[FFmpeg-user] Extract image every x frames?

[Peter B.]

Quoting Nicolas George <nicolas.george at normalesup.org>:

> Le primidi 21 thermidor, an CCXX, Peter B. a écrit :
>> However, it's incredibly slow!
>> When I extract the keyframes using the looping method, it's done in a
>> second, whereas using the "-vf select" filter it takes almost 15
>> minutes (!) to process a 30 minute ffv1 PAL/SD file.
>>
>> The image file for the first frame (frame #0) is written after a few
>> seconds, whereas I'd assume that it should be instant, no?
>>
>> Am I doing something wrong?
>
> You are doing nothing wrong: filters work with decoded frames, so even if
> you want to extract only one frame out of 1500, all must be decoded, even
> those that do not change the output at all. Doing things more efficiently is
> possible, but would be more complex, and nothing is implemented in ready to
> use solutions.

Ah, ok!
Makes perfect sense now. Thank you very much for the explanation.

Thanks,
Pb