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79 lines
2.9 KiB
Markdown
79 lines
2.9 KiB
Markdown
# GstPipeline
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A GstPipeline is usually a toplevel bin and provides all of its children
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with a clock.
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A GstPipeline also provides a toplevel GstBus (see [gstbus](design/gstbus.md))
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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 [synchronisation](design/synchronisation.md)).
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The pipeline will calculate a global latency for the elements in the
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pipeline. (See also [latency](design/latency.md)).
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## State changes
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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
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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 state
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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
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units, that the pipeline spent in the PLAYING state (see
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[synchronisation](design/synchronisation.md)). The running_time is set to 0 after a
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flushing seek.
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## Clock selection
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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
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pipeline 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.
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This selection is performed by iterating the element starting from
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the 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
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sure 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
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yet be prerolled and the selection process will select the clock
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of 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
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pipeline regardless of what clock providers are children of the
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pipeline. Setting NULL disables the clock completely and makes the
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pipeline run as fast as possible.
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The `_auto_clock()` method removes the fixed clock and reactivates the
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auto- matic clock selection algorithm described above.
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## GstBus
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A GstPipeline provides a GstBus to the application. The bus can be
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retrieved with `gst_pipeline_get_bus()` and can then be used to
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retrieve messages posted by the elements in the pipeline (see
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[gstbus](design/gstbus.md)).
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