How to mix live and non-live sources

[Sebastian Dröge]

On Fri, 2021-09-24 at 12:59 +0200, Marianna Smidth Buschle via
gstreamer-devel wrote:
> I have a use case where I have N live sources (v4l2src) from which I
> can 
> select to be the output.
> 
> I also need to be able to make a replay (seek from a filesrc).
> 
> My question is how to properly handle the mixing of the live and 
> non-live sources since the timestamps don't match.
> 
> The filesrc starts from zero (or the seeking point) and the live
> sources 
> have a running-time based on the clock.

This is actually simpler than what you wrote. What you would do is to
call gst_pad_set_offset() on the source pad of the non-live streams
that you want to connect to the mixer.
That would then make sure to update the running time accordingly.

The offset would in the simplest case be the current running time when
connecting the non-live stream.
You can get that from the current pipeline clock's time minus the
current pipeline's base time.

Alternatively you might want to add some more time to that to schedule
the non-live stream into the future, or do more complicated
calculations to schedule the start of the stream to a specific time.


Note that after a (flushing) seek the running time starts again at 0,
so you might need to re-offset here after a seek.


-- 
Sebastian Dröge, Centricular Ltd · https://www.centricular.com

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