[FFmpeg-user] No duration with pipe:1 as output

[Robert Nagy]

>Not if there are filters that alter the timestamps, nor if the source file
> duration is not accurate enough or completely unknown.

True, but if that is not the case then it would? In theory it should
be possible to make a conservative guess.

Would be nice to be able to at least specify it manually if known in
advance:

e.g.

-metadata duration=00:00:25:00

or something...