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I am trying to do something similar to the questions here and here, but with a small difference. I am trying to flip the video along the Y or X axes instead of doing a rotation.
The solution in those threads to modify the rotation matrix seems like the perfect approach, and I am able to modify the parameters to perform the rotation. But I cannot figure out what the values should be for flipping the video.
To quote from those threads:
- no rotation:00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 40- 180°:FF FF 00 00 00 00 00 00 00 00 00 00 00 00 00 00 FF FF 00 00 00 00 00 00 00 00 00 00 00 00 00 00 40- 90° cw:00 00 00 00 00 01 00 00 00 00 00 00 FF FF 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 40- 90° ccw:00 00 00 00 FF FF 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 40
I was unable to find documentation online that talks about what the values in those two lines indicate, hence I was not able to achieve the flip.
I tried alternatives with ffmpeg as well, but I run into issues:
ffmpeg -i input.mp4 -vf hflip output.mp4
... This re-encodes the file because of the "vf" flag that applies filtering.
ffmpeg -i input.mp4 -vf "transpose=0,transpose=1" output.mp4
... This too re-encodes the file because of the "vf" flag.
ffmpeg -i input.mp4 -metadata:s:v rotate="90" -codec copy output.mp4
... This works without re-encoding (and is almost as fast as directly editing the file), but obviously rotation does not achieve a flip. Is there a way I could use the metadata option with transpose?
ffmpeg -i input.mp4 -vf "transpose=0,transpose=1" -codec copy output.mp4
... This gives me an error saying that filtering and streamcopy cannot be used together.
Is it possible to do what I am trying to do? How would I achieve a video flip without encoding? Even if I end up encoding, how can I do it in a lossless manner?
I would have posted on the threads I have referenced above for help, but I am a new user here and cannot post comments on threads as a result.
Sayontan
Posted 2019-01-13T20:14:28.563
Reputation: 1
In terms of the matrix mathematics, for horz. flip, you have to change, in
no rotation, the first00 01toFF FF. For vert. flip, change the 2nd00 01toFF FF. However, it seems most players only check if the matrix values correspond to a pure rotation transform. If not, the image is untouched. – Gyan – 2019-01-14T05:08:32.907@Gyan - So the interpretation of this is really player-specific? In other words the only true way to do this is to re-encode? – Sayontan – 2019-01-14T18:49:20.930
Short answer: yes. Longer answer: it's not really the interpretation that changes but whether the player checks for pure flips. Generally, they don't. – Gyan – 2019-01-15T04:14:55.517