Quote Originally Posted by Melterx12 View Post
All games using the Doom3 engine including Doom 3 and quake 4 run on openGL 2.1, because the Doom3 Engine is based on the structure for the Quake 3 Arena Engine, which uses a pre-2.0 version of OpenGL. However DirectX is still needed, for what i am not sure.
As far as I know OpenGL only replaces the DirectX Drawing part.

there are more...
DirectX Input
DirectX Sound etc