The core OpenGL function calls are the same between Windows
and Linux. So what is the difficulty to support it in Wine ?
Well, there are two different problems :
the interface to the windowing system is different for
each OS. It's called 'GLX' for Linux (well, for X Window)
and 'wgl' for Windows. Thus, one need first to emulate one
(wgl) with the other (GLX).
the calling convention between Windows (the 'Pascal'
convention or 'stdcall') is different from the one used on
Linux (the 'C' convention or 'cdecl'). This means that
each call to an OpenGL function must be 'translated' and
cannot be used directly by the Windows program.
Add to this some brain-dead programs (using GL calls without
setting-up a context or deleting three time the same context)
and you have still some work to do :-)
At GDI level for all pixel format selection routines (ie
choosing if one wants a depth / alpha buffer, the size
of these buffers, ...) and to do the 'page flipping' in
double buffer mode. This is implemented in
dlls/x11drv/opengl.c (all these
functions are part of Wine's graphic driver function
pointer table and thus could be reimplemented if ever Wine
works on another Windowing system than X).
In the OpenGL32.DLL itself for all
other functionalities (context creation / deletion,
querying of extension functions, ...). This is done in
dlls/opengl32/wgl.c.
The thunks are the Wine code that does the calling
convention translation and they are auto-generated by a Perl
script. In Wine's Git tree, these thunks are already
generated for you. Now, if you want to do it yourself, there
is how it all works....
The script is located in dlls/opengl32
and is called make_opengl. It requires
Perl5 to work and takes two arguments :
The first is the path to the OpenGL registry. Now, you
will all ask 'but what is the OpenGL registry ?' :-)
Well, it's part of the OpenGL sample implementation
source tree from SGI (more information at this URL :
http://oss.sgi.com/projects/ogl-sample/.
To summarize, these files contain human-readable but
easily parsed information on ALL OpenGL core functions
and ALL registered extensions (for example the
prototype, the OpenGL version, ...).
the second is the OpenGL version to 'simulate'. This
fixes the list of functions that the Windows application
can link directly to without having to query them from
the OpenGL driver. Windows is based, for now, on OpenGL
1.1, but the thunks that are in the Git tree are
generated for OpenGL 1.2.
This option can have three values:
1.0, 1.1 and
1.2.
This script generates three files :
opengl32.spec gives Wine's linker
the signature of all function in the
OpenGL32.DLL library so that the
application can link them. Only 'core' functions are
listed here.
opengl_norm.c contains all the
thunks for the 'core' functions. Your OpenGL library
must provide ALL the function used in this file as these
are not queried at run time.
opengl_ext.c contains all the
functions that are not part of the 'core' functions.
Contrary to the thunks in
opengl_norm.c, these functions do
not depend at all on what your libGL provides.
In fact, before using one of these thunks, the Windows
program first needs to 'query' the function pointer. At
this point, the corresponding thunk is useless. But as
we first query the same function in libGL and store the
returned function pointer in the thunk, the latter
becomes functional.
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