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