2 KiB
Preroll
A sink element can only complete the state change to PAUSED after a
buffer has been queued on the input pad or pads. This process is called
prerolling and is needed to fill the pipeline with buffers so that the
transition to PLAYING goes as fast as possible with no visual delay for
the user.
Preroll is also crucial in maintaining correct audio and video synchronisation and ensuring that no buffers are dropped in the sinks.
After receiving a buffer (or EOS) on a pad the chain/event function should wait to render the buffers or in the EOS case, wait to post the EOS message. While waiting, the sink will wait for the preroll cond to be signalled.
Several things can happen that require the preroll cond to be signalled. This include state changes or flush events. The prerolling is implemented in sinks (see element-sink
Committing the state
When going to PAUSED and PLAYING a buffer should be queued in the pad.
We also make this requirement for going to PLAYING since a flush event
in the PAUSED state could unqueue the buffer again.
The state is commited in the following conditions:
- a buffer is received on a sinkpad
- an GAP event is received on a sinkpad.
- an EOS event is received on a sinkpad.
We require the state change to be commited in EOS as well since an EOS means by definition that no buffer is going to arrive anymore.
After the state is commited, a blocking wait should be performed for the next event. Some sinks might render the preroll buffer before starting this blocking wait.
Unlocking the preroll
The following conditions unlock the preroll:
- a state change
- a flush event
When the preroll is unlocked by a flush event, a return value of
GST_FLOW_FLUSHING is to be returned to the peer pad.
When preroll is unlocked by a state change to PLAYING, playback and
rendering of the buffers shall start.
When preroll is unlocked by a state change to READY, the buffer is to be
discarded and a GST_FLOW_FLUSHING shall be returned to the peer
element.