Hello, I'd like to know the difference between:
1) change the fps of a video without re-encoding it, so changing timestamps, for example using mkvtoolnix (mkvmerge --default-duration)
2) change the fps of a video re-encoding it
What is the best choice in term of preserving quality? There will be problems when playing the file modified losslessly?
if I understand correctly, when I encode a video in order to change fps, I "produce" frames and I assign to each frame a timestamp, maybe frames are lost or duplicated. When I use a tool like mkvtoolnix to change fps, I change all timestamps without touching the frames, so quality is preserved, there aren't lost frames and there shouldn't be problems when playing the video: in fact the procedure should be exactly equivalent. Is it correct?
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No, but better you try it itself.
Re-encoding with lossy codecs always loses quality. Changing timestamps doesn't change the quality (though there may be a perceived "quality" difference depending on the frame rate of the video vs. the frame rate of the TV/monitor -- i.e. jerkiness with a frame rate mismatch).
Depending on the original source timebase, and whether this is a "correction" back to that timebase or not, it would also affect the apparent motion consistency of the action, either for better or worse. Depending on the action, it may not be noticeable.