[Bug 731373] New: [pitivi] Temporary live playback restriction caps

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=731373
  GStreamer | gst-editing-services | git

           Summary: [pitivi] Temporary live playback restriction caps
    Classification: Platform
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: Normal
         Component: gst-editing-services
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: nekohayo at gmail.com
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


In Pitivi, we encounter the usecase where we would need to downscale videos
during live playback, so that they are only processed at the resolution of UI
widget displaying the sink/previewer.

This approach has been demonstrated to significantly improve performance when
applying effects on top of HD footage, with this proof of concept branch
branch:
https://github.com/nekohayo/pitivi/commits/low-quality-preview

GES provides a set_restriction_caps method for tracks, allowing to set a width
and height to clamp to. However, this API is insufficient because it was made
to set permanent restriction caps (ex: rendering resolution scaling) and they
get saved into the file format. Notwithstanding the fact that it interferes
with the real rendering restriction caps, trying to manage this temporary stuff
in Pitivi means piles upon piles of nasty hacks that conflict with each other.

- Trying to work around the current API means tracking the state globally,
setting restriction caps only in certain situations and then trying to
surgically remove them right before rendering and right before any project file
saving. This doesn't work.

- Changes to restriction caps will make the pipeline go to pause. Since Pitivi
autosaves a backup copy of the GES Project periodically, and GES serializes the
restriction caps, Pitivi will uncontrollably stop playback whenever an
automatic backup file save occurs.


Hence I have come to the conclusion that trying to work around this in Pitivi
is impossible and that it is the responsibility of GES to provide a way to
(un)set temporary/live/playback restriction caps globally. You need to be able
to (un)inhibit those playback restriction caps (easily toggle them on/off) at
any time, and since they're UI-dependent (they don't make sense outside of this
context), they should never be saved into the project file.

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