1. A method of generating a keypoint descriptor for identifying an object in an image or in a sequence of images, the keypoint descriptor being substantially invariant to a transformation of the object in the image, wherein the method is performed using processing apparatus and includes: a) receiving object data representing an object for identification in an image; b) processing said object data to generate at least one basis function representing a feature having undergone at least one transformation or a transformation sequence across several consecutive frames, optionally using transformations that are out of the image plane to recognise objects from multiple views; c) modifying a prototype wavelet function based on said at least one basis function to generate a plurality of modified wavelet functions; d) comparing the plurality of modified wavelet functions with the at least one basis function; e) selecting a modified wavelet function of said plurality of modified wavelet functions based on the comparison of step d); and f) processing an input image or input orientation field according to said selected modified wavelet function to generate the keypoint descriptor.