Updated OpenGL documentation.

documentation/opengl

@@ -95,7 +95,91 @@ and to run Wine with the '--desktop' option.
 III How it all works
 ====================
 
-(to be done later)
+The core OpenGL function calls are the same between Windows and
+Linux. So what is the difficulty to support it in Wine ? Well, there
+is 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 braindead programs (using GL calls without setting-up
+a context or deleting three time the same context) and you have still
+some work to do :-)
+
+
+III.1 The Windowing system integration
+--------------------------------------
+
+This integration is done at two levels : 
+
+ - 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 'graphics/x11drv/opengl.c' (all these functions are
+   part of Wine's graphic driver function pointer table and thus could
+   be reimplented 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'.
+
+
+III.2 The thunks
+----------------
+
+The thunks are the Wine code that does the calling convention
+translation and they are auto-generated by a Perl script. In Wine's
+CVS 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
+   informations at this URL : http://oss.sgi.com/projects/ogl-sample/). 
+
+   To summarize, these files contains human-readable but easily parsed
+   informations 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 CVS
+   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.
 
 
 
@@ -155,6 +239,32 @@ If you have this, run with --debugmsg +opengl and send me
 (lionel.ulmer@free.fr) the TRACE.
 
 
+IV.5 libopengl32.so is built but it is still not working
+--------------------------------------------------------
+
+This may be caused by some missing functions required by opengl_norm.c
+but that your Linux OpenGL library does not provide.
+
+To check for this, do the following steps :
+
+ - create a dummy .c file :
+
+int main(void) {
+    return 0;
+}
+
+ - try to compile it by linking both libwine and libopengl32 (this
+   command line supposes that you installed the Wine libraries in
+   /usr/local/lib, YMMV) :
+
+gcc dummy.c -L/usr/local/lib -lwine -lopengl32
+
+ - if it works, the problem is somewhere else (and you can send me an
+   email). If not, you could re-generate the thunk files for OpenGL
+   1.1 for example (and send me your OpenGL version so that this
+   problem can be detected at configure time).
+
+
 
                          Lionel Ulmer (lionel.ulmer@free.fr)
-           last modification : 2000/06/12
+                         last modification : 2000/06/13