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71 lines
3.3 KiB
Text
71 lines
3.3 KiB
Text
GST State Bits and Transition Rules (graph to follow)
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-----------------------------------
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These are the 'new' state bits and what they mean.
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What the state bits are:
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GST_STATE_COMPLETE: if the element has enough data, but is not in any kind
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of running or explicitely stopped state. ready to be used.
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GST_STATE_RUNNING: this is the normal state of the pipeline, where data
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goes all the way through the pipeline normally.
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GST_STATE_DISCOVERY: anything the element does in this state must be reset
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after discovery. any data read from sync source must be cached.
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GST_STATE_PREROLL: not a lot different from PLAYING, except sinks don't
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render what they're getting. useful for elements that require
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data to get in sync, such as an MPEG video decoder that needs
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IBBPBB before starting at the next P.
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Basic transition rules:
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Completeness is based on the element having enough information to actually
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do something. GST_STATE_COMPLETE is required for any other state to be
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valid, though the only invariant is that you can't be RUNNING unless
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you're COMPLETE. In fact, AFAICT, that's the *only* invariant.
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The element is entirely in control of this bit at all times. There is no
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way to externally change this bit except by changing the state of the
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element in such a way as to effect a change.
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|= GST_STATE_COMPLETE
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setting whatever the last bit of info the element was looking for
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(gst_object_set)
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&= ~GST_STATE_COMPLETE
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changing anything that invalidates the complete state of the element
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Whether the element is running or not, on the other hand, is almost
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entirely out of the hands of the individual element. This is generally
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turned on by way of gst_element_run() as called by the parent (ultimately
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by the Pipeline), which happens to optionally call a function private to
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the element to prepare it. As per docs/random/gboolean, very likely this
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function should return a TRUE/FALSE.
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Generally, I think if there is no such function, the generic element code
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should go ahead and set the state, and trigger the state_changed signal,
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returning TRUE. If there is a function, call it. If it returns TRUE,
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fire off the signal (since the signal is actually an Element signal
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anyway, why eat another function call?). Return the result regardless.
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|= GST_STATE_RUNNING
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starting up the pipeline with gst_pipeline_start
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~= ~GST_STATE_RUNNING
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stopping the pipeline with gst_pipeline_stop, or some error state
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gst_pipeline_start() simply calls the gst_element_start() function on each
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of the elements in it. This sets the RUNNING bit of each element, and for
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GstBin's it loops through that list. gst_pipeline_start() is just a
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special case version of gst_bin_start(). All start() functions are
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GstElementClass functions, meaning you can start any element the same way.
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The pipeline can be stopped the same way, but more likely the pipeline
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will be stopped due to some stoppage condition, such as EOF on the source
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file, or the parser being told to stop the stream. In the EOF case, it
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would turn its RUNNING bit off, then call the stop() class function on its
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parent. This would trigger an up-hill, breath-first traversal of the
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whole graph. Alternately, if each element lists its uber-parent (the
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Pipeline) it can simply inform the pipeline directly, causing a
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depth-first traversal just like the start() case.
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