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77 lines
2.9 KiB
Text
77 lines
2.9 KiB
Text
Source elements
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---------------
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A source element is an element that provides data to the pipeline. It
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does typically not have any sink (input) pads.
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Typical source elements include:
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- file readers
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- network elements
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- capture elements (video/audio/...)
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- generators (signals/video/audio/...)
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A source element can operate in three ways:
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- it is fully seekable, this means that random access can be performed
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on it in an efficient way. (a file reader,...). This also typically
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means that the source is not live.
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- data can be obtained from it with a variable size. This means that
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the source can give N bytes of data. An example is an audio source.
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A video source always provides the same amount of data (one video
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frame). Note that this is not a fully seekable source.
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- it is a live source, this means that data arrives when it is ready.
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An example of this is a video or network source.
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When writing a source, one has to look at how the source can operate to
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decide on the scheduling methods to implement on the source.
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- fully seekable sources implement a getrange function on the source pad.
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- sources that can give N bytes but cannot do seeking also implement a
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getrange function but state that they cannot do random access.
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- sources that are purely live sources implement a task to push out
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data.
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Any source that has a getrange function must also implement a push based
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scheduling mode. In this mode the source starts a task that gets N bytes
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and pushes them out. Whenever possible, the peer element will select the
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getrange based scheduling method of the source, though.
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A source with a getrange function must activate itself in the pad activate
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function. This is needed because the downstream peer element will decide
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and activate the source element in its state change function before the
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source's state change function is called.
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Source base classes
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-------------------
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GstBaseSrc:
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This base class provides an implementation of a random access source and
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is very well suited for file reader like sources.
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GstPushSrc:
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Base class for block-based sources. This class is mostly useful for
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elements that cannot do random access, or at least very slowly. The
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source usually prefers to push out a fixed size buffer.
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Classes extending this base class will usually be scheduled in a push
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based mode. It the peer accepts to operate without offsets and withing
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the limits of the allowed block size, this class can operate in getrange
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based mode automatically.
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The subclass should extend the methods from the baseclass in
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addition to the create method. If the source is seekable, it
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needs to override GstBaseSrc::event() in addition to
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GstBaseSrc::is_seekable() in order to retrieve the seek offset,
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which is the offset of the next buffer to be requested.
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Flushing, scheduling and sync is all handled by this base class.
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