1GL_ATI_texture_env_combine3 2http://oss.sgi.com/projects/ogl-sample/registry/ATI/texture_env_combine3.txt 3 GL_MODULATE_ADD_ATI 0x8744 4 GL_MODULATE_SIGNED_ADD_ATI 0x8745 5 GL_MODULATE_SUBTRACT_ATI 0x8746 6