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66 lines
2.4 KiB
Text
66 lines
2.4 KiB
Text
Context
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-------
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GstContext is a container object, containing a type string and a
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generic GstStructure. It is used to store and propagate context
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information in a pipeline, like device handles, display server
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connections and other information that should be shared between
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multiple elements in a pipeline.
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For sharing context objects and distributing them between application
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and elements in a pipeline, there are downstream queries, upstream
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queries, messages and functions to set a context on a complete pipeline.
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Context types
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~~~~~~~~~~~~~
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Context type names should be unique and be put in appropiate namespaces,
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e.g. "gst.egl.EGLDisplay", go prevent name conflicts. Only one specific
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type is allowed per context type name.
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Elements
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~~~~~~~~
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Elements that need a specific context for their operation would
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do the following steps until one succeeds:
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1) Check if the element already has a context of the specific type,
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i.e. it was previoulsy set via gst_element_set_context().
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2) Query downstream with GST_QUERY_CONTEXT for the context and check if
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downstream already has a context of the specific type
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3) Query upstream with GST_QUERY_CONTEXT for the context and check if
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upstream already has a context of the specific type
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4) Post a GST_MESSAGE_NEED_CONTEXT message on the bus with the required
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context types and afterwards check if a usable context was set now
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as in 1). The message could be handled by the parent bins of the
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element and the application.
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4) Create a context by itself and post a GST_MESSAGE_HAVE_CONTEXT message
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on the bus.
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Bins will propagate any context that is set on them via
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gst_element_set_context() to their child elements, including newly added
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elements after the context was set.
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Bins can handle the GST_MESSAGE_NEED_CONTEXT message, can filter both
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messages and can also set different contexts for different pipeline parts.
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Applications
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~~~~~~~~~~~~
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Applications can set a specific context on a pipeline or elements inside
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a pipeline with gst_element_set_context().
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If an element inside the pipeline needs a specific context, it will post
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a GST_MESSAGE_NEED_CONTEXT message on the bus. The application can now
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create a context of the requested type or pass an already existing
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context to the element (or the complete pipeline).
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Whenever an element creates a context internally it will post a
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GST_MESSAGE_HAVE_CONTEXT message on the bus. Bins will cache these
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contexts and pass them to any future elements that request them.
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