A method of compressing digital ink input including a sequence of strokes is provided. Each stroke is represented as a series of digital words representing x and y co-ordinates. The compressed digital ink data includes, for each stroke, at least one reference co-ordinate expressed in absolute terms, with subsequent co-ordinates being expressed in terms of an offset from a co-ordinate, estimated using linear prediction, such that a given co-ordinate may be determined on the basis of the previous m co-ordinates as:.alpha..times..times..alpha. ##EQU00001## where .alpha. represents either an x or a y co-ordinate of the digital ink input and c.sub.i are coefficients calculated as: c=A.sup..dagger..alpha. where A.sup..dagger. is the pseudo inverse matrix of sample co-ordinates used for the linear prediction.