Clocks ------ 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 an element is added or removed from the pipeline. Whever the clock changes in a pipeline, a message is posted on the bus signaling the new clock to the application. 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 expessed in nanoseconds. Since the baseline of the clock is undefined, the clock time returned is not meaningfull in itself, what matters are the deltas between two clock times. The time reported by the clock is called the absolute time. Time in GStreamer ----------------- The absolute time is used to calculate the stream time. The stream time is defined as follows: - If the pipeline is NULL/READY, the stream time is undefined. - In PAUSED, the stream time remains at the time when it was last PAUSED. When the stream is PAUSED for the first time, the stream time is 0. - In PLAYING, the stream time is the delta between the absolute time and the base time. The base time is defined as the absolute time minus the stream time at the time when the pipeline is set to PLAYING. - after a seek, the stream time is set to 0 (see part-seeking.txt) The stream time is completely managed by the GstPipeline object using the GstClock absolute time. Timestamps ---------- The combination of the last DISCONT event and the buffer timestamps express the presentation stream time of the buffer. The stream time of a buffer is calculated as follows: ST = TS - DT where: TS = buffer timestamp DT = DISCONT timestamp ST = buffer stream time The reason for not making the buffer times express the stream time directly is for the following reasons: - demuxers are easier if they can just copy the timestamps as encoded in the file. The initial discont event would contain the lowest timestamp in the stream which makes the stream time start from 0. - pipelines requiring retimestamping of buffers can efficiently adjust the timestamp in the discont events and have all buffers retimestamped automatically. - resync after different kinds of seeks is easier. If an element wants to synchronize a buffer to the clock it needs to first calculate the buffer stream time and then bring the stream time to the absolute clock time. Converting a timestamp (in stream time) to absolute time is performed using the following formula: AT = BT + ST where: AT = absolute time BT = base time ST = stream time The pipeline base time is propagated to all the element during the PAUSED to PLAYING state change. All elements are therefore able to convert the stream time to the absolute time. It is possible to specify an aditional delay to the base time to compensate for the delay it takes to perform the state change using the GstPipeline "delay" property. 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 return 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 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. 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.