[Bug 701430] For audio / video mixing in gst-editing-services, we need to use metadata.

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=701430
  GStreamer | gst-plugins-base | git

--- Comment #2 from Thibault Saunier <thibault.saunier at collabora.com> 2013-06-04 14:50:28 UTC ---
I think the description of the issue is not 100% clear here. I will try to
clarify it:

We need to expose the x-y-alpha properties of some gnlsource so that the
information will be used by the videomixer that is inside the pipeline. But
those gnlobject are not linked to the mixer at the time the user want to set
those properties (as the linking is dynamical), and thus there is no pad to set
the properties on.

We could have dummy properties but then making the properties controllable will
make the code really ugly (we have gone that way first and it is just not a
solution).

Having an element in the source itself to which to set those properties on and
that attaches metas on buffer make the code way simpler/cleaner in our case,
and I believe this is not a design issue on our part (we have beeen thinking a
lot about the various solutions and this on seem to be the cleanest/simplest
one to me). 

We ended up implementing exactly that in GES[0] itself and the result in terme
of code is quite nice to me. I believe the same issue will have to be solve in
other projects at some point, but anyway.

[0] https://github.com/MathieuDuponchelle/PitiviGes/commits/video_mixing

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