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50 lines
2.6 KiB
Text
50 lines
2.6 KiB
Text
Dynamic pads are pads that are created while the element is playing.
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This means that the element will create a new pas depending on the
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media type it is handling. For example, the mpeg1pdemuxer element will
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create one or more video pads and one or more audio pads depending
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on the number of elementary streams found in the media stream.
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The element will present its dynamic pads with the padtemplate list
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attached to the elementfactory. both the MIME type, direction, presence
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and properties of a pad can be obtained from the elementfactory using
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the padtemplates.
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Dynamic pad usually have the presence indicator set to
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GST_PAD_SOMETIMES. This indicated that the pad might become
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available at runtime. When the pad is actually created in the
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element, the element will signal the new_pad signal. This signal can
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then be used to attach a desired (compatible) element to the
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pad.
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For certain elements, like a tee and a muxer, we need another pad
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presence flag: GST_PAD_REQUEST. With this flag, the
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elementfactory will announce that some of the pads are available on
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request. For the tee element, for example, one might obtain a new output
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pad by looking up a suitable padtemplate (temp) and performing:
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gst_element_request_new_pad (element, temp). The element will then
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create a new pad from the template (sink or source) and will return
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a handle to it. You can then connect elements to this new pad.
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The muxer element (an avi encoder, for example) might expose several
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padtemplates for audio and video. The typical usage pattern for the
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muxer would then be: create a compressor element (JPEG). Get the
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src pad of the compressor, Request a new pad from the element using the
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padtemplate provided by the compressor src pad and connect the
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compressor element to this pad.
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An element that can be requested for a new pad has to implement the
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gst_element_request_new_pad method and perform the necessary steps
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to create a pad from that template.
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This interesting behaviour can be extended to ghostpads too. A
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compound element (a bin with internal elements) will also expose some
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padtemplates, either from the internal elements or from itself. Along
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with a new GstGhostPad class, this also solves the naming conflicts
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we might have with the ghostpads. The compound element will override
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the request_new_pad function and figure out which element it needs
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to get a pad from.
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Related to this solution: we will move the tee element to the
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gst/elements/ directory because there is no point in having the
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header files anymore. The new pad request API will become a feature
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of gstelement so gsttee becomes a real element too.
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