Patent ID: 6950094

Claim:
An optical motion detector circuit for use in a hand held pointing device for a computer system, said device being movable across an illuminated work surface having surface height irregularities, said illumination producing highlights upon surface height irregularities that extend out of the desktop surface and that intercept the illumination and shadows upon surface height irregularities that extend into the desktop surface and whose illumination is blocked by adjacent surface height irregularities that are illuminated, the highlights and shadows forming a pattern that varies as a function of rotations and translations of the device relative to the work surface, the circuit comprising: an array of photo detectors, each photo detector having an output on which an output value is provided; a memory containing a reference frame of digitized photo detector output values that is stored in a reference array of memory locations corresponding to the array of photo detectors and a sample frame of digitized photo detector output values obtained subsequent to the reference frame and that is stored in a sample array of memory locations corresponding to the array of photo detectors; arithmetic comparison circuits coupled to the reference and sample frames in the memory of the optical motion detector circuit for measuring an amount of correlation between the reference frame and the sample frame; wherein a plurality of comparison frames, each being a shifted version of one of the reference frame or the sample frame, is correlated with the other of the reference frame or the sample frame to ascertain motion in directions along first and second axes, the correlation producing motion signals indicative of motion of the device across the work surface in directions along first and second axes and relative to the surface height irregularities of the illuminated portion of the surface; and a circuit for inhibiting production of motion signals when the arithmetic comparison circuits measure an amount of correlation between the reference frame and the sample frame that is less than an amount of correlation corresponding to a predetermined distance from the work surface.