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53 lines
2.1 KiB
Markdown
53 lines
2.1 KiB
Markdown
# Live sources
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A live source is a source that cannot be arbitrarily `PAUSED` without
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losing data.
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A live source such as an element capturing audio or video, needs to be
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handled in a special way. It does not make sense to start the dataflow
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in the `PAUSED` state for those devices as the user might wait a long time
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between going from `PAUSED` to `PLAYING`, making the previously captured
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buffers irrelevant.
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A live source therefore only produces buffers in the `PLAYING` state. This
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has implications for sinks waiting for a buffer to complete the preroll
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state since such a buffer might never arrive.
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Live sources return `NO_PREROLL` when going to the `PAUSED` state to inform
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the bin/pipeline that this element will not be able to produce data in
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the `PAUSED` state. `NO_PREROLL` should be returned for both `READY→PAUSED`
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and `PLAYING→PAUSED`.
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When performing a `get_state()` on a bin with a non-zero timeout value,
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the bin must be sure that there are no live sources in the pipeline
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because otherwise, `get_state()` would block on the sinks.
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A `GstBin` therefore always performs a zero-timeout `get_state()` on its
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elements to discover the `NO_PREROLL` (and `ERROR`) elements before
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performing a blocking wait.
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## Scheduling
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Live sources will not produce data in the `PAUSED` state. They block in
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`get_range()` or in the loop function until they go to `PLAYING`.
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## Latency
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The live source timestamps its data with the time of the clock when
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the data was captured. Normally, it will take some time to capture
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the first sample of data and the last one. This means that when the
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buffer arrives at the sink, it will already be late and will be dropped.
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The latency is the time it takes to construct one buffer of data and it's
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exposed with a `LATENCY` query.
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See [latency](design/latency.md)
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## Timestamps
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Live sources always timestamp their buffers with the `running_time` of
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the pipeline. This is needed to be able to match the timestamps of
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different live sources in order to synchronize them.
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This is in contrast to non-live sources, which timestamp their buffers
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starting from `running_time` 0.
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