gstreamer/README

.. gst-python README
.. This file writen with docutils markup (http://docutils.sourceforge.net/)

About
=====

This is **gst-python**, the Python_ bindings for the GStreamer_ project.  The
bindings provide access to almost all of the GStreamer C API through an object
oriented Python API.

.. _Python: http://www.python.org/
.. _GStreamer: http://www.gstreamer.net/


Requirements
============

* Python_ 2.2
* GStreamer_ 0.6.0
* PyGTK_ 1.99.14

.. _PyGTK: http://www.daa.com.au/~james/pygtk/


Build/Install
=============

For build and install information please refer to the "``INSTALL``" file.
Installation is optional, gst-python can be used from the build directory.  The
quick instructions: build and install PyGTK and GStreamer then build
gst-python::

 $ ./configure && make


Using
=====

You either need to install the package or add the root directory to your
Python path::

 $ export PYTHONPATH=$PYTHONPATH:`pwd`

Try running examples::

 $ cd examples/gstreamer/
 $ python cp.py <input file> <output file>
 $ cmp <input file> <output file>
 $ python vorbisplay.py <a vorbis file>


Documentation
=============

General/API
-----------

The gst-python bindings are directly generated from the GStreamer headers.
Look at the GStreamer documentation_ for general API and programming issues.
In most cases the GStreamer classes and boxed types map directly to Python
classes.  The function-based methods also map onto Python object methods.

.. _documentation: http://www.gstreamer.net/docs/


Divergence From C API
---------------------

Due to the nature of C and Python some of the GStreamer API is handled slightly
different in Python than C.  There are a few of the GStreamer C functions that
are not yet provided in gst-python.  These are mostly related to creating
`Python Elements`_.  A few others remain that return GList* or return values in
their parameters.  These have been wrapped as needed.  Please file a bug_ if
you need one of the unwrapped functions.

API changes:

* ``gst_props_entry_get_type`` is accessed through
  ``PropsEntry.get_props_type()``.  This is due to the ``_get_type`` function
  extention being normally used for ``GType`` access and is inaccessable
  otherwise.

* Special `Pipeline Iteration`_ support through the following functions:

  * ``add_iterate_bin(bin) -> id``: used to iterate a bin with a C idle loop
    callback instead of a Python callback.
  * ``remove_iterate_bin(id)``: used to remove the ``add_iterate_bin``
    idle loop callback id.
  * ``iterate_bin_all(bin)``: releases locks, calls ``gst_bin_iterate``
    until it returns 0, reacquires locks and completes

* `Python Elements`_ support through the following horribly inefficient
  functions:

  * ``Buffer.get_data() -> string``: converts buffer data to a string and
     returns it.
  * ``Buffer.set_data(string)``: sets the buffer data from a string.


Examples
--------

The best documentation right now is the examples in
"``./examples/gstreamer/``".  Read them.


Threads
-------

Threading is a tricky subject for gst-python.  There are a few lock you need to
be aware of:

* GIL

  The CPython interpreter is single threaded.  Code execution in the
  interpreter is protected by a Global Interpreter Lock (GIL).  This means that
  C code can run in other threads in parallel but only one thread will be
  running Python code at any one point.  Most of this is handled internally by
  means of locking and unlocking the GIL at appropriate times.  Callback code
  and other various code paths between Python and C *should* be setup to do
  proper GIL handling.
  
  However, it is possible that you may encounter a situation where proper
  locking is not done.  This is most likely due to calling a wrapper function
  that follows a sequence like this:

  - Python -> wrapper function
  - wrapper function -> C GStreamer function
  - C GStreamer function -> side effect code
  - side effect code -> callback
  - callback -> tries to acquire Python GIL but it's already locked
  - deadlocked...

  This has been fixed for commonly called functions that have side effects
  which are likely to re-enter the interpreter.  It just involves lock/unlock
  around the call to the C gst function.  But doing it for every function could
  have performance issues and, more importantly, is not an automated process.

  Please file a bug_ if you have problems related to this and need other
  functions to be specially handled.

* Gdk lock

  If you are using PyGTK you will have to deal with Gdk locking.  Make sure
  you're holding the Gdk lock while executing Gdk/Gtk calls.  See PyGTK
  documentation and FAQ list for more information.


Pipeline Iteration
------------------

There are a number of ways to iterate pipelines.  ./examples/gstreamer/bps.py
is a small test program to measure the performance in buffers per second of
these various techniques.  Please see the example for how to use these
techniques.

* Bin.iterate() in Python from the gtk idle loop
* gst_bin_iterate() in C from gtk idle loop
* Bin.iterate() in a Python loop
* gst_bin_iterate() in a C loop

The method you chose depends on your application.  The idle loop methods are
slightly slower yet more flexible.  Probably useful for interactive GUI
applications.

The basic loop methods are faster but probably more use for non-interactive
applications.  A variation on these loops would be to also check for a stop
condition which may provide performance increase and some level of control.


Python Elements
---------------

It is possible to write Python subclasses of GstElement.  This support is very
primitive and likely to change.  See "``./examples/gstreamer/rot13.py``" for an
example.


Bugs
====

*Please* submit gst-python bugs, patches, or suggestions to GNOME Bugzilla_,
Product: GStreamer, Component: gst-python.  Thank you.

.. _Bugzilla: http://bugzilla.gnome.org/
.. _bug: `Bugs`_