GL_ATI_texture_env_combine3 http://www.opengl.org/registry/specs/ATI/texture_env_combine3.txt GL_ATI_texture_env_combine3 GL_MODULATE_ADD_ATI 0x8744 GL_MODULATE_SIGNED_ADD_ATI 0x8745 GL_MODULATE_SUBTRACT_ATI 0x8746