[Bug 651806] New: We need to add layer mixing

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=651806
  GStreamer | gst-editing-services | unspecified

           Summary: We need to add layer mixing
    Classification: Platform
           Product: GStreamer
           Version: unspecified
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: Normal
         Component: gst-editing-services
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: tsaunier at gnome.org
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


Until now, there is no way to mix audios/videos from various layers. We need to
get something with a similar behavior as what now have in pitivi.

The solution in pitivi is:
 * Have mixer elements smartVideomixerBin and SmartAdderBin (in
pitivi/elements/mixer.py)

 * All tracks add this mixer in a gnloperation with a priority of 0 and which
is expandable so it is in the whole duration of the track so any trackobject
added to track is plugged on the mixer in pitivi/timeline/track.py

This result in:
    + layers (audio/video from different layer) are mixed when the alpha
channel of objects on higher layers are < 100

We also currently have an optimization that take advantage of the alpha
passthrough capabilities of the videomixer, we should try to keep it.

If we want to reproduce something similar, we re-implement those custom
GstElement-s and modify ges/ges-track.c to handle it.

In GES, we should use the new video mixer: videomixer2

Also I guess this is something we might want to handle directly in
GnlComposition, passing a videomixer or adder to it?

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