Contributing to PyOpenGL

This document describes how to get started with contributing to the PyOpenGL 3.x release series.  It outlines the basic architecture of the system and how to begin work on the system for a new developer.  It assumes familiarity with Python, Numpy and ctypes.

The History of PyOpenGL 3.x

PyOpenGL 3.x is a re-implementation of the OpenGL bindings for Python.  Historically there were two other mainline implementations of OpenGL for Python.

If you are interested in the rationale for the rewrite, see this posting, which outlines why the reimplementation was undertaken.

Getting the Code

PyOpenGL 3.x is developed and maintained on the LaunchPad site.  To branch the current version of PyOpenGL:

bzr branch lp:pyopengl
bzr branch lp:pyopengl-demo

You will need to add the pyopengl/OpenGL directory to your PYTHONPATH to work directly from the checkout.

When you have changes you would like to work on, either post them to your LaunchPad account and request a merge into PyOpenGL's trunk, or generate a merge-set with:

bzr send -o my-patch.diff

And send it to the PyOpenGL-dev mailing list or Mike Fletcher.

The Architecture of PyOpenGL 3.x

Here are the loose design goals of PyOpenGL 3.x:

Platform Abstraction

PyOpenGL is exposing "platform" (Operating System and Hardware) functionality to the Python environment.  Differences among the various platforms are abstracted such that porting PyOpenGL to a new platform is largely a matter of implementing a small module in the "platform" sub-package.

Each platform gets their own OpenGL.platform.baseplatform.BasePlatform subclass.  OpenGL.platform.baseplatform.BasePlatform classes provide:

New platform implementations are registered in using pkgtools entry points.  We use the sys.platform and (in that order of preference) to decide which entry point to load.

Autogenerated Wrappers

There are two major wrapper generator systems available for use with ctypes.  Originally PyOpenGL 3.x used the ctypes generator module (based on GCC-XML) to produce its wrappers.  We are switching to using the pyglet/tools/wraptypes module, but have not yet got a working version of this generator.  Extension modules are currently created via a regex-based generator, eventually we'll switch to using wraptypes for that work as well.

wraptypes Generator (new method, not finished)

The primary value of wraptypes is that it is easy to install and configure, and allows for parsing headers which are not "native" to the platform on which they are being run.  The wrapper generator ( is not as advanced/finished as the module, but it can generate the platform-specific module quite nicely, and the underlying code is under active development with a focus on GL-related operations.

To run this generator, simple check out pyglet's svn repository and add the tools/wraptypes package to your PYTHONPATH, then run the src/ module in PyOpenGL's source tree.

ctypes Wrapper Generator (old method, broken)

ctypes includes a mechanism based on GCC-XML which will autogenerate wrappers for many C libraries. PyOpenGL 3.x uses an extended version of this autogenerator (in the src subdirectory of the CVS repository, see and to produce the C-style "raw" API for the core libraries.  These are the modules in the OpenGL.raw packages.  If you wish to use ctypes directly with a C-style API it is possible to directly import and use these modules.

The generator also produces "annotations" modules in each of the raw.* packages.  These contain calls which wrap the base functions in size-of-array-aware wrappers.  The constants from the module are also split into a separate module for easier reading (this module is then imported into the main module).

Much of the API for the core libraries can be used as-is from the raw packages, so the main package modules normally import all symbols from their raw.XXX and raw.XXX.annotations module before importing from modules providing customised functionality.

This method has been broken as the ctypes generator has changed since we produced our (extensive diffs).

OpenGL Extension Generation

Unlike the core modules, extension modules are auto-generated using a low-level hack that downloads the OpenGL extension registry's current header file and parses the file with regexes, matching the two definitions so that it can generate a single createExtensionFunction call that has all of the data-type and naming information for that function.

The OpenGL extension definitions in the glext.h header are split into two parts, a macro which retrieves the pointer to the function, and the function itself.  One of the definitions provides the parameter list, the other provides the properly-capitalised name.  We need both to use the function.

A consequence of this approach is that PyOpenGL 3.x can remain up-to-date for OpenGL extensions.  Producing the raw versions of all new extensions is normally just a single command away.

glGet() output arrays are handled specially by the auto-generation.  It produces calls which register constants against array sizes so that the glGet* family of calls can return the correctly-sized array for a given constant.  It does this by combining information from the specification documents and the size specifications stored in the glgetsizes.csv document in the source directory.

The autogeneration process also attempts to copy the "Overview" section of the specification for each module into the docstring of the module, so that users and coders can readily identify the purpose of the module.

The extension modules are written as a single file, with the code for the auto-generated material placed above a comment line which tells you not to edit above it.

### DO NOT EDIT above the line "END AUTOGENERATED SECTION" below!

Customisations of the extension are done after the auto-generated section.  This (single file approach) is done mostly to reduce the number of files in the project and to make it easier to hack on a single extension.

It is expected and encouraged that users will hack on an extension module they care about to make it more Pythonic and then contribute the changes back to the project.

If you remove the autogenerated comment then further autogeneration passes will not process the module, keep in mind, however, that improvements to the extension autogeneration will likely occur over time.

Converters and Wrappers

When a method cannot use the autogenerated ctypes wrapper as-is, we normally fall back to the OpenGL.wrapper.Wrapper class, and the converters defined in the OpenGL.converters module.  The Wrapper class provides a set of argument transformation stages which allow for composing most functions from a common set of simple operations.

This approach will seem familiar to those who have looked at the source code generated by systems such as SWIG.  There you define a set of matching rules which include snippets of code which are composed into the C function being compiled.  Instead of rule-based matching, we use explicit specification.

In some cases it's just easier to code up a custom wrapper function that uses raw ctypes.  We can do so without a problem simply by including the code in the namespace with the appropriate name.  The lazy wrapper (described below) can make this easier.

Wrapper Objects

The stages in the Wrapper call are as follows:

  1. pyConverters -- accept (or suppress) incoming Python arguments
  2. cConverters -- pull C-compatible argument out of the Python argument list
  3. cResolvers -- take the C-compatible Python objects and turn them into the low-level data-type required by the ctypes call
  4. ctypes call, (with error checking, errors are annotated with the arguments used during the call)
  5. storeValues -- called to store a C-compatible Python object, for instance to prevent garbage collection of an array that is in-use
  6. returnValues -- determines what result is required from the wrapped function

Of particular interest is the method wrapper.Wrapper.setOutput which allows you to generate output arrays for a function using a passed-in size tuple, dictionary or function to determine the appropriate size for the array.  See the OpenGL.GL.glget module for examples of usage.


The OpenGL.converters module provides a number of conversion "functions" for use with the wrapper module's Wrapper objects.  The idea of these converter functions is to produce readily re-used code that describes a common idiom in wrapping a function.  The core libraries and extensions then use these idioms to simplify the wrapping of their functions.

Lazy Wrapping

It is often desirable to write a small piece of wrapper code in python and then just call the base operation.  You can use the OpenGL.lazywrapper module's "lazy" wrapper to accomplish this.  It passes the base operation as the first argument when calling the function which is decorated.  Example usage:

@lazy( glGetInfoLogARB )
def glGetInfoLogARB( baseOperation, obj ):
"""Retrieve the program/shader's error messages as a Python string

returns string which is '' if no message
length = int(glGetObjectParameterivARB(obj, GL_INFO_LOG_LENGTH_ARB))
if length > 0:
log = ctypes.create_string_buffer(length)
baseOperation(obj, length, None, log)
return log.value.strip('\000') # null-termination
return ''

Array Handling

While you can do a great deal of work with OpenGL without array operations, Python's OpenGL interfaces are all fastest when you use array (or display-list) techniques to push as much of your rendering work into the platform implementation as possible.  As such, the natural handling of arrays is often a key issue for OpenGL programmers.  Display-lists are deprecated, so their use isn't advisable.

Perhaps the most complex mechanisms in PyOpenGL 3.x are those which implement the array-based operations which allow for using low-level blocks of formatted data to communicate with the OpenGL implementation.  PyOpenGL 3.x preferred basic array implementation is the (new) numpy reimplementation of the original Numeric Python.

The array handling functionality provided within PyOpenGL 3.x is localised to the OpenGL.arrays sub-package.  Within the package, there are two major classes, one (the FormatHandler) which implements an interface to a way of storing data in Python, and another (the ArrayDatatype) which models an OpenGL array format.  The ArrayDatatype classes use FormatHandlers to manipulate array-compatible objects for use in the system.


ArrayDatatype classes provide an API composed primarily of classmethods (that is, methods which are called directly on the class, rather than requiring an instance of the class).  The classmethods are used throughout PyOpenGL 3.x to provide array-format-specific handling of Python arguments.

Currently we have the following array types defined:

When you are coding new PyOpenGL 3.x modules, you should always use the ArrayDatatype interfaces.  These interfaces allow us to code generic operations such that they dispatch to the appropriate format handlers.

Format Handlers

Each format handler is responsible for implementing an API that ArrayDatatypes can use to work with the Python data-format.  Data-formats can support a subset of the API, they only need to support those aspects of the data-format which make sense.  For instance, a write-only array data-type (such as a Python string) doesn't need to implement the zeros method.

At the moment we have the following Format Handlers:

Registering new FormatHandlers

PyOpenGL 3.x uses the simplistic OpenGL.plugins module which allows you to register a plugin instance which defines a class which is to be loaded to handle a given data format.

from OpenGL.plugins import FormatHandler
FormatHandler( 'numpy', 'OpenGL.arrays.numpymodule.NumpyHandler', ['numpy.ndarray'] )

The first parameter is just a name used to refer to the plugin.  The second is the actual class to load.  If there is not third parameter, then the plugin will automatically load.  If there is a value, then the value is a list of module.classname values which will be matched against incoming array-parameter values.

PyOpenGL 3.x delays resolving the FormatHandler set until the last possible moment (i.e. the first call is made which requires a FormatHandler).  Any time before this you can use code like this to declare your application's preference for the handler to be used for creating output argument (this handler must define a zeros(...) method):

from OpenGL.arrays import formathandler
formathandler.FormatHandler.chooseOutput( 'ctypesarrays' )

Where the strings passed are those under which the handler was registered (see previous section).

There are currently no C-level extension modules in PyOpenGL 3.x.  However, we have (disabled) implementations for a few format handlers which are C extensions.  It should be possible to rewrite each of these as pure Python code using ctypes eventually.  They were written as C extensions simply because I had the code handy and I didn't want to have to re-specify the structures for every release of Python or numpy.  The module is an example of a how such a rewrite could be done.  It does a test at run-time to determine the offset required to get a data-pointer from a Python string.

Image Handling

Most of the complexity of Image handling is taken care of by the Array Handling functionality, as most image data-types are simply arrays of data in a given format.  Beyond that, it is necessary to set various OpenGL parameters so that the data-format assumptions of most Python users (e.g. tightly packed image data) will be met.

The OpenGL.images module has the basic functions and data-tables which allow for processing image data (both input and output).  Eventually we will add APIs to support registering new image-types, but for now we have to directly modify the data-tables to register a new data-type.

The OpenGL.GL.images module has implementations of the core OpenGL image-manipulation functions which use the OpenGL.images module.  It can serve as an example of how to use the image handling mechanisms.

Error Handling

As with previous versions of PyOpenGL, PyOpenGL 3.x tries to follow Python's

Errors should never pass silently.

philosophy, rather than OpenGL's philosophy of always requiring explicit checks for error conditions.  PyOpenGL 3.x functions run the function OpenGL.error.glCheckError after each function call.  This function is glBegin/glEnd aware, that is, the glBegin and glEnd functions enable and disable the checking of errors (because error checking doesn't work between those calls).

You can override the error-handler, either to provide your own custom functionality, or to disable checking entirely.  For instance, if you will always have a valid context, you could register the raw glGetError function as the error checker to avoid the overhead of the context-validity checks:

from OpenGL import error
error.ErrorChecker.registerChecker( myAlternateFunction )

PyOpenGL 3.x has a set of errors defined in the OpenGL.error module.  It can also raise standard Python exceptions, such as ValueError or TypeError.  Finally, it can raise ctypes errors when argument conversion fails. (XXX that's sub-optimal, it has implementation details poking out to user code).

Wrapper objects catch OpenGL errors and annotate the error with extra information to make it easier to debug failures during the wrapping process.

Context-specific Data

Because of the way OpenGL and ctypes handle, for instance, pointers, to array data, it is often necessary to ensure that a Python data-structure is retained (i.e. not garbage collected).  This is done by storing the data in an array of data-values that are indexed by a context-specific key.  The functions to provide this functionality are provided by the OpenGL.contextdata module.

The key that is used to index the storage array is provided by the platform module's GetCurrentContext() function.  The current context is used if the context argument is passed in as None.

You can store a new value for the current context with a call to:

def setValue( constant, value, context=None, weak=False ):
"""Set a stored value for the given context"""

You can retrieve the current value (which will return None if there is no currently set value) with a call to:

def getValue( constant, context = None ):
"""Get a stored value for the given constant"""

Lastly, you can delete any currently set value with:

def delValue( constant, context=None ):
"""Delete the specified value for the given context"""

which will return a boolean telling you whether an existing value was found.

Keep in mind that you must either explicitly clear out each stored value or explicitly clear out the stored data with a call to OpenGL.contextdata.cleanupContext when you destroy a rendering context.


The OpenGL extension mechanism is quite well developed, with most new functionality appearing as an extension before it migrates into the OpenGL core.  There are hundreds of registered extensions to OpenGL, with a large fraction of the extensions simply introducing new constants or a few new simple functions.

A few of the largest extensions, such as the GL.ARB.shader_object or GL.ARB.vertex_buffer_object extensions are more involved in their effects on the system.  These extensions require considerable custom code beyond that generated by the auto-generation system.  These wrapper modules are stored in the OpenGL.GL.* hierarchy, while the raw generated APIs are in the OpenGL.raw.* hierarchy.