A method of compressing a digital ink input including a sequence of strokes is provided in which each stroke is represented as a series of digital words representing x and y co-ordinates, at least one reference co-ordinate of each stroke is expressed in absolute terms, and a co-ordinate other than the at least one reference co-ordinate is expressed as an offset from a preceding co-ordinate. The co-ordinate other than the at least one reference co-ordinate is represented as a combination of an estimate determined from the previous m co-ordinates and a residual error value.