mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-24 10:41:04 +00:00
88 lines
3.5 KiB
Text
88 lines
3.5 KiB
Text
Clocks
|
|
------
|
|
|
|
The GstClock returns a monotonically increasing time with the method
|
|
_get_time(). Its accuracy and base time depends on the specific clock
|
|
implementation but time is always expressed in nanoseconds. Since the
|
|
baseline of the clock is undefined, the clock time returned is not
|
|
meaningful in itself, what matters are the deltas between two clock
|
|
times.
|
|
The time reported by the clock is called the absolute_time.
|
|
|
|
|
|
Clock Selection
|
|
~~~~~~~~~~~~~~~
|
|
|
|
To synchronize the different elements, the GstPipeline is responsible for
|
|
selecting and distributing a global GstClock for all the elements in it.
|
|
|
|
This selection happens whenever the pipeline goes to PLAYING. Whenever an
|
|
element is added/removed from the pipeline, this selection will be redone in the
|
|
next state change to PLAYING. Adding an element that can provide a clock will
|
|
post a GST_MESSAGE_CLOCK_PROVIDE message on the bus to inform parent bins of the
|
|
fact that a clock recalculation is needed.
|
|
|
|
When a clock is selected, a NEW_CLOCK message is posted on the bus signaling the
|
|
clock to the application.
|
|
|
|
When the element that provided the clock is removed from the pipeline, a
|
|
CLOCK_LOST message is posted. The application must then set the pipeline to
|
|
PAUSED and PLAYING again in order to let the pipeline select a new clock
|
|
and distribute a new base time.
|
|
|
|
The clock selection is performed as part of the state change from PAUSED to
|
|
PLAYING and is described in part-states.txt.
|
|
|
|
|
|
Clock features
|
|
~~~~~~~~~~~~~~
|
|
|
|
The clock supports periodic and single shot clock notifications both
|
|
synchronous and asynchronous.
|
|
|
|
One first needs to create a GstClockID for the periodic or single shot
|
|
notification using _clock_new_single_shot_id() or _clock_new_periodic_id().
|
|
|
|
To perform a blocking wait for the specific time of the GstClockID use the
|
|
gst_clock_id_wait(). To receive a callback when the specific time is reached
|
|
in the clock use gst_clock_id_wait_async(). Both these calls can be interrupted
|
|
with the gst_clock_id_unschedule() call. If the blocking wait is unscheduled
|
|
a value of GST_CLOCK_UNSCHEDULED is returned.
|
|
|
|
The async callbacks can happen from any thread, either provided by the
|
|
core or from a streaming thread. The application should be prepared for this.
|
|
|
|
A GstClockID that has been unscheduled cannot be used again for any wait
|
|
operation.
|
|
|
|
It is possible to perform a blocking wait on the same ID from multiple
|
|
threads. However, registering the same ID for multiple async notifications is
|
|
not possible, the callback will only be called once.
|
|
|
|
None of the wait operations unref the GstClockID, the owner is
|
|
responsible for unreffing the ids itself. This holds true for both periodic and
|
|
single shot notifications. The reason being that the owner of the ClockID
|
|
has to keep a handle to the ID to unblock the wait on FLUSHING events
|
|
or state changes and if we unref it automatically, the handle might be
|
|
invalid.
|
|
|
|
These clock operations do not operate on the stream time, so the callbacks
|
|
will also occur when not in PLAYING state as if the clock just keeps on
|
|
running. Some clocks however do not progress when the element that provided
|
|
the clock is not PLAYING.
|
|
|
|
|
|
Clock implementations
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The GStreamer core provides a GstSystemClock based on the system time.
|
|
Asynchronous callbacks are scheduled from an internal thread.
|
|
|
|
Clock implementors are encouraged to subclass this systemclock as it
|
|
implements the async notification.
|
|
|
|
Subclasses can however override all of the important methods for sync and
|
|
async notifications to implement their own callback methods or blocking
|
|
wait operations.
|
|
|
|
|