RFC - possible new vmethod for GstBaseSink

[Tom Bailey]

Hi all,

I would like to see if there is any support in the developer community for a new vmethod on GstBaseSink to allow derived sinks to indicate their pre-roll status to the base sink.

I've made this change as a patch to GstBaseSink and it seems to work well for our application. Our application runs on a set-top box and uses a custom sink element which wraps some proprietary APIs which give access to a hardware decoder which accepts MPEG TS or ES. Our custom sink is derived from GstBaseSink and runs unsynchronized (sync = FALSE).

We were sometimes seeing unacceptably long delays when the pipeline transitioned from PLAYING to PAUSED, which I tracked down to the fact that when the sink was asked to transition to PAUSED it would sometimes report that it needed to wait for preroll, even though video was currently playing. This meant that the state transition was delayed until a buffer arrived, which could be several seconds in some cases.

I tried to work out the logic in GstBaseSink by which it decides whether the element needs to preroll or not, and there doesn't seem to be any way for a derived sink to indicate that following a call to the render method it still has frames of video queued for display, and therefore doesn't need to pre-roll. So I tried adding a new vmethod, is_prerolled to GstBaseSink, and changed the logic in gst_base_sink_needs_preroll to look like this:

static gboolean
gst_base_sink_needs_preroll (GstBaseSink * basesink)
{
  gboolean is_prerolled, res, derived_needs_preroll = FALSE;
  GstBaseSinkClass *bclass;

  bclass = GST_BASE_SINK_GET_CLASS (basesink);

  /* we have 2 cases where the PREROLL_LOCK is released:
   *  1) we are blocking in the PREROLL_LOCK and thus are prerolled.
   *  2) we are syncing on the clock
   */
  is_prerolled = basesink->have_preroll || basesink->priv->received_eos ||
      basesink->priv->received_segment_done;

  if (!is_prerolled && bclass->is_prerolled) {
    derived_needs_preroll = !bclass->is_prerolled (basesink);
    res = derived_needs_preroll;
  } else {
    res = !is_prerolled;
  }

  GST_DEBUG_OBJECT (basesink, "have_preroll: %d, EOS: %d SEGMENT_DONE: %d "
      "derived_needs_preroll: %d => needs preroll: %d", basesink->have_preroll,
      basesink->priv->received_eos, basesink->priv->received_segment_done,
      derived_needs_preroll, res);

  return res;
}

The implementation of the is_prerolled vmethod in our derived sink now queries the hardware decoder, and if there are decoded frames of video queued for display it returns true. This has massively improved the performance of the application when transitioning from playing to paused.

Does this seem like a sensible enhancement to GstBaseSink for derived sinks which run with sync=FALSE? If so I can submit a patch.

If people don't think the change makes sense I'd be interested to hear what other approaches are available for sink elements which perform their own synchronization and which have to manage a queue of decoded frames.

Best regards

Tom


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