mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-16 21:36:35 +00:00
1cda8197e9
Original commit message from CVS: Added support for live sources and other elements that cannot do preroll. Updated design docs, added live-source design doc. Implemented live source functionality in basesrc Fix error condition in _bin_get_state() Implement live source handling in -launch. Added check for live sources. Fixed case in GstBin where elements were changed state multiple times.
44 lines
1.6 KiB
Text
44 lines
1.6 KiB
Text
Live sources
|
|
------------
|
|
|
|
A live source such as an element capturing audio or video need to be handled
|
|
in a special way. It does not make sense to start the dataflow in the PAUSED
|
|
state for those devices as the user might wait a long time between going from
|
|
PAUSED to PLAYING, making the previously captured buffers irrelevant.
|
|
|
|
A live source therefore only produces buffers in the PLAYING state. This has
|
|
implications for sinks waiting for a buffer to complete the preroll state
|
|
since such a buffer might never arrive.
|
|
|
|
Live sources return NO_PREROLL when going to the PAUSED state to inform the
|
|
bin/pipeline that this element will not be able to produce data in the
|
|
PAUSED state.
|
|
|
|
When performing a get_state() on a bin with a non-zero timeout value, the
|
|
bin must be sure that there are no live sources in the pipeline because else
|
|
the get_state() function would block on the sinks.
|
|
|
|
A gstbin therefore always performs a zero timeout get_state() on its
|
|
elements to discover the NO_PREROLL (and ERROR) elements before performing
|
|
a blocking wait on all elements.
|
|
|
|
|
|
Scheduling
|
|
----------
|
|
|
|
Live sources can not produce data in the paused state. They block in the
|
|
getrange function or in the loop function until they go to PLAYING.
|
|
|
|
|
|
Latency
|
|
-------
|
|
|
|
The live source timestamps its data with the time of the clock at the
|
|
time the data was captured. Normally it will take some time to capture
|
|
the first sample of data and the last sample. This means that when the
|
|
buffer arrives at the sink, it will already be late and will be dropped.
|
|
|
|
The latency is the time it takes to construct one buffer of data.
|
|
|
|
|
|
|