mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-03 16:09:39 +00:00
111 lines
3.8 KiB
Text
111 lines
3.8 KiB
Text
Seeking
|
|
-------
|
|
|
|
Seeking in GStreamer means configuring the pipeline for playback of the
|
|
media between a certain start and stop time, called the playback segment.
|
|
By default a pipeline will play from position 0 to the total duration of the
|
|
media at a rate of 1.0.
|
|
|
|
A seek is performed by sending a seek event to the sink elements of a
|
|
pipeline. Sending the seek event to a bin will by default forward
|
|
the event to all sinks in the bin.
|
|
|
|
When performing a seek, the start and stop values of the segment can be
|
|
specified as absolute positions or relative to the currently configured
|
|
playback segment. Note that it is not possible to seek relative to the current
|
|
playback position. To seek relative to the current playback position, one must
|
|
query the position first and then perform an absolute seek to the desired
|
|
position.
|
|
|
|
Feedback of the seek operation can be immediately using the GST_SEEK_FLAG_FLUSH
|
|
flag. With this flag, all pending data in the pipeline is discarded and playback
|
|
starts from the new position immediately.
|
|
|
|
When the FLUSH flag is not set, the seek will be queued and executed as
|
|
soon as possible, which might be after all queues are emptied.
|
|
|
|
Seeking can be performed in different formats such as time, frames
|
|
or samples.
|
|
|
|
The seeking can be performed to a nearby key unit or to the exact
|
|
(estimated) unit in the media (GST_SEEK_FLAG_KEY_UNIT).
|
|
|
|
The seeking can be performed by using an estimated target position or in an
|
|
accurate way (GST_SEEK_FLAG_ACCURATE). For some formats this can result in
|
|
having to scan the complete file in order to accurately find the target unit.
|
|
|
|
Non segment seeking will make the pipeline emit EOS when the configured
|
|
segment has been played.
|
|
|
|
Segment seeking (using the GST_SEEK_FLAG_SEGMENT) will not emit an EOS at
|
|
the end of the playback segment but will post a SEGMENT_DONE message on the
|
|
bus. This message is posted by the element driving the playback in the
|
|
pipeline, typically a demuxer. After receiving the message, the application
|
|
can reconnect the pipeline or issue other seek events in the pipeline.
|
|
Since the message is posted as early as possible in the pipeline, the
|
|
application has some time to issue a new seek to make the transition seamless.
|
|
Typically the allowed delay is defined by the buffer sizes of the sinks as well
|
|
as the size of any queues in the pipeline.
|
|
|
|
The seek can also change the playback speed of the configured segment.
|
|
A speed of 1.0 is normal speed, 2.0 is double speed. Negative values
|
|
mean backward playback.
|
|
|
|
When performing a seek with a playback rate different from 1.0, the
|
|
GST_SEEK_FLAG_SKIP flag can be used to instruct decoders and demuxers that they
|
|
are allowed to skip decoding. This can be useful when resource consumption is
|
|
more important than accurately producing all frames.
|
|
|
|
|
|
Seeking in push based elements
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Generating seeking events
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A seek event is created with gst_event_new_seek ().
|
|
|
|
|
|
|
|
Seeking variants
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
The different kinds of seeking methods and their internal workings are
|
|
described below.
|
|
|
|
|
|
FLUSH seeking
|
|
^^^^^^^^^^^^^
|
|
|
|
This is the most common way of performing a seek in a playback application.
|
|
The application issues a seek on the pipeline and the new media is immediately
|
|
played after the seek call returns.
|
|
|
|
|
|
seeking without FLUSH
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
This seek type is typically performed after issuing segment seeks to finish
|
|
the playback of the pipeline.
|
|
|
|
Performing a non-flushing seek in a PAUSED pipeline blocks until the pipeline
|
|
is set to playing again since all data passing is blocked in the prerolled
|
|
sinks.
|
|
|
|
|
|
segment seeking with FLUSH
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
This seek is typically performed when starting seamless looping.
|
|
|
|
|
|
segment seeking without FLUSH
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
This seek is typically performed when continuing seamless looping.
|
|
|