A
SourceForge Open-Source project: This
document describes how to get started with
contributing to the OpenGL-ctypes project, the project which is creating the PyOpenGL 3.0.0 release. 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.
OpenGL-ctypes is a re-implementation of the OpenGL bindings for Python. Historically there were two other mainline implementations of OpenGL for Python.
OpenGL-ctypes is intended to become the 3.x release of "PyOpenGL", that is, it will eventually replace the 2.x stream as the "standard" PyOpenGL. However, to get there, it needs developer attention. In particular, it needs some porting work for new architectures, testing and debugging on all architectures, and lots of work on extension development. If you are interested in the rationale for OpenGL-ctypes, see this posting, which outlines why the reimplementation is being undertaken.
OpenGL-ctypes is developed and maintained within the PyOpenGL CVS repository. To check out the current version of OpenGL-ctypes:
cvs -z3 -d:pserver:anonymous@pyopengl.cvs.sourceforge.net:/cvsroot/pyopengl co -P OpenGL-ctypes
You can install the checkout to your path for further development as
follows (from the OpenGL-ctypes checkout directory):
./setup.py develop --install-dir=~/YOUR-WORKING-DIRECTORY-ON-PYTHONPATH-HERE
As of 3.0.0a3 OpenGL-ctypes is dependant on the setuptools
package. You cannot run without the setuptools support, as it is
used to provide the plugin mechanism used by array data-type plugin
mechanism. You likely already have setuptools installed, but if not, download the ez_setup.py script and run it to install the package. You will probably want to install numpy as well, and in case you missed it, ctypes
is a dependency (for Python 2.4 and below), it is declared in the
setuptools dependencies for version 3.0.0a5 and above so that it should
automatically be installed for you when you "develop" your PyOpenGL
working directory.
When you make a change, run cvs
diff on the OpenGL-ctypes
directory to produce a patch file and upload it to the PyOpenGL Patch Tracker as an attachment. I prefer "context" diffs (cvs
diff -c) for contributed
code, as it makes it easier to see where the code fits in.
That said, I'm happy to get code in any readily integrated format. We discuss PyOpenGL development on the PyOpenGL-dev mailing list.
Here are the loose design goals of OpenGL-ctypes:
OpenGL-ctypes is exposing "platform" (Operating System and Hardware) functionality to the Python environment. Differences among the various platforms are abstracted such that porting OpenGL-ctypes 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:
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.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
implementationgetExtensionProcedure(
name ) to retrieve
an OpenGL extension function by namegetGLUTFontPointer(
constant ) to retrieve
a void *
to a GLUT font, different platforms use very
different conventions for these valuesHAS_DYNAMIC_EXT
and EXT_DEFINES_PROTO
which tell OpenGL-ctypes whether the platform has the ability to load
dynamic extensions and whether it uses prototype definitionsdef 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.
There are two major wrapper generator systems available for use with
ctypes. Originally OpenGL-ctypes used the ctypes generator module
(based on GCC-XML) to produce its wrappers. We are switching to
using the pyglet/tools/wraptypes module. Extension
modules are currently created via a regex-based generator, eventually
we'll switch to using wraptypes for that work as well.
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 (gengl.py) is not as advanced/finished as the openglgenerator.py
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/gengl.py module in PyOpenGL's source tree.
ctypes includes a mechanism
based on GCC-XML which will autogenerate
wrappers for many C libraries. OpenGL-ctypes uses an
extended version of this autogenerator (in the src subdirectory of
the CVS
repository, see generateraw.py and openglgenerator.py) 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.
If you want to create new core modules (see below for OpenGL extension modules), you can use the ctypes_codegen module from ctypes svn. This module is dependant on having a very up-to-date (i.e. not yet officially released) gccxml. Windows users can download this gccxml from the ctypes SourceForge download page where it was released with the 0.9.6 ctypes release (note that the release requires the Visual C runtime from VC6, which is not included in the download and may not be present on your machine if you have not installed a VC6-compiled product).
You can check out the ctypeslib project from the Python svn tree like so:
svn co http://svn.python.org/projects/ctypes/trunk ctypes
cd ctypes/ctypeslib
python setup.py install
To create a new PyOpenGL sub-package, such as one to support your platform-native GLX work-alike (e.g. WGL or AGL), you first create an XML description of your library. Here's an example:
python ~/site-packages/ctypes_codegen/h2xml.py /usr/include/GL/glx.h -o glx.xml -c
which produces an XML file in your local directory (here glx.xml). The -c
flag tells GCC-XML to attempt to include preprocessor definitions in
the XML file, which is normally desirable for GL-related systems, as
they tend to have a lot of constants defined in preprocessor directives.
Once we have the XML file, we need to turn it into Python source files. The src/generateraw.py
script in the OpenGL-ctypes source distribution is an example of how to
create new wrappers. Keep in mind that the output here is only
going to include those
functions which are defined on your system (particularly, in the
DLL/.so that you specify with the -l flag). That means that locally unavailable functions may not show up in the resulting files.
Autogeneration has gone through many revisions over the course of the project. The newest version requires a heavily restructured version of the ctypes codegenerator module to work.
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
OpenGL-ctypes 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!
...
### 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.
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 stages in the Wrapper call are as follows:
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.
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.
Perhaps the most complex mechanisms in OpenGL-ctypes are those which implement the array-based operations which allow for using low-level blocks of formatted data to communicate with the OpenGL implementation. OpenGL-ctypes preferred basic array implementation is the (new) numpy reimplementation of the original Numeric Python.
The array handling
functionality provided within OpenGL-ctypes 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 OpenGL-ctypes to provide array-format-specific handling of Python arguments.
Currently we have the following array types defined:
When you are coding new OpenGL-ctypes 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.
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.NoneHandlernumpymodule.NumpyHandler,
(numarrays.NumarrayHandler, numeric.NumericHandler)numbers.NumberHandlerstrings.StringHandlerOpenGL-ctypes 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.
OpenGL-ctypes 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 OpenGL-ctypes. 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.
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.
As with previous versions of PyOpenGL, OpenGL-ctypes tries to follow Python's
Errors should never pass silently.
philosophy, rather than
OpenGL's philosophy of always requiring explicit checks for error
conditions. OpenGL-ctypes 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 )
OpenGL-ctypes 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.
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.
We have included the Python wrapper for the Tk Togl widget in the OpenGL-ctypes package. We do not, however, currently include the Togl widget itself. If you would like to use Togl in your package, please use your system's package manager to install the Togl package (or compile from source). You may have to recompile Python with Tk support as well.
Historically, Togl support was one of the most complex and error-prone aspects of the PyOpenGL installation procedure. Although there are lots of scripts which use Togl from PyOpenGL, we just don't have the personel to maintain it as part of the PyOpenGL project. It would likely be best if we could simple include a redistributable Togl package alongside the OpenGL-ctypes package.
OpenGL-ctypes is still far from finished. Here is a (partial) list of tasks still needing to be completed, I'll add more here as I discover it:
A
SourceForge Open-Source project: