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62 lines
2.5 KiB
Text
62 lines
2.5 KiB
Text
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Segments
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--------
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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
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always 0 and the stop time is the duration (or -1 if unknown, for example
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a 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 rate.
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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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The playback of a segment starts with a source or demuxer element pushing a
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newsegment event containing the start time, stop time and rate of the segment.
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The purpose of this newsegment is to inform downstream elements of the
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requested segment positions. Some elements might produce buffers that fall
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outside of the segment and that might therefore be discarded or clipped.
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ex.
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filesrc ! avidemux ! videodecoder ! videosink
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When avidemux starts playback of the segment from second 1 to 5, it pushes
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out a newsegment with 1 and 5 as start and stop times.
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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 stop time is important when the video format contains B frames. The
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video decoder receives a P frame first, which is 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 frame with timestamp 5 and pushes
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an EOS event downstream to finish playback.
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