[FFmpeg-devel] [PATCH] lavfi: add asendcmd and sendcmd filters

Nicolas George nicolas.george at normalesup.org
Tue Sep 11 21:49:05 CEST 2012

Le sextidi 26 fructidor, an CCXX, Clément Bœsch a écrit :
> OK, assuming we are sure to have different PTS set for every single frame
> (so monotically increasing, no NOPTS, ...).

A lot of filters rely on that assumption. The whole logic of sendcmd also
assumes that timestamps are strictly increasing.

>					      A counter is way easier to
> deal with, and is sure to not differ between different apps (contrary to
> timestamps where you could have rounding and stuff).

The counter could also differ: does it start at 0 or 1? Does it increment
when there are skipped frames?

> The SMPTE timecode is just a frame counter, expressed as a timestamp +
> frame-id-in-the-current-second. It gives a hint about the time, but you
> are also sure to not have any duplicate. It applies to only a few
> restrictive frame rate thought. That's what is being used in editing to
> identify frames.

This is rather ugly.

> Maybe. I think users are not confident with it, maybe for specific issues
> such as -ss not being very exact for example. "Why will the sendcmd filter
> have exact timing but -ss won't?".

Then the users need to be educated.

> While it may not be justified to use it all the time, what's the problem
> about allowing another unit, which is more spatial than temporal?

Stefano gave an two arguments: he has trouble implementing it and it does
not work with seeking.

I have another: this is comforting users who mistakenly think that counting
frames is more accurate. Just look at the recent ticket about overlay with
image2 (25 FPS by default) on top of a 24 FPS video for an example of the
kind of confusion this can lead to.


  Nicolas George
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