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43 lines
2.1 KiB
Markdown
43 lines
2.1 KiB
Markdown
# push-pull
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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
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the 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
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element. The downstream element does this by calling
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`gst_pad_pull_range()` on one of its sinkpads. In this mode, the
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downstream element is the driving force in the pipeline as it initiates
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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
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elements this means that all downstream elements should be in the
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correct state and for `pull_range()` based elements this means the
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upstream elements should 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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The `GstBin` sets the state of all the sink elements. These are the
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elements without source pads.
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Setting the state on an element will first activate all the srcpads and
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then the sinkpads. For each of the sinkpads,
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`gst_pad_check_pull_range()` is performed. If the sinkpad supports a
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loopfunction and the peer pad returns TRUE from the `GstPadCheckPullRange`
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function, then the peer pad is activated first as it must be in the
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right state to handle a `_pull_range()`. Note that the state change of
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the element is not yet performed, just the activate function is called
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on the source pad. This means that elements that implement a getrange
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function must be prepared to get their activate function called before
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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
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does not support pull based mode.
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