1. A method of compressing digital ink input including 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: α ~ n = ∑ i = 1 m ⁢ c i ⁢ α n - i where α represents either an x or a y co-ordinate of the digital ink input and c i are coefficients calculated as: 
 c=A † α where A † is the pseudo inverse matrix of sample co-ordinates used for the linear prediction.