gstreamer/docs/design/part-element-source.txt

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.