1. A method of processing data, comprising the steps of: (a) comparing first and second samples of a space, said first and second samples being related by a set of transformations, to provide a set of pairs of signals, each of said pairs of signals corresponding to substantially a same feature in said space in said first and second samples; (b) for each of said pairs of the signals, evaluating residuals corresponding to differences between values of said space at the location of said feature in each said sample; (c) evaluating a first estimate of said set of transformations from said residuals; (d) estimating a tensor field representing orientation of flow in said space, said tensor field covering at least one point in said space; and (e) recalculating, from said first estimate and said tensor field, said residuals for each of said pairs of signals to improve said estimate of said set of transformations.