A method of user command or data acquisition in a processing system that allows for analysis of human hand and finger motions. The activities are then interpreted as operations on a physically non-existent input device. The resulting optical user/computer interface complements the visual screen-to-user path with a hand-to-image data acquisition link from the user to the processor unit. It allows for the emulation of different types of manually operated input devices (mouse, keyboard, etc.). Mechanical input units can be replaced by virtual devices, optimized for the current application and for the user's physiology. The speed, simplicity and avoidance of ambiguity of manual data input will be maintained. Sensing of finger motions can generate feedback to reduce the hazard of repetitive strain injury (RSI) syndrome.