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61 lines
2.7 KiB
Markdown
61 lines
2.7 KiB
Markdown
# GstElement
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The Element is the most important object in the entire GStreamer system,
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as it defines the structure of the pipeline. Elements include sources,
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filters, sinks, and containers (Bins). They may be an intrinsic part of
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the core GStreamer library, or may be loaded from a plugin. In some
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cases they’re even fabricated from completely different systems (see the
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LADSPA plugin). They are generally created from a `GstElementFactory`,
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which will be covered in another chapter, but for the intrinsic types
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they can be created with specific functions.
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Elements contains `GstPads` (also covered in another chapter), which are
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subsequently used to connect the Elements together to form a pipeline
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capable of passing and processing data. They have a parent, which must
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be another Element. This allows deeply nested pipelines, and the
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possibility of "black-box" meta-elements.
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## Name
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All elements are named, and while they should ideally be unique in any
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given pipeline, they do not have to be. The only guaranteed unique name
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for an element is its complete path in the object hierarchy. In other
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words, an element’s name is unique inside its parent. (This follows from
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GstObject’s name explanation)
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This uniqueness is guaranteed through all functions where either
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parentage or name of an element is changed.
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## Pads
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`GstPads` are the property of a given `GstElement`. They provide the
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connection capability, with allowing arbitrary structure in the graph.
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For any Element but a source or sink, there will be at least 2 Pads
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owned by the Element. These pads are stored in a single `GList` within the
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Element. Several counters are kept in order to allow quicker
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determination of the type and properties of a given Element.
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Pads may be added to an element with `_add_pad()`. Retrieval is done via
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`_get_static_pad()`, which operates on the name of the Pad (the unique
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key). This means that all Pads owned by a given Element must have unique
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names. A pointer to the `GList` of pads may be obtained with
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`_iterate_pads()`.
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`gst_element_add_pad(element,pads)`: Sets the element as the parent of
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the pad, then adds the pad to the element’s list of pads, keeping the
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counts of total, src, and sink pads up to date. Emits the `new_pad`
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signal with the pad as argument. Fails if either the element or pad are
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NULL or not what they claim to be. Should fail if the pad already
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has a parent. Should fail if the pad is already owned by the element.
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Should fail if there’s already a pad by that name in the list of pads.
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`pad = gst_element_get_pad(element, "padname")`: Searches through the
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list of pads
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## Ghost Pads
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More info in [ghostpad](design/gstghostpad.md).
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## State
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An element has a state. More info in [state](design/states.md).
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