Could you clarify what you mean with "use output seeking for -to"?
From your Python script it seems that you're just using input seeking and then specifying the duration in seconds with `-t`, which is actually the same as using `-to` when doing input seeking.
Also, input seeking should be inaccurate when doing stream copy, so I'm not sure your script actually works as expected?
(And unless I'm missing something, it seems that all of this is well-explained in the ffmpeg guide linked above.)
Thanks!
In my Python script, I did input seeking for -ss (start point) part, and then output seeking for -t part (end point). As you can see, the -ss part is before -i {inputfile}, and -t is after.
-ss 1:00 -i file -t 5
is NOT the same as
-ss 1:00 -t 5 -i file.
The latter has a bug that happens frequently when I'm trimming MPEG-TS files recorded from HDTV. It literally doesn't stop at the -t/-to timestamp for reason I don't know. And it only happens when stream copy.
Below is a quick showcase: t.ts is the source, and the filenames show how I generate them with FFMPEG (for example, ss_t_i means input seeking -ss first, then -i t.ts, then -t 1:00).
https://i.imgur.com/lLUSzEM.png
As you can see, if I use -t/-to before -i, it doesn't cut the file properly.
>Also, input seeking should be inaccurate when doing stream copy
Yeah, it's not frame accurate, can only cut at keyframes, but enough for my application. By the way the same inaccuracy exists for output seeking if you're doing stream copy.
There are however, plenty of great software that can but at any frame and only re-encode the frames that are outside the whole GOP. Most of them are commercial though, I haven't find one that is free and good.
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Also, seeking in FFMPEG in practice, is actually more complicated than the guide [1] you linked. Below is a note I keep for own reference for keyframe-copy. Hope someone will find it useful.
How to keyframe-cut video properly with FFMPEG
FFMPEG supports "input seeking" and "output seeking". The output seeking is very slow (it needs to decode the whole video until the timestamp of your -ss) so you want to avoid it if unnecessary.
However, while -ss (seek start) works fine with input seeking, "-to/-t" (seek ending) is somehow vastly inaccurate in input seeking for FFMPEG. It could be off by a few seconds, or sometimes straight up does not work (for some mepeg-ts files recorded from TV).
The best of the two worlds is to use input seeking for -ss and then output seeking for -to. However, this way, the timestamp will restart from 0 in output seeking. So instead of using -to, you should calculate -t (duration) yourself by subtracting -ss from -to, and use `-t duration` instead. Below is a quick Python script to do so.
https://gist.github.com/fireattack/9a100c5a200154937babd1823...
(You can also try to use -copyts to keep timestamp, but not recommended because it doesn't work if the video file has non-zero start time.)