Abstract: A computer control includes a control member movably mounted for movement in two dimensions wherein orthogonally related gratings are moved to the corresponding directions to generate pairs of quadrature related signals. Circuitry produces binary signals defining four states for each cycle of quadrature signals to change a corresponding count indicating position of the control member. In another variation patterns each having a plurality of spots with a unique combination of gray levels are sensed by a rectangular array of photosensers to determine the position of the movable member.