Contributing to PyOpenGL 3.x

The Architecture of PyOpenGL 3.x

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

Access to the all of OpenGL's functionality

Allow access to OpenGL 1.x through 4.x
Access to (all) modern extensions

Backward compatability with PyOpenGL code

Not a fanatic requirement, but most legacy code should run without changes
All legacy code should run with minor changes only
Insulate the PyOpenGL developer from the implementation (ctypes) so that code can continue to work across multiple implementations

Play nicely with others

Alternate Python implementations (PyPy, IronPython)
GUI libraries
Numeric libraries
Threading behaviour
Multimedia libraries

Flexibility and robustness

Graceful degradation in the face of missing functionality
Ease of debugging
Informative error reports

Ease of installation

Portability (note, we are limited to ctypes ported platforms, but that should grow faster than we grow, given that ctypes is now part of Python)
Simplicity of installation process (no hacked up distutils!)
Use of eggs and easy_install
Ability to plug in support for new data-types and the like

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:

ctypes library objects for GL, GLU, GLUT and possibly GLE. How we find these libraries, the names of the libraries, flags needed for loading and the like tends to be system specific
definition of the ctypes FunctionType used for calling functions in the libraries, i.e. what calling convention to use for calling the functions. On Windows, for instance, we have to use windows, rather than C calling conventions.
functions GetCurrentContext() and CurrentContextIsValid() to allow code to retrieve and/or test whether we have a current context. This is used to implement context-specific data-storage. These are normally exposed by the platform's OpenGL implementation
function getExtensionProcedure( name ) to retrieve an OpenGL extension function by name
function getGLUTFontPointer( constant ) to retrieve a void * to a GLUT font, different platforms use very different conventions for these values
flags HAS_DYNAMIC_EXT and EXT_DEFINES_PROTO which tell PyOpenGL 3.x whether the platform has the ability to load dynamic extensions and whether it uses prototype definitions
function safeGetError() to retrieve OpenGL's error state if doing so would be safe (i.e. we have a valid context or the platform can handle having a call to glGetError when there is no valid context)

functions/methods to create/retrieve function pointers from libraries (createBaseFunction and createExtensionFunction), which insulate us from changes in ctypes as well as giving us a convenient place to add functionality to any function.

def createBaseFunction( 
    functionName, dll=OpenGL, 
    resultType=ctypes.c_int, argTypes=(),
    doc = None, argNames = (),
):

and

def createExtensionFunction( 
	functionName, dll=OpenGL,
	resultType=ctypes.c_int, 
	argTypes=(),
	doc = None, argNames = (),
):

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

Autogenerated Wrappers

PyOpenGL 3.x is generated from the official upstream Khronos repositories/definitions in XML format for all GL and EGL entry points. Legacy generators were used to produce the GLE and GLUT extensions, which are mostly static these days.

Regenerating/updating the wrappers

  cd src
  ./xml_generate.py

This produces 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.

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.

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!
...
### END AUTOGENERATED SECTION

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:

pyConverters -- accept (or suppress) incoming Python arguments

Used to convert a Python argument to a type which is compatible with the expected data-type
Can be used to eliminate an argument from the argument list by passing no function, this is useful when we want to imply an argument from the structure of another argument, for instance

cConverters -- pull C-compatible argument out of the Python argument list

Looks at the whole set of Python argument and produces a 1:1 argument list of Python objects for the ctypes call

cResolvers -- take the C-compatible Python objects and turn them into the low-level data-type required by the ctypes call
ctypes call, (with error checking, errors are annotated with the arguments used during the call)
storeValues -- called to store a C-compatible Python object, for instance to prevent garbage collection of an array that is in-use
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.

Converters

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.

ArrayDatatypes

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:

ArrayDatatype

Used for generic array operations

GLclampdArray
GLclampfArray

Clamped floating-point types

GLfloatArray
GLdoubleArray

Un-clamped floating-point types

GLbyteArray
GLcharArray (GLcharARBArray)
GLubyteArray

Character arrays

GLshortArray
GLintArray
GLsizeiArray
GLenumArray

Integer array types

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:

nones.NoneHandler

Supports passing the None object as an array data-pointer, quite limited in its functionality
Implemented in Python
registered as: 'nones'

numpymodule.NumpyHandler, (numarrays.NumarrayHandler, numeric.NumericHandler)

Full data-format handlers for the three Numeric Python implementations
numpy module is implemented in Python using a stable API
Numeric module is implemented in Python using a bit of a hack
Numarray currently requires a C extension and, at the moment, is not built by default
registered as: 'numpy' and 'numeric' ('numarray')

numbers.NumberHandler

Supports passing single Python integers/floats in as array data-types
Does not currently support use for output arguments (since Numeric arrays are currently used for single-value output arrays)
Implemented in Python
registered as: 'numbers'

strings.StringHandler

Supports using Python strings as data-sources, no output support
Implemented in Python (with a bit of a hack in _strings.py)
registered as: 'strings'

ctypesarrays.CtypesArrayHandler, ctypespointers.CtypesPointerHandler

Full data-format handler for ctypes arrays, input-only for pointers
Implemented in Python
registered as: 'ctypesarrays','ctypespointers'

lists.ListHandler

Backup supports for using Python lists/tuples as data-sources if there is no numpy module available
registered as 'lists'

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 _strings.py 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.

Extensions

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.

Contributing to PyOpenGL

Getting the Code