[Bug 697327] New: RFC: syncing messages with rendering

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=697327
  GStreamer | gstreamer (core) | git

           Summary: RFC: syncing messages with rendering
    Classification: Platform
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: Normal
         Component: gstreamer (core)
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: ensonic at sonicpulse.de
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


Right now we don't offer a good solution with the existing level and spectrum
elements. They post a message with the analysis data containing a timestamp
with the presentation-time. Scheduling the on-screen update of a widget or a
graph is left to the developer. And is actually complicated and inefficient to
do.

This is what I do right now in buzztard:
// listen for message::element on the bus
// in the signal handler:
  // get pts from the message
  pts += gst_element_get_base_time (level);
  GstClockId clock_id = gst_clock_new_single_shot_id (clock, pts);
  gst_clock_id_wait_async (clock_id, on_delayed_analyzer_change, message,
NULL);

// on_delayed_analyzer_change
  g_idle_add_full (G_PRIORITY_HIGH, on_delayed_idle_analyzer_change, data,
NULL);

// on_delayed_idle_analyzer_change
  gtk_level_meter_set (....)
  gst_message_unref (message);


Here the control-flow goes from streaming thread (level) via bus to the
application's main thread. Then a new handler is added to the clock (for each
message). This gets fired from the streaming thread of the sink. Thus I marshal
things back to the main thread via g_idle_add. (I could probably skip the first
context marshaling by using a sync message.)

Using a sink-message in the element is not good enough, as this will only sync
with dataflow, but not rendering. Now a few proposals.

1) Add a boolean "post-sink-message" property to level/spectrum. Add the PTS to
the sink-message. In basesink check if the message has a PTS, if not post
immediately from the sink (as now). If it has a PTS, dispatch to render().

2) Add a boolean "sync-message" property to level/spectrum. If set the element
will do the gst_clock_new_single_shot_id() + gst_clock_id_wait_async() before
posting the message.

Any opinions, other ideas?

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