[Libav-user] Encoding with variable frame rate

Kalileo kalileo at universalx.net
Tue May 21 16:26:53 CEST 2013

On May 21, 2013, at 20:35 , Brad O'Hearne wrote:

> On May 21, 2013, at 12:12 AM, Robert Krüger <krueger at lesspain.de> wrote:
>> are you talking about using the ffmpeg as a command line application?
> No, using libraries programmatically in an app.
>> you elaborate what exactly you mean by "but FFmpeg must have fixed
>> frame rate"?
> In my testing, you have to feed the encoder the exact number of FPS set in time_base.den. You cannot give the encoder some lesser or greater FPS and think that accommodating this by setting pts will fix the timing.

I disagree (again). If your player synchronizes audio and video using dts/pts, and as far as I know most do, you can indeed create the weirdest frame rate changes by manipulating dts/pts. 

> Playback is going to process time_base.den frames per second, so if you give it fewer FPS than time_base.den, then your video is going to play back faster than intended, and if you give it greater FPS than time_base.den, then your video is going to play back slower than intended. 

Which "Playback" , which player ? Every player can have it's own playback logic, nevertheless I don't see where the player logic would require fps to be fixed (unless it ignores dts/pts).

time_base might play a role in the formulas used by the player, so it makes sense to match the dts/pts manipulations to the timebase used. 
If players base the playback on fps, they will run out of sync sooner or later. That's why we have dts/pts.

> The upshot is that you are pinned to a fixed frame rate while encoding. If your source data provides are a variable frame rate, then you have some work to do to feed a fixed frame rate to the encoder.

Again i disagree. Just set DTS/PTS accordingly, to compensate for the different fps rate. Been there, done that. Works.

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