# Wine and OpenGL

## What is needed to have OpenGL support in Wine

Basically, if you have a Linux OpenGL ABI compliant libGL ( http://oss.sgi.com/projects/ogl-sample/ABI/) installed on your computer, you should have everything that is needed.

To be more clear, I will detail one step after another what the `configure` script checks.

If, after Wine compiles, OpenGL support is not compiled in, you can always check `config.log` to see which of the following points failed.

### Header files

The needed header files to build OpenGL support in Wine are :

`gl.h:`

- the definition of all OpenGL core functions, types and enumerants

`glx.h:`

- how OpenGL integrates in the X Window environment

`glext.h:`

- the list of all registered OpenGL extensions

The latter file (`glext.h`) is, as of now, not necessary to build Wine. But as this file can be easily obtained from SGI ( http://oss.sgi.com/projects/ogl-sample/ABI/glext.h), and that all OpenGL should provide one, I decided to keep it here.

### OpenGL library itself

To check for the presence of 'libGL' on the system, the script checks if it defines the `glXCreateContext` function.

### glXGetProcAddressARB function

The core of Wine's OpenGL implementation (at least for all extensions) is the `glXGetProcAddressARB` function. Your OpenGL library needs to have this function defined for Wine to be able to support OpenGL.

## How it all works

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 :

1. 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).

2. 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 :-)

### The Windowing system integration

This integration is done at two levels :

1. 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).

2. 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

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 :

1. 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 contain human-readable but easily parsed information on ALL OpenGL core functions and ALL registered extensions (for example the prototype, the OpenGL version, ...).

2. 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 :

1. `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.

2. `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.

3. `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.

## Known problems

### When running an OpenGL application, the screen flickers

Due to restrictions (that do not exist in Windows) on OpenGL contexts, if you want to prevent the screen to flicker when using OpenGL applications (all games are using double-buffered contexts), you need to set the following option in your `~/.wine/config` file in the `[x11drv]` section:

    DesktopDoubleBuffered = Y

and to run Wine in desktop mode.

### Unknown extension error message:

    Extension defined in the OpenGL library but NOT in opengl_ext.c...
    Please report (lionel.ulmer@free.fr) !

This means that the extension requested by the application is found in the libGL used by Linux (ie the call to `glXGetProcAddressARB` returns a non-`NULL` pointer) but that this string was NOT found in Wine's extension registry.

This can come from two causes:

1. The `opengl_ext.c` file is too old and needs to be generated again.

2. Use of obsolete extensions that are not supported anymore by SGI or of 'private' extensions that are not registered. An example of the former are `glMTexCoord2fSGIS` and `glSelectTextureSGIS` as used by Quake 2 (and apparently also by old versions of Half Life). If documentation can be found on these functions, they can be added to Wine's extension set.

If you have this, run with `WINEDEBUG=+opengl` and send me lionel.ulmer@free.fr the TRACE.

### `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 :

1. create a dummy `.c` file :

       int main(void)
       {
           return 0;
       }

2. 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

3. 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).