decoded, nothing is re-encoded, not one pixel changes, and a two-hour file rotates as fast as a two-second one.

Every current player, phone, television and editor honours the flag. A few older programs do not, and that is exactly the situation people arrive here with: a clip that looks upright everywhere except in the one place it needs to be. Baking the turn in solves it permanently by decoding the video and encoding it genuinely rotated, so there is no note left to ignore. It costs time and one generation of quality, and it is the right answer when something downstream has already proved it cannot read the flag.

Mirroring always re-encodes, because there is no flag for it. A horizontal flip is not a rotation and the transformation matrix has no way to express one that players honour reliably, so the pixels have to move. It is done at high quality. Worth remembering that most phone selfie cameras have already mirrored the picture on your behalf, which is why text in a selfie video reads backwards and why flipping it is usually the correction rather than the effect.

AVI, WMV and MPEG have no rotation flag at all — the formats predate the idea — so a turn there is always baked in, and the file comes back as an MP4. Quarter turns only: an arbitrary angle means resampling every frame and cropping the corners, which is an editor's job rather than this one.

When you want something else

ffmpeg sets the same flag without touching the video: ffmpeg -i in.mp4 -c copy -metadata:s:v:0 rotate=90 out.mp4, and -vf transpose=1 bakes the turn in when something downstream insists. For a folder of sideways clips from the same camera, a loop over either is the whole job.

When a clip needs straightening by a few degrees rather than turning by ninety — a handheld shot with a tilted horizon — nothing here will help. Shotcut and Kdenlive have rotation and stabilisation filters, are free, and will show you the crop the rotation forces before you commit to it.