mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-22 08:17:01 +00:00
50 lines
2.6 KiB
Text
50 lines
2.6 KiB
Text
Dynamic pads are pads that are created while the element is playing.
|
|
This means that the element will create a new pas depending on the
|
|
media type it is handling. For example, the mpeg1pdemuxer element will
|
|
create one or more video pads and one or more audio pads depending
|
|
on the number of elementary streams found in the media stream.
|
|
|
|
The element will present its dynamic pads with the padtemplate list
|
|
attached to the elementfactory. both the MIME type, direction, presence
|
|
and properties of a pad can be obtained from the elementfactory using
|
|
the padtemplates.
|
|
|
|
Dynamic pad usually have the presence indicator set to
|
|
GST_PAD_SOMETIMES. This indicated that the pad might become
|
|
available at runtime. When the pad is actually created in the
|
|
element, the element will signal the new_pad signal. This signal can
|
|
then be used to attach a desired (compatible) element to the
|
|
pad.
|
|
|
|
For certain elements, like a tee and a muxer, we need another pad
|
|
presence flag: GST_PAD_REQUEST. With this flag, the
|
|
elementfactory will announce that some of the pads are available on
|
|
request. For the tee element, for example, one might obtain a new output
|
|
pad by looking up a suitable padtemplate (temp) and performing:
|
|
gst_element_request_new_pad (element, temp). The element will then
|
|
create a new pad from the template (sink or source) and will return
|
|
a handle to it. You can then connect elements to this new pad.
|
|
|
|
The muxer element (an avi encoder, for example) might expose several
|
|
padtemplates for audio and video. The typical usage pattern for the
|
|
muxer would then be: create a compressor element (JPEG). Get the
|
|
src pad of the compressor, Request a new pad from the element using the
|
|
padtemplate provided by the compressor src pad and connect the
|
|
compressor element to this pad.
|
|
|
|
An element that can be requested for a new pad has to implement the
|
|
gst_element_request_new_pad method and perform the necessary steps
|
|
to create a pad from that template.
|
|
|
|
This interesting behaviour can be extended to ghostpads too. A
|
|
compound element (a bin with internal elements) will also expose some
|
|
padtemplates, either from the internal elements or from itself. Along
|
|
with a new GstGhostPad class, this also solves the naming conflicts
|
|
we might have with the ghostpads. The compound element will override
|
|
the request_new_pad function and figure out which element it needs
|
|
to get a pad from.
|
|
|
|
Related to this solution: we will move the tee element to the
|
|
gst/elements/ directory because there is no point in having the
|
|
header files anymore. The new pad request API will become a feature
|
|
of gstelement so gsttee becomes a real element too.
|