Difference between lossless and lossy changing fps (timestamp vs encoding)?

[incredibilePino]

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?

[benwaggoner]

Not reencoding preserves the quality perfectly. Any solution that's reencoding can, at best, only be as good as the original, and often won't be.

[shae]

What sort and purpose of FPS changing?

[mastrboy]

If preserving quality is more important than storage space you could encode is using a lossless encoder like x264 with --qp 0 or something like Huffyuv (I'm not really up2date on lossless encoders, so might be better alternatives)

[kolak]

Changing fps is last thing which you want to do.
No idea why would you do it? It may have some sense if you have eg. 60p masters and want to make them 30p due to bandwidth etc needs.

[Balling]

Best way is ffmpeg. Lossless and stuff.

ffmpeg -itsscale 0.01666 -i input.mkv -c copy output.mkv

Or those https://trac.ffmpeg.org/wiki/How%20to%20speed%20up%20/%20slow%20down%20a%20video

[kolak]

Best in what way?

There is no such a thing like best :)

[Balling]

Best in what way?

There is no such a thing like best :)

Lossless is best.

[Sharc]

Lossless is best.
Apart from filesize :)

[Balling]

Apart from filesize :)

Same filesize, because no reencoding happens.