mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-25 17:50:36 +00:00
77 lines
2.9 KiB
Text
77 lines
2.9 KiB
Text
Source elements
|
|
---------------
|
|
|
|
A source element is an element that provides data to the pipeline. It
|
|
does typically not have any sink (input) pads.
|
|
|
|
Typical source elements include:
|
|
|
|
- file readers
|
|
- network elements
|
|
- capture elements (video/audio/...)
|
|
- generators (signals/video/audio/...)
|
|
|
|
A source element can operate in three ways:
|
|
|
|
- it is fully seekable, this means that random access can be performed
|
|
on it in an efficient way. (a file reader,...). This also typically
|
|
means that the source is not live.
|
|
|
|
- data can be obtained from it with a variable size. This means that
|
|
the source can give N bytes of data. An example is an audio source.
|
|
A video source always provides the same amount of data (one video
|
|
frame). Note that this is not a fully seekable source.
|
|
|
|
- it is a live source, this means that data arrives when it is ready.
|
|
An example of this is a video or network source.
|
|
|
|
When writing a source, one has to look at how the source can operate to
|
|
decide on the scheduling methods to implement on the source.
|
|
|
|
- fully seekable sources implement a getrange function on the source pad.
|
|
|
|
- sources that can give N bytes but cannot do seeking also implement a
|
|
getrange function but state that they cannot do random access.
|
|
|
|
- sources that are purely live sources implement a task to push out
|
|
data.
|
|
|
|
Any source that has a getrange function must also implement a push based
|
|
scheduling mode. In this mode the source starts a task that gets N bytes
|
|
and pushes them out. Whenever possible, the peer element will select the
|
|
getrange based scheduling method of the source, though.
|
|
|
|
A source with a getrange function must activate itself in the pad activate
|
|
function. This is needed because the downstream peer element will decide
|
|
and activate the source element in its state change function before the
|
|
source's state change function is called.
|
|
|
|
|
|
Source base classes
|
|
-------------------
|
|
|
|
GstBaseSrc:
|
|
|
|
This base class provides an implementation of a random access source and
|
|
is very well suited for file reader like sources.
|
|
|
|
|
|
GstPushSrc:
|
|
|
|
Base class for block-based sources. This class is mostly useful for
|
|
elements that cannot do random access, or at least very slowly. The
|
|
source usually prefers to push out a fixed size buffer.
|
|
|
|
Classes extending this base class will usually be scheduled in a push
|
|
based mode. It the peer accepts to operate without offsets and withing
|
|
the limits of the allowed block size, this class can operate in getrange
|
|
based mode automatically.
|
|
|
|
The subclass should extend the methods from the baseclass in
|
|
addition to the create method. If the source is seekable, it
|
|
needs to override GstBaseSrc::event() in addition to
|
|
GstBaseSrc::is_seekable() in order to retrieve the seek offset,
|
|
which is the offset of the next buffer to be requested.
|
|
|
|
Flushing, scheduling and sync is all handled by this base class.
|
|
|