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46 lines
2.1 KiB
Text
46 lines
2.1 KiB
Text
push-pull
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---------
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Normally a source element will push data to the downstream element using
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the gst_pad_push() method. The downstream peer pad will receive the
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buffer in the Chain function. In the push mode, the source element is the
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driving force in the pipeline as it initiates data transport.
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It is also possible for an element to pull data from an upstream element.
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The downstream element does this by calling gst_pad_pull_range() on one
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of its sinkpads. In this mode, the downstream element is the driving force
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in the pipeline as it initiates data transfer.
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It is important that the elements are in the correct state to handle a
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push() or a pull_range() from the peer element. For push() based elements
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this means that all downstream elements should be in the correct state and
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for pull_range() based elements this means the upstream elements should
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be in the correct state.
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Most sinkpads implement a chain function. This is the most common case.
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sinkpads implementing a loop function will be the exception. Likewise
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srcpads implementing a getrange function will be the exception.
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state changes
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~~~~~~~~~~~~~
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The GstBin sets the state of all the sink elements. These are the elements
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without source pads.
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Setting the state on an element will first activate all the srcpads and then
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the sinkpads. For each of the sinkpads, gst_pad_check_pull_range() is
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performed. If the sinkpad supports a loopfunction and the peer pad returns TRUE
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from the GstPadCheckPullRange function, then the peer pad is activated first as
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it must be in the right state to handle a _pull_range(). Note that the
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state change of the element is not yet performed, just the activate function
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is called on the source pad. This means that elements that implement a
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getrange function must be prepared to get their activate function called
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before their state change function.
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Elements that have multiple sinkpads that require all of them to operate
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in the same mode (push/pull) can use the _check_pull_range() on all
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their pads and can then remove the loop functions if one of the pads does
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not support pull based mode.
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