Get instant bitrate from H.264 encoder ?

[Jean-Michel Hautbois]

2012/1/6 Michael Smith <msmith at xiph.org>:
> You can use a buffer probe for this. Look at the size (in bytes) of
> each buffer coming out of the encoder. You know the duration of the
> frame (from the framerate, even if the buffer itself doesn't have a
> duration set), so you can trivially calculate what you call the
> "instant bitrate" from this.

Thanks for this idea :).
But it won't be frame accurate, because I can't AFAIK know the frame
related to an output buffer.
How can I know the accuracy of this measure ?

JM