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77 lines
3.8 KiB
Text
77 lines
3.8 KiB
Text
gstelement
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name
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pads
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state
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loopfunction
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threadstate
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manager
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ghostpads
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GstElement
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==========
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The Element is the most important object in the entire GStreamer system, as it defines the structure of the pipeline.
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Elements include sources, filters, sinks, and containers (Bins). They may be an intrinsic part of the core GStreamer
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library, or may be loaded from a plugin. In some cases they're even fabricated from completely different systems (see
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the LADSPA plugin). They are generally created from a GstElementFactory, which will be covered in another chapter, but
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for the intrinsic types they can be created with specific functions.
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Elements contains GstPads (also covered in another chapter), which are subsequently used to connect the Elements
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together to form a pipeline capable of passing and processing data. They have a parent, which must be another Element.
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This allows deeply nested pipelines, and the possibility of "black-box" meta-elements.
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Name
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----
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All elements are named, and while they should ideally be unique in any given pipeline, the do not have to be. The only
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guaranteed unique name for an element is its complete path in the object hierarchy. Functions are provided to set and
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get the name of the element. _set_name() creates a local copy of the string passed. _get_name() returns the actual
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element's pointer. Therefore, the argument to _set_name() is the responsibility of the caller to free if necessary,
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but the return from _get_name() is definitely not to be messed with by the caller. Accordingly, _get_name() returns an
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const gchar *.
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Providing a new name to an element causes it to free its internal copy of the name and make a copy of the new name.
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This means that you must consider the pointer returned by _get_name() to be short-lived. If you must make use of the
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name beyond the immediate scope, it is suggested that you make yourself a copy of it. If you know for a fact neither
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the pointer nor its contents will change, you may retain the original pointer. If you get odd results when using the
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returned string, that's the first thing to check.
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Pads
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----
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GstPads are the property of a given GstElement. They provide the connection capability, with allowing arbitrary
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structure in the graph. For any Element but a source or sink, there will be at least 2 Pads owned by the Element.
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These pads are stored in a single GList within the 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 via _get_pad(), which operates on the name of the Pad (the
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unique key). This means that all Pads owned by a given Element must have unique names (FIXME we don't verify this at
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_add time). A pointer to the GList of pads may be obtained with _get_pad_list. As with the element's name,
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precaution must be taken with all these pointers, as they are the same pointer that the Element uses internally. One
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must be especially careful not to manipulate the list of pads.
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gst_element_add_pad(element,pads):
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Sets the element as the parent of the pad, then adds the pad to the element's list of pads, keeping the counts
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of total, src, and sink pads up to date. Emits the "new_pad" signal with the pad as argument. Fails if either
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the element or pad are either NULL or not what they claim to be. Should fail if the pad already has a parent.
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Should fail if the pad is already owned by the element. Should fail if there's already a pad by that name in
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the the list of pads.
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pad = gst_element_get_pad(element,"padname"):
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Searches through the list of pads
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Ghost Pads
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----------
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State
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-----
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Elements use state to determine what the are capable of doing at any given moment. The states are defined as follows:
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NULL No state is held for the element
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READY Devices are open
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PLAYING
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PAUSED
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