Anton Khirnov (12023-04-19):
> Previously they would only be used with trivial filtergraphs, because
> filters did not handle frame durations. That is no longer true - most
> filters process frame durations properly (there may still be some that
> don't - this change will help finding and fixing them).
> 
> Improves output video frame durations in a number of FATE tests.

I do not object to this patch nor the one to concat, but I want to state
once again:

The authoritative source of information about frame durations in
libavfilter, be it internally for the caller, is the timestamp of the
next frame, or the timestamp of EOF for the last frame.

Filtered frame durations are only a hint.

-- 
  Nicolas George