[FFmpeg-user] Recording a video when framerate is unknown before recording starts

[John Dexter]

I am dynamically recording the output of a 3D rendering app to video.
Although we seek a locked framerate of 30fps, on slower PCs it can be
less (it IS pretty constant). As each frame is rendered, it is passed
into ffmpeg as the next video frame.

So this raises an issue recording video - until we start recording we
don't know what the fps will be. I don't know enough about ffmpeg to
know if the fps is just a value in a header, or is baked into how
every frame is stored - so any advice or tips or discussion on how to
handle this are welcome. If we assume my fps is stable but not known
in advance, what can/should I do?

I'm using libav programatically, not using the ffmpeg command-line
tool(s). Should this be on the dev list instead?

Thanks,
John.