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Original commit message from CVS: * docs/design/part-live-source.txt: * gst/gstclock.h: Small docs fixes.
58 lines
2.4 KiB
Plaintext
58 lines
2.4 KiB
Plaintext
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.
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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. This latency
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could be exposed by latency queries.
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Theses latency queries could to be done by the managing pipeline for all sinks.
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In that case they can only be done after the meassurements have been taken (all
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are prerolled). Thus in pipeline:state_changed:PAUSED_TO_PLAYING we need
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get the max-latency and set this as a sync-offset in all sinks. The problem is
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that in a live pipeline, we set the pipeline to PLAYING before waiting for the
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sinks to preroll.
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Another possibility would be to configure a fixed latency in a pipeline that
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would automatically be configured on any sink in the case of a NO_PREROLL
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element. For decoupled elements this is practically the only viable way to
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introduce enough latency that does not starve the sinks.
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The current latency can also be measured in the sinks and the pipeline could
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optimize it to the lowest possible latency.
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