gstreamer/markdown/design/streams.md

# 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
  - `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 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`**
   - 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
   - marks valid buffer timestamp range (start, stop)
   - marks `stream_time` of buffers (time). This is the stream time of buffers
     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
     rate. (See also [trickmodes](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`:

```
        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
```

  - a monotonically increasing value that can be used to synchronize
    against the clock (See also
    [synchronisation](design/synchronisation.md)).

  - `stream_time`:
    *  current position in stream between 0 and duration.

```
        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.