Clock


Object Hierarchy:

Object hierarchy for Clock

Description:

public abstract class Clock : Object

GStreamer uses a global clock to synchronize the plugins in a pipeline.

Different clock implementations are possible by implementing this abstract base class or, more conveniently, by subclassing SystemClock.

The Clock returns a monotonically increasing time with the method get_time. Its accuracy and base time depend 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 returned by a clock is called the absolute time.

The pipeline uses the clock to calculate the running time. Usually all renderers synchronize to the global clock using the buffer timestamps, the newsegment events and the element's base time, see Pipeline.

A clock implementation can support periodic and single shot clock notifications both synchronous and asynchronous.

One first needs to create a ClockID for the periodic or single shot notification using new_single_shot_id or new_periodic_id.

To perform a blocking wait for the specific time of the ClockID use the id_wait. To receive a callback when the specific time is reached in the clock use id_wait_async. Both these calls can be interrupted with the id_unschedule call. If the blocking wait is unscheduled a return value of UNSCHEDULED is returned.

Periodic callbacks scheduled async will be repeatedly called automatically until it is unscheduled. To schedule a sync periodic callback, id_wait should be called repeatedly.

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 ClockID that has been unscheduled cannot be used again for any wait operation, a new ClockID should be created and the old unscheduled one should be destroyed with id_unref.

It is possible to perform a blocking wait on the same ClockID from multiple threads. However, registering the same ClockID for multiple async notifications is not possible, the callback will only be called for the thread registering the entry last.

None of the wait operations unref the ClockID, the owner is responsible for unreffing the ids itself. This holds for both periodic and single shot notifications. The reason being that the owner of the ClockID has to keep a handle to the ClockID to unblock the wait on FLUSHING events or state changes and if the entry would be unreffed automatically, the handle might become invalid without any notification.

These clock operations do not operate on the running 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.

When a clock has the CAN_SET_MASTER flag set, it can be slaved to another Clock with the set_master. The clock will then automatically be synchronized to this master clock by repeatedly sampling the master clock and the slave clock and recalibrating the slave clock with set_calibration. This feature is mostly useful for plugins that have an internal clock but must operate with another clock selected by the Pipeline. They can track the offset and rate difference of their internal clock relative to the master clock by using the get_calibration function.

The master/slave synchronisation can be tuned with the timeout, window_size and window_threshold properties. The timeout property defines the interval to sample the master clock and run the calibration functions. window_size defines the number of samples to use when calibrating and window_threshold defines the minimum number of samples before the calibration is performed.

All known sub-classes:

Namespace: Gst
Package: gstreamer-1.0

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