1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
|
Context
-------
GstContext is a container object, containing a generic GstStructure.
It is used to store and propagate context information in a pipeline,
like device handles, display server connections and other information
that should be shared between multiple elements in a pipeline.
For sharing context objects and distributing them between application
and elements in a pipeline, there are downstream queries, downstream
events, messages and functions to set a context on a complete pipeline.
Context types
~~~~~~~~~~~~~
Context type names should be unique and be put in appropiate namespaces,
e.g. "gst.egl.EGLDisplay", go prevent name conflicts. Only one specific
type is allowed per context type name.
Elements
~~~~~~~~
Elements that need a specific context for their operation would
do the following steps until one succeeds:
1) Check if the element already has a context of the specific type,
i.e. by checking the context returned by gst_element_get_context()
2) Query downstream with GST_QUERY_CONTEXT for the context and check if
downstream already has a context of the specific type
3) Post a GST_MESSAGE_NEED_CONTEXT message on the bus with the required
context types and afterwards check if a usable context was set now
as in 1). The message could be handled by the parent bins of the
element and the application.
4) Create a context by itself and post a GST_MESSAGE_HAVE_CONTEXT message
and send a GST_EVENT_CONTEXT event downstream, containing the complete
context information at this time.
Bins will propagate any context that is set on them via
gst_element_set_context() to their child elements, including newly added
elements after the context was set.
Bins can handle the GST_MESSAGE_NEED_CONTEXT message, can filter both
messages and can also set different contexts for different pipeline parts.
Applications
~~~~~~~~~~~~
Applications can set a specific context on a pipeline or elements inside
a pipeline with gst_element_set_context().
If an element inside the pipeline needs a specific context, it will post
a GST_MESSAGE_NEED_CONTEXT message on the bus. The application can now
create a context of the requested type or pass an already existing
context to the element (or the complete pipeline).
Whenever an element creates a context internally it will post a
GST_MESSAGE_HAVE_CONTEXT message on the bus. Applications should store
the context of these messages, for example by creating a GstContext
containing all the contexts of the pipeline by merging the structures.
Applications can also just set the context contained in the
GST_MESSAGE_HAVE_CONTEXT message on the complete pipeline to make sure it
is shared between all elements.
|
