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