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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 total 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
segment event containing the start time, stop time and rate of the segment.
The purpose of this segment 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.
Use case: FLUSHING seek
~~~~~~~~~~~~~~~~~~~~~~~
ex.
filesrc ! avidemux ! videodecoder ! videosink
When doing a seek in this pipeline for a segment 1 to 5 seconds, avidemux
will perform the seek.
Avidemux starts by sending a FLUSH_START event downstream and upstream. This
will cause its streaming task to PAUSED because _pad_pull_range() and
_pad_push() will return WRONG_STATE. It then waits for the STREAM_LOCK,
which will be unlocked when the streaming task pauses. At this point no
streaming is happening anymore in the pipeline and a FLUSH_STOP is sent
upstream and downstream.
When avidemux starts playback of the segment from second 1 to 5, it pushes
out a segment with 1 and 5 as start and stop times. The stream_time in
the segment is also 1 as this is the position we seek to.
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 video sink receives a frame of timestamp 1. It takes the start value of
the previous segment and aplies the folowing (simplified) formula:
render_time = BUFFER_TIMESTAMP - segment_start + element->base_time
It then syncs against the clock with this render_time. Note that
BUFFER_TIMESTAMP is always >= segment_start or else it would fall outside of
the configure segment.
Videosink reports its current position as (simplified):
current_position = clock_time - element->base_time + segment_time
See part-synchronisation.txt for a more detailed and accurate explanation of
synchronisation and position reporting.
Since after a flushing seek the stream_time is reset to 0, the new buffer
will be rendered immediately after the seek and the current_position will be
the stream_time of the seek that was performed.
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 a frame with timestamp 5 and
returns GST_FLOW_UNEXPECTED from the chain function to make the upstream
elements perform the EOS logic.
Use case: live stream
~~~~~~~~~~~~~~~~~~~~~
Use case: segment looping
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the case of a wav file with raw audio.
filesrc ! wavparse ! alsasink
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