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 event 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 cases

FLUSHING seek

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 FLUSHING. 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 applies the following (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 configured segment.

Videosink reports its current position as (simplified):

current_position = clock_time - element->base_time + segment_time

See synchronisation 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 it 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_EOS from the chain function to make the upstream elements perform the EOS logic.

Live stream

Segment looping

Consider the case of a wav file with raw audio.

filesrc ! wavparse ! alsasink

FIXME!