mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-10-01 16:22:25 +00:00
80 lines
2.9 KiB
Markdown
80 lines
2.9 KiB
Markdown
|
# 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](design/gstbus.md))
|
||
|
|
|
||
|
|
The pipeline also calculates the running\_time based on the selected
|
||
|
|
clock (see also clocks.txt and [synchronisation](design/synchronisation.md)).
|
||
|
|
|
||
|
|
The pipeline will calculate a global latency for the elements in the
|
||
|
|
pipeline. (See also [latency](design/latency.md)).
|
||
|
|
|
||
|
|
## 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](design/synchronisation.md)). 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
|
||
|
|
[gstbus](design/gstbus.md)).
|