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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:
- set the bus to non-flushing
- reset the running_time to 0
- 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.
- calculate the running_time when the pipeline was PAUSED.
- 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
The _auto_clock() method removes the fixed clock and reactivates the auto-
matic clock selection algorithm described above.
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).