Method and apparatus for removing noise at the ends of a stroke caused

Abstract

A method and apparatus are described for processing a series of digital signals representing a stroke of a stylus on a tablet to remove signals at the ends of the stroke caused by retracing, the series of signals corresponding to the X and Y coordinates of a series of points along the direction of travel of the stroke. The signals for each point in the series are compared to the signals of the points adjacent to it to form a second series of signals which contain only those signals corresponding to points having an ordinate value more extreme than the corresponding ordinate value of its adjacent points. The first three points in the second series of signals are then processed to determine the ratio of the distance from the first point to the second point to the distance from the second point to the third point and to determine the size of the angle formed by the three points. The distance ratio is fed into a table-look-up which outputs a reference angle signal which is compared to the processed angle signal. If the processed angle signal is less than the reference angle, the second point in the second series is used as the end point and all points in the first series before that point are discarded as being caused by retracing. In the processed angle is equal to or greater than the reference angle, the first point in the second series is used as the end point. The last three points in the second series are processed in a similar manner to determine the correct end point at the finish of the stroke. If the second series of signals consists of only two points, further processing is not performed since each one of the two points is an end point of the stroke.