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45 lines
1.6 KiB
Text
45 lines
1.6 KiB
Text
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Live sources
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------------
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A live source such as an element capturing audio or video need to be handled
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in a special way. It does not make sense to start the dataflow in the PAUSED
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state for those devices as the user might wait a long time between going from
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PAUSED to PLAYING, making the previously captured buffers irrelevant.
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A live source therefore only produces buffers in the PLAYING state. This has
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implications for sinks waiting for a buffer to complete the preroll state
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since such a buffer might never arrive.
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Live sources return NO_PREROLL when going to the PAUSED state to inform the
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bin/pipeline that this element will not be able to produce data in the
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PAUSED state.
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When performing a get_state() on a bin with a non-zero timeout value, the
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bin must be sure that there are no live sources in the pipeline because else
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the get_state() function 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 performing
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a blocking wait on all elements.
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Scheduling
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----------
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Live sources can not produce data in the paused state. They block in the
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getrange function or in the loop function until they go to PLAYING.
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Latency
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-------
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The live source timestamps its data with the time of the clock at the
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time the data was captured. Normally it will take some time to capture
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the first sample of data and the last sample. 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.
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