gstreamer/markdown/design/gstpipeline.md

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 gstbus)

The pipeline also calculates the running_time based on the selected clock (see also clocks.txt and synchronisation).

The pipeline will calculate a global latency for the elements in the pipeline. (See also latency).

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 synchronisation). 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 one. 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 the most upstream element that can provide one. 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 gstbus).