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Original commit message from CVS: * docs/design/part-TODO.txt: * docs/design/part-events.txt: * docs/design/part-gstbin.txt: * docs/design/part-gstbus.txt: * docs/design/part-gstpipeline.txt: * docs/design/part-messages.txt: * gst/gstbus.c: * gst/gstmessage.c: Docs updates
89 lines
2.6 KiB
Plaintext
89 lines
2.6 KiB
Plaintext
GstPipeline
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-----------
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A GstPipeline is usually a toplevel bin an provides all of its
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children with a clock and a bus.
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The GstPipeline will also collect EOS messages from its children and
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will forward the EOS message to the application when all of the
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sinks are in EOS.
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The pipeline also calculates the stream time based on the selected
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clock (see part-clocks.txt).
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The pipeline manages the seek operation for the application.
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GstBus
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------
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The pipeline creates a GstBus and attaches a sync handler to receive
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the EOS events.
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Since the pipeline subclasses GstBin, all of its children will receive
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the same bus when added to the Gstbin.
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The application can retrieve the GstBus and integrate it in the
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mainloop or it can just _pop() messages off in its own thread.
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Clock selection
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---------------
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Since all of the children of a GstPipeline must use the same clock, the
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pipeline must select a clock.
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The default clock selection algorithm works as follows:
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- If the application selected a clock, use that clock. (see below)
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- use clock of source elements (*)
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- use clock of other element, starting from the sinks going upstream.
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(+)
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- use GstSystemClock.
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(*) currently not implemented.
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(+) traversing the graph upstream to find the best clock is not implemented,
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currently the first element found that provides a clock is used.
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The application can influence this clock selection with two methods:
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gst_pipeline_use_clock() and gst_pipeline_auto_clock().
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The _use_clock() method forces the use of a specific clock on the pipeline
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regardless of what clock providers are children of the pipeline. Setting
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NULL disables the clock completely and makes the pipeline run as fast as
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possible.
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The _auto_clock() method removes the fixed clock and reactivates the auto-
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matic clock selection algorithm described above.
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EOS
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---
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The sink elements will post an EOS event on the bus when they reach EOS. The
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EOS message is only posted to the bus when the element is in PLAYING.
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The pipeline collects all EOS messages and forwards it to the application as
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soon as all the sinks have posted an EOS.
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The list of queued EOS messages is cleared when the pipeline goes to PAUSED
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again. This means that all elements should repost the EOS message when going
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to PLAYING again.
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Seeking
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-------
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When performing a seek on the pipeline element using gst_element_send_event(),
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the pipeline performs the following actions:
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- record the current state of the pipeline.
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- set the pipeline to paused
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- send the seek event to all sinks
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- update the stream time with the time of the seek
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- restore old state of the pipeline.
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