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design: segments: section breakdown, markup and readability fixes

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Reynaldo H. Verdejo Pinochet 2016-12-29 22:48:32 -08:00
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markdown/design/segments.md
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@ -26,25 +26,28 @@ pushing a segment event containing the start time, stop time and rate of
the segment. The purpose of this segment is to inform downstream the segment. The purpose of this segment is to inform downstream
elements of the requested segment positions. Some elements might produce elements of the requested segment positions. Some elements might produce
buffers that fall outside of the segment and that might therefore be buffers that fall outside of the segment and that might therefore be
discarded or discarded or clipped.
clipped.
## Use case: FLUSHING seek ## Use cases
### FLUSHING seek
```
ex. `filesrc ! avidemux ! videodecoder ! videosink` ex. `filesrc ! avidemux ! videodecoder ! videosink`
```
When doing a seek in this pipeline for a segment 1 to 5 seconds, avidemux When doing a seek in this pipeline for a segment 1 to 5 seconds, avidemux
will perform the seek. will perform the seek.
Avidemux starts by sending a FLUSH_START event downstream and upstream. This Avidemux starts by sending a `FLUSH_START` event downstream and upstream. This
will cause its streaming task to PAUSED because \_pad_pull_range() and will cause its streaming task to `PAUSED` because `_pad_pull_range()` and
\_pad_push() will return FLUSHING. It then waits for the STREAM_LOCK, `_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 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 streaming is happening anymore in the pipeline and a `FLUSH_STOP` is sent
upstream and downstream. upstream and downstream.
When avidemux starts playback of the segment from second 1 to 5, it pushes 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 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 segment is also 1 as this is the position we seek to.
The video decoder stores these values internally and forwards them to the The video decoder stores these values internally and forwards them to the
@ -65,25 +68,25 @@ The video sink receives a frame of timestamp 1. It takes the start value of
the previous segment and aplies the following (simplified) formula: the previous segment and aplies the following (simplified) formula:
``` ```
render_time = BUFFER_TIMESTAMP - segment_start + element->base_time render_time = BUFFER_TIMESTAMP - segment_start + element->base_time
``` ```
It then syncs against the clock with this render_time. Note that 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 `BUFFER_TIMESTAMP` is always >= `segment_start` or else it would fall outside of
the configure segment. the configure segment.
Videosink reports its current position as (simplified): Videosink reports its current position as (simplified):
``` ```
current_position = clock_time - element->base_time + segment_time current_position = clock_time - element->base_time + segment_time
``` ```
See [synchronisation](design/synchronisation.md) for a more detailed and accurate explanation of See [synchronisation](design/synchronisation.md) for a more detailed and
synchronisation and position reporting. accurate explanation of synchronisation and position reporting.
Since after a flushing seek the stream_time is reset to 0, the new buffer 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 will be rendered immediately after the seek and the `current_position` will be
the stream_time of the seek that was performed. the `stream_time` of the seek that was performed.
The stop time is important when the video format contains B frames. The 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. video decoder receives a P frame first, which it can decode but not push yet.
@ -92,17 +95,17 @@ 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. segment, the decoder knows it should not send the P frame.
Avidemux stops sending data after pushing a frame with timestamp 5 and Avidemux stops sending data after pushing a frame with timestamp 5 and
returns GST_FLOW_EOS from the chain function to make the upstream returns `GST_FLOW_EOS` from the chain function to make the upstream
elements perform the EOS logic. elements perform the EOS logic.
## Use case: live stream ### Live stream
## Use case: segment looping ### Segment looping
Consider the case of a wav file with raw audio. Consider the case of a wav file with raw audio.
``` ```
filesrc ! wavparse ! alsasink filesrc ! wavparse ! alsasink
``` ```
FIXME! FIXME!