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