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Original commit message from CVS: * docs/design/part-TODO.txt: Remove item from the todo list because it was fixed with the latency state change rewrites. * docs/design/part-seeking.txt: * docs/design/part-segments.txt: Update some docs. * gst/gstevent.c: (gst_event_new_new_segment_full), (gst_event_parse_new_segment_full), (gst_event_new_buffer_size), (gst_event_parse_buffer_size), (gst_event_new_qos), (gst_event_parse_qos), (gst_event_new_seek), (gst_event_parse_seek), (gst_event_new_latency), (gst_event_parse_latency): Use quarks to construct and parse events. * gst/gstquark.c: (_priv_gst_quarks_initialize): * gst/gstquark.h: Add some more quarks to the table. Emit a warning when the quark tables are not in sync. * tests/check/gst/gstbus.c: (GST_START_TEST): Add an assert.
99 lines
3.5 KiB
Text
99 lines
3.5 KiB
Text
Seeking
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-------
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Seeking in GStreamer means configuring the pipeline for playback of the
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media between a certain start and stop time, called a segment. By default a
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pipeline will play from position 0 to the total duration of the media at a rate
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of 1.0.
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A seek is performed by sending a seek event to the sink elements of a
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pipeline. Sending the seek event to a bin will by default forward
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the event to all sinks in the bin.
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When performing a seek, the start and stop values of the segment can be
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specified as absoulte positions or relative to the currently configured
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playback segment. Note that it is not possible to seek relative to the current
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playback position.
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Feedback of the seek operation can be immediatly using the GST_SEEK_FLAG_FLUSH
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flag. With this flag, all pending data in the pipeline is discarded and playback
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starts from the new position immediatly.
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When the FLUSH flag is not set, the seek will be queued and executed as
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soon as possible, which might be after all queues are emptied.
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Seeking can be performed in different formats such as time, frames
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or samples.
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The seeking can be performed to a nearby key unit or to the exact
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(estimated) unit in the media (GST_SEEK_FLAG_KEY_UNIT).
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The seeking can be performed by using an estimated target position or in an
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accurate way (GST_SEEK_FLAG_ACCURATE). For some formats this can result in
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having to scan the complete file in order to accurately find the target unit.
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Non segment seeking will make the pipeline emit EOS when the configured
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segment has been played.
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Segment seeking (using the GST_SEEK_FLAG_SEGMENT) will not emit an EOS at
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the end of the playback segment but will post a SEGMENT_DONE message on the
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bus. This message is posted by the element driving the playback in the
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pipeline, typically a demuxer. After receiving the message, the application
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can reconnect the pipeline or issue other seek events in the pipeline.
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Since the message is posted as early as possible in the pipeline, the
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application has some time to issue a new seek to make the transition seamless.
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Typically the allowed delay is defined by the buffer sizes of the sinks as well
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as the size of any queues in the pipeline.
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The seek can also change the playback speed of the configured segment.
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A speed of 1.0 is normal speed, 2.0 is double speed. Negative values
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mean backward playback.
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When performing a seek with a playback rate different from 1.0, the
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GST_SEEK_FLAG_SKIP flag can be used to instruct decoders and demuxers that they
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are allowed to skip decoding. This can be useful when resource consumption is
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more important than accurately producing all frames.
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Generating seeking events
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-------------------------
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A seek event is created with gst_event_new_seek ().
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The different kinds of seeking methods and their internal workings are
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described below.
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FLUSH seeking
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-------------
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This is the most common way of performing a seek in a playback application.
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The application issues a seek on the pipeline and the new media is immediatly
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played after the seek calls returns.
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seeking without FLUSH
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---------------------
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This seek type is typically performed after issuing segment seeks to finish
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the playback of the pipeline.
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Performing a non-flushing seek in a PAUSED pipeline blocks until the pipeline
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is set to playing again since all data passing is blocked in the prerolled
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sinks.
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segment seeking with FLUSH
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--------------------------
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This seek is typically performed when starting seamless looping.
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segment seeking without FLUSH
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-----------------------------
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This seek is typically performed when continuing seamless looping.
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