A method of compressing a sequence of strokes input at an input device of a processing system is provided in which each stroke input at the input device is represented as a series of digital words representing x and y co-ordinates, and a co-ordinate is expressed as an offset from a preceding co-ordinate by combining a residual error value and an estimate calculated from the previous m co-ordinates using the formula: a ~ n = ∑ i = 1 m ⁢ c i ⁢ α n - i where ci are coefficients selected to model characteristics of the input strokes, and α represents either an x or a y co-ordinate.