let null_engine =
        object
                inherit fps_engine
                method set_position _ = ()
                method rise _ = ()
                method rotate _ = ()
                method tilt _ = ()
                method forward _ = ()
                method strafe _ = ()
                method get_state = {position=zero_vector ;  
                                    tilt=0.0 ;
                                                        rotation=0.0}
        end