A method of compressing a digital ink input is disclosed. The digital ink includes a sequence of strokes, each stroke being represented as a series of digital words representing x and y co-ordinates, the compressed digital ink data including, 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 formula (1); where are coefficients dependent on the nature of the digital ink, and .alpha. represents either an x or a y co-ordinate.