gstreamer/subprojects/gst-docs/markdown/additional/design/streams.md

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# Streams
This document describes the objects that are passed from element to
element in the streaming thread.
## Stream objects
The following objects are to be expected in the streaming thread:
- events, including:
- `STREAM_START` (START)
- `SEGMENT` (SEGMENT)
- `EOS` * (EOS)
- `TAG` (T)
- buffers * (B)
Objects marked with * need to be synchronised to the clock in sinks and
live sources.
## Typical stream
A typical stream starts with a `STREAM_START` event that marks the start of the
stream, followed by a `SEGMENT` event that marks the buffer timestamp
range. After that buffers are sent one after the other. After the last buffer an
`EOS` event marks the end of the stream. No more buffers are to be processed
after the `EOS` event.
```
+-----+-------+ +-++-+ +-+ +---+
|START|SEGMENT| |B||B| ... |B| |EOS|
+-----+-------+ +-++-+ +-+ +---+
```
1) **`STREAM_START`**
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- marks the start of a stream; unlike the `SEGMENT` event, there
will be no `STREAM_START` event after flushing seeks.
2) **`SEGMENT`**, rate, start/stop, time. (See also
[Segments](additional/design/segments.md))
- marks valid buffer timestamp range (start, stop)
- marks `stream_time` of buffers (time). This is the stream time of buffers
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with a timestamp of `S.start`.
- marks playback rate (rate). This is the required playback rate.
- marks applied rate (`applied_rate`). This is the already applied playback
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rate. (See also [trickmodes](additional/design/trickmodes.md))
- marks `running_time` of buffers. This is the time used to synchronize
against the clock.
3) **N buffers**
- displayable buffers are between start/stop of the `SEGMENT` (S). Buffers
outside the segment range should be dropped or clipped.
- `running_time`: For each buffer, a monotonically increasing value that can
be used to synchronize against the clock (See also
[synchronisation](additional/design/synchronisation.md)).
``` c
if (S.rate > 0.0)
running_time = (B.timestamp - S.start) / ABS (S.rate) + S.base;
else
running_time = (S.stop - B.timestamp) / ABS (S.rate) + S.base;
```
- `stream_time`: The current position in the stream, between 0 and duration.
``` c
stream_time = (B.timestamp - S.start) * ABS (S.applied_rate) + S.time;
```
4) **`EOS`**
- marks the end of data, nothing is to be expected after `EOS`, elements
should refuse more data and return `GST_FLOW_EOS`. A `FLUSH_STOP`
event clears the `EOS` state of an element.
## Elements
These events are generated typically either by the `GstBaseSrc` class for
sources operating in push mode, or by a parser/demuxer operating in
pull-mode and pushing parsed/demuxed data downstream.