Fixed bug 1161 (Setting GL_ACCELERATED_VISUAL to 1 forces software rendering in Windows XP)

 Jesse Anders      2011-03-05 23:30:09 PST

It seems that in Windows XP, setting SDL_GL_ACCELERATED_VISUAL to 1 actually
disables hardware acceleration and puts OpenGL in software mode.

In the source code, the corresponding WGL attribute is first set here:

*iAttr++ = WGL_ACCELERATION_ARB;
*iAttr++ = WGL_FULL_ACCELERATION_ARB;

Later, this code:

if (_this->gl_config.accelerated >= 0) {
    *iAttr++ = WGL_ACCELERATION_ARB;
    *iAttr++ =
        (_this->gl_config.accelerated ? WGL_GENERIC_ACCELERATION_ARB :
         WGL_NO_ACCELERATION_ARB);
}

Sets it again if SDL_GL_ACCELERATED_VISUAL has a value other than the default.

More importantly, the documentation I found states that
WGL_GENERIC_ACCELERATION_ARB asks for an MDC driver, which, although I don't
know much about this topic, doesn't seem like the correct choice here. As
mentioned previously, the end effect is that requesting hardware acceleration
in Windows XP actually forces the renderer into software mode (on my system at
least), which I'm guessing isn't the desired behavior.

src/video/windows/SDL_windowsopengl.c

@@ -409,8 +409,6 @@ WIN_GL_SetupWindow(_THIS, SDL_Window * window)
 
     *iAttr++ = WGL_DRAW_TO_WINDOW_ARB;
     *iAttr++ = GL_TRUE;
-    *iAttr++ = WGL_ACCELERATION_ARB;
-    *iAttr++ = WGL_FULL_ACCELERATION_ARB;
     *iAttr++ = WGL_RED_BITS_ARB;
     *iAttr++ = _this->gl_config.red_size;
     *iAttr++ = WGL_GREEN_BITS_ARB;
@@ -469,12 +467,9 @@ WIN_GL_SetupWindow(_THIS, SDL_Window * window)
         *iAttr++ = _this->gl_config.multisamplesamples;
     }
 
-    if (_this->gl_config.accelerated >= 0) {
-        *iAttr++ = WGL_ACCELERATION_ARB;
-        *iAttr++ =
-            (_this->gl_config.accelerated ? WGL_GENERIC_ACCELERATION_ARB :
-             WGL_NO_ACCELERATION_ARB);
-    }
+    *iAttr++ = WGL_ACCELERATION_ARB;
+    *iAttr++ = (_this->gl_config.accelerated ? WGL_FULL_ACCELERATION_ARB :
+                                               WGL_NO_ACCELERATION_ARB);
 
     *iAttr = 0;