Patent ID: 7536051

Claim:
A method of calibrating a digital pen, which includes a pen tip and a camera, and of mapping a location of a center of a camera-captured image to a location of the pen tip, the method comprising: generating a calibration parameter based on a plurality of images captured by the digital pen's camera by placing the pen in a plurality of different orientations while keeping the pen tip at a substantially fixed location L pentip on a positionally encoded surface, wherein the calibration parameter is generated based on a plurality of iteratively generated estimates of the calibration parameter, wherein the iteratively generated estimates are based on a gradient of an error function of the iteratively generated estimates; using the calibration parameter to map the location of the center of the camera-captured image to the location of the pen tip so that a stroke written by a user on the positionally encoded surface can be recovered at a location within an electronic copy of the positionally encoded surface, wherein the position of the recovered stroke within the electronic copy corresponds to a plurality of locations of the pen tip as the user was writing the stroke on the positionally encoded surface; and finding a fixed point p =( p 1 p 2 1) t , such that g ⁡ ( p ) = 1 2 ⁢ n ⁢ ⁢ ∑ i = 1 n ⁢  λ i ⁢ H i ⁢ p - p _  2 , is minimized, given H i = ( h 11 ( i ) h 12 ( i ) h 13 ( i ) h 21 ( i ) h 22 ( i ) h 23 ( i ) h 31 ( i ) h 32 ( i ) h 33 ( i ) ) , for each of the plurality of captured images.