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108 lines
4 KiB
Markdown
108 lines
4 KiB
Markdown
# Segments
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A segment in GStreamer denotes a set of media samples that must be
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processed. A segment has a start time, a stop time and a processing
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rate.
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A media stream has a start and a stop time. The start time is always 0
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and the stop time is the total duration (or -1 if unknown, for example a
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live stream). We call this the complete media stream.
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The segment of the complete media stream can be played by issuing a seek
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on the stream. The seek has a start time, a stop time and a processing
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rate.
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```
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complete stream
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+------------------------------------------------+
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0 duration
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segment
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|--------------------------|
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start stop
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```
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The playback of a segment starts with a source or demuxer element
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pushing a segment event containing the start time, stop time and rate of
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the segment. The purpose of this segment is to inform downstream
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elements of the requested segment positions. Some elements might produce
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buffers that fall outside of the segment and that might therefore be
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discarded or
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clipped.
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## Use case: FLUSHING seek
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ex. `filesrc ! avidemux ! videodecoder ! videosink`
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When doing a seek in this pipeline for a segment 1 to 5 seconds, avidemux
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will perform the seek.
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Avidemux starts by sending a FLUSH_START event downstream and upstream. This
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will cause its streaming task to PAUSED because \_pad_pull_range() and
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\_pad_push() will return FLUSHING. It then waits for the STREAM_LOCK,
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which will be unlocked when the streaming task pauses. At this point no
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streaming is happening anymore in the pipeline and a FLUSH_STOP is sent
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upstream and downstream.
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When avidemux starts playback of the segment from second 1 to 5, it pushes
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out a segment with 1 and 5 as start and stop times. The stream_time in
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the segment is also 1 as this is the position we seek to.
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The video decoder stores these values internally and forwards them to the
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next downstream element (videosink, which also stores the values)
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Since second 1 does not contain a keyframe, the avi demuxer starts sending
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data from the previous keyframe which is at timestamp 0.
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The video decoder decodes the keyframe but knows it should not push the
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video frame yet as it falls outside of the configured segment.
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When the video decoder receives the frame with timestamp 1, it is able to
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decode this frame as it received and decoded the data up to the previous
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keyframe. It then continues to decode and push frames with timestamps >= 1.
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When it reaches timestamp 5, it does not decode and push frames anymore.
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The video sink receives a frame of timestamp 1. It takes the start value of
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the previous segment and aplies the following (simplified) formula:
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```
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render_time = BUFFER_TIMESTAMP - segment_start + element->base_time
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```
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It then syncs against the clock with this render_time. Note that
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BUFFER_TIMESTAMP is always >= segment_start or else it would fall outside of
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the configure segment.
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Videosink reports its current position as (simplified):
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```
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current_position = clock_time - element->base_time + segment_time
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```
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See [synchronisation](design/synchronisation.md) for a more detailed and accurate explanation of
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synchronisation and position reporting.
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Since after a flushing seek the stream_time is reset to 0, the new buffer
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will be rendered immediately after the seek and the current_position will be
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the stream_time of the seek that was performed.
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The stop time is important when the video format contains B frames. The
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video decoder receives a P frame first, which it can decode but not push yet.
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When it receives a B frame, it can decode the B frame and push the B frame
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followed by the previously decoded P frame. If the P frame is outside of the
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segment, the decoder knows it should not send the P frame.
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Avidemux stops sending data after pushing a frame with timestamp 5 and
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returns GST_FLOW_EOS from the chain function to make the upstream
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elements perform the EOS logic.
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## Use case: live stream
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## Use case: segment looping
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Consider the case of a wav file with raw audio.
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```
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filesrc ! wavparse ! alsasink
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```
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FIXME!
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