mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-30 13:41:48 +00:00
88 lines
2.9 KiB
Text
88 lines
2.9 KiB
Text
GstPipeline
|
|
-----------
|
|
|
|
A GstPipeline is usually a toplevel bin and provides all of its
|
|
children with a clock.
|
|
|
|
A GstPipeline also provides a toplevel GstBus (see part-gstbus.txt)
|
|
|
|
The pipeline also calculates the running_time based on the selected
|
|
clock (see also clocks.txt and part-synchronisation.txt).
|
|
|
|
The pipeline will calculate a global latency for the elements in the pipeline.
|
|
(See also part-latency.txt).
|
|
|
|
State changes
|
|
~~~~~~~~~~~~~
|
|
|
|
In addition to the normal state change procedure of its parent class
|
|
GstBin, the pipeline performs the following actions during a state change:
|
|
|
|
- NULL -> READY:
|
|
- set the bus to non-flushing
|
|
|
|
- READY -> PAUSED:
|
|
- reset the running_time to 0
|
|
|
|
- PAUSED -> PLAYING:
|
|
- Select and a clock.
|
|
- calculate base_time using the running_time.
|
|
- calculate and distribute latency.
|
|
- set clock and base_time on all elements before performing the
|
|
state change.
|
|
|
|
- PLAYING -> PAUSED:
|
|
- calculate the running_time when the pipeline was PAUSED.
|
|
|
|
- READY -> NULL:
|
|
- set the bus to flushing (when auto-flushing is enabled)
|
|
|
|
The running_time represents the total elapsed time, measured in clock units,
|
|
that the pipeline spent in the PLAYING state (see part-synchronisation.txt).
|
|
The running_time is set to 0 after a flushing seek.
|
|
|
|
|
|
Clock selection
|
|
~~~~~~~~~~~~~~~
|
|
|
|
Since all of the children of a GstPipeline must use the same clock, the
|
|
pipeline must select a clock. This clock selection happens when the pipeline
|
|
goes to the PLAYING state.
|
|
|
|
The default clock selection algorithm works as follows:
|
|
|
|
- If the application selected a clock, use that clock. (see below)
|
|
|
|
- Use the clock of most upstream element that can provide a clock. This
|
|
selection is performed by iterating the element starting from the
|
|
sinks going upstream.
|
|
|
|
* since this selection procedure happens in the PAUSED->PLAYING
|
|
state change, all the sinks are prerolled and we can thus be sure
|
|
that each sink is linked to some upstream element.
|
|
* in the case of a live pipeline (NO_PREROLL), the sink will not yet
|
|
be prerolled and the selection process will select the clock of
|
|
a more upstream element.
|
|
|
|
- use GstSystemClock, this only happens when no element provides a
|
|
usable clock.
|
|
|
|
The application can influence this clock selection with two methods:
|
|
gst_pipeline_use_clock() and gst_pipeline_auto_clock().
|
|
|
|
The _use_clock() method forces the use of a specific clock on the pipeline
|
|
regardless of what clock providers are children of the pipeline. Setting
|
|
NULL disables the clock completely and makes the pipeline run as fast as
|
|
possible.
|
|
|
|
The _auto_clock() method removes the fixed clock and reactivates the auto-
|
|
matic clock selection algorithm described above.
|
|
|
|
|
|
GstBus
|
|
~~~~~~
|
|
|
|
A GstPipeline provides a GstBus to the application. The bus can be retrieved
|
|
with gst_pipeline_get_bus() and can then be used to retrieve messages posted by
|
|
the elements in the pipeline (see part-gstbus.txt).
|
|
|