GstPushSrc needs to execute part of its _fill routine under the main thread.

[Sebastian Dröge]

On Fr, 2013-12-13 at 12:51 -0500, Stirling Westrup wrote:
> On Wed, Dec 11, 2013 at 12:33 PM, Sebastian Dröge
> <sebastian at centricular.com> wrote:
>         On Mi, 2013-12-11 at 12:13 -0500, Stirling Westrup wrote:
>         
>  
>         > Ideally, I imagine a solution to this problem would be to
>         have a
>         > non-blocking routine inserted into the main gst event loop,
>         and each time
>         > around the loop that routine would check if there was
>         rendering to do, and
>         > if so would perform the render operation.
>         >
>         > Then the _fill routine would need to set things up for the
>         render
>         > operation, somehow trigger the main event loop, and then
>         wait on a 'done'
>         > signal of some sort.
>         
>         
>         You could trigger things to be called from the main thread
>         from the
>         GStreamer threads, and in the GStreamer thread then wait until
>         the main
>         thread has provided the results (make sure that the waiting
>         can be
>         interrupted by GstBaseSrc::unlock() vfunc implementation).
>         
>         In Qt IIRC you would do that with signals/slots, in GLib you
>         would use
>         things like g_idle_add() (and that would also work on Qt if it
>         is built
>         with GLib mainloop support).
> 
> 
> Thanks! Your advice pushed me in the right direction. I built a custom
> event queue out of a pipe, and turned it into a GSource to register
> with the main gstreamer loop. Now in my _fill routine I send a
> 'Render' event down the pipe and wait on a 'completion' signal. When
> the GSource wakes up, it calls a callback in my element that decodes
> the Render event, performs the render operation, and signals
> completion, all of the above protected with the appropriate semphores
> and locks.
> 
> 
> Was a bit convoluted to write, but it works like a charm!

Great to hear :) You could also take a look at the WebKit GStreamer
video sink, it does something very similar too.

Note that using a GSource only works as long as Qt is compiled with GLib
mainloop support. On Windows and OSX this is not the default IIRC.

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