Using input-selector

[Sebastian Dröge]

On Di, 2014-01-21 at 16:00 +0100, Jan Alexander Steffens wrote:
> On Sun, Jan 19, 2014 at 10:56 AM, Sebastian Dröge
> <sebastian at centricular.com> wrote:
> > On Do, 2014-01-16 at 09:55 +0100, Jan Alexander Steffens wrote:
> >> Hello List,
> >>
> >> I'm trying to use input-selector to switch between two streams.
> >> Streams that are unconnected should be paused (in case of a file) or
> >> continue running as normal (except without output; in case of a
> >> livestream). How do I do this? I think my previous attempts fail
> >> because the sink gets confused when the timestamps suddenly change.
> >>
> >> For the big picture; I'm trying to get the bits and pieces together to
> >> build a live video composer.
> >
> > That's actually not so trivial to do unfortunately. The default case is
> > that input-selector just drops data from every unselected stream, so
> > that should be no problem.
> >
> > However pausing the file-based streams is, as the pausing is usually
> > only implemented in the sinks and for other elements there's usually no
> > difference between PLAYING and PAUSED state. Also you will need to
> > adjust the timestamp after the pausing to be at the exact current
> > running time, not the running time when you paused. For the latter you
> > should be able to use the pad offsets (gst_pad_set_offset()), for the
> > pausing you could just block the pads of the file-based streams
> > (gst_pad_add_probe() with a blocking probe type).
> 
> I've restricted myself to live sources at the moment and I've managed
> to produce a prototype that switches back and forth between two video
> streams coming from RTMP sources.
> 
> Now I want to do the same with audio, too. My naive approach simply used
> two input-selectors, however the switch is not simultaneous—even though
> I'm switching the audio selector first, the audio switch is late by
> half a second
> or so, probably because the flow through the selectors is not synchronized.
> 
> Will I need to implement my own multi-input-selector (-bin?) to synchronize the
> switch properly? I imagine this involves blocking flow through the early
> streams until the late ones have caught up.

Yes, something like that will be necessary. Also note that after the
input-selectors there will be some buffering (as in queue elements or
the ringbuffer in the audio sinks for example), so it might not be too
bad to do this.

> I've also tried putting a multiqueue in front of the selectors -
> however I'm pretty
> sure I'm not using it right as I'm back to my old problem of the output hanging
> after the switch.

Not sure what exactly you did, but there are many ways to use multiqueue
the wrong way and cause deadlocks like this :) If you explain what
exactly you did maybe I can help, but multiqueue is not the solution for
your problem here.

-- 
Sebastian Dröge, Centricular Ltd - http://www.centricular.com
Expertise, Straight from the Source
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