OpenGL Interface Implementation
--------------------------------

See `OpenGL Interface` for details on the publically-visible modules.

See `ctypes Wrapper Generation` for details on some of these modules are
generated.

ctypes linkage
==============

Most functions link to libGL.so (Linux), opengl32.dll (Windows) or
OpenGL.framework (OS X).  ``pyglet.gl.lib`` provides some helper types then
imports linker functions for the appropriate platform: one of
``pyglet.gl.lib_agl``, ``pyglet.gl.lib_glx``, ``pyglet.gl.lib_wgl``.

On any platform, the following steps are taken to link each function during
import:

1. Look in the appropriate library (e.g. libGL.so, libGLU.so, opengl32.dll,
   etc.) using ``cdll`` or ``windll``.

2. If not found, call ``wglGetProcAddress`` or ``glxGetProcAddress`` to try to
   resolve the function's address dynamically.  On OS X, skip this step.

3. On Windows, this will fail if the context hasn't been created yet.  Create
   and return a proxy object ``WGLFunctionProxy`` which will try the same
   resolution again when the object is ``__call__``'d.

   The proxy object caches its result so that subsequent calls have only a
   single extra function-call overhead.

4. If the function is still not found (either during import or proxy call),
   the function is replaced with ``MissingFunction`` (defined in
   ``pyglet.gl.lib``), which raises an exception.  The exception message
   details the name of the function, and optionally the name of the extension
   it requires and any alternative functions that can be used.

   The extension required is currently guessed by ``gengl.py`` based on nearby
   ``#ifndef`` declarations, it is occasionally wrong.

   The suggestion list is not currently used, but is intended to be
   implemented such that calling, for example, ``glCreateShader`` on an
   older driver suggests ``glCreateShaderObjectARB``, etc.

To access the linking function, import ``pyglet.gl.lib`` and use one of
``link_AGL``, ``link_GLX``, ``link_WGL``, ``link_GL`` or ``link_GLU``.  This
is what the generated modules do.

Missing extensions
==================

The latest ``glext.h`` on opengl.org and nvidia does not include some recent
extensions listed on the registry.  These must be hand coded into
``pyglet.gl.glext_missing``.  They should be removed when ``glext.h`` is
updated.



