mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-23 18:21:04 +00:00
84770e8437
Original commit message from CVS: * docs/design/part-events.txt: * docs/design/part-gstbus.txt: * docs/design/part-gstpipeline.txt: * docs/design/part-messages.txt: * docs/design/part-overview.txt: * docs/design/part-segments.txt: * gst/gstbin.c: * gst/gstbuffer.c: * gst/gstclock.c: * gst/gstelement.c: * gst/gstevent.c: * gst/gstfilter.c: * gst/gstiterator.c: Various documentation updates.
61 lines
2.5 KiB
Text
61 lines
2.5 KiB
Text
Segments
|
|
--------
|
|
|
|
A segment in GStreamer denotes a set of media samples that must be
|
|
processed. A segment has a start time, a stop time and a processing
|
|
rate.
|
|
|
|
A media stream has a start and a stop time. The start time is
|
|
always 0 and the stop time is the duration (or -1 if unknown, for example
|
|
a live stream). We call this the complete media stream.
|
|
|
|
The segment of the complete media stream can be played by issuing a seek
|
|
on the stream. The seek has a start time, a stop time and a processing rate.
|
|
|
|
|
|
complete stream
|
|
+------------------------------------------------+
|
|
0 duration
|
|
segment
|
|
|--------------------------|
|
|
start stop
|
|
|
|
|
|
The playback of a segment starts with a source or demuxer element pushing a
|
|
newsegment event containing the start time, stop time and rate of the segment.
|
|
The purpose of this newsegment is to inform downstream elements of the
|
|
requested segment positions. Some elements might produce buffers that fall
|
|
outside of the segment and that might therefore be discarded or clipped.
|
|
|
|
ex.
|
|
|
|
filesrc ! avidemux ! videodecoder ! videosink
|
|
|
|
When avidemux starts playback of the segment from second 1 to 5, it pushes
|
|
out a newsegment with 1 and 5 as start and stop times.
|
|
|
|
The video decoder stores these values internally and forwards them to the
|
|
next downstream element (videosink, which also stores the values)
|
|
|
|
Since second 1 does not contain a keyframe, the avi demuxer starts sending
|
|
data from the previous keyframe which is at timestamp 0.
|
|
|
|
The video decoder decodes the keyframe but knows it should not push the
|
|
video frame yet as it falls outside of the configured segment.
|
|
|
|
When the video decoder receives the frame with timestamp 1, it is able to
|
|
decode this frame as it received and decoded the data up to the previous
|
|
keyframe. It then continues to decode and push frames with timestamps >= 1.
|
|
When it reaches timestamp 5, it does not decode and push frames anymore.
|
|
|
|
The stop time is important when the video format contains B frames. The
|
|
video decoder receives a P frame first, which is can decode but not push yet.
|
|
When it receives a B frame, it can decode the B frame and push the B frame
|
|
followed by the previously decoded P frame. If the P frame is outside of the
|
|
segment, the decoder knows it should not send the P frame.
|
|
|
|
Avidemux stops sending data after pushing frame with timestamp 5 and pushes
|
|
an EOS event downstream to finish playback.
|
|
|
|
|
|
|