1. A method of tracking a feature across a sequence of image frames, each said image frame comprising a two-dimensional array of pixel data, said method comprising the steps of: (a) estimating a current feature position in a current frame from at least a previous feature position in a previous frame; (b) extracting feature data from pixel data of said current frame that arc substantially around said current feature position; (c) comparing said feature data with reference data, wherein a difference between said feature data and said reference data that is larger than a first predetermined number indicates that track of said feature has been lost; and (d) updating said reference data periodically with feature data of a plurality of frames.