a virtual keyboard input system using three - dimensional motion detecting by variable focal length mmal is provided . the compactness of the virtual keyboard input system is accomplished by the variable focal length mmal performing three - dimensional imaging function . since the three - dimensional imaging process occurs in a very short time scale , the system can determine the motion of the input by the user and track the motion of the user in real - time based response . image processor can determine which input is occurred at the moment and the time - dependent profile of the motion itself . the input system gives an output just like a conventional keyboard and a mouse system .