1. A method of analysing an image represented by an ordered set of elements (x i ) each having a value comprising, for each of a plurality of root positions (x 0 ), repeatedly performing the steps of: (i) selecting a first plurality of elements (x i ) from the ordered set; (ii) for each selected element (x i ), selecting a second element (u i ) such that the selected second element has a vector position relative to the root position that is scaled by a factor (α) in comparison with the position of the first selected element; (iii) determining whether the selected elements meet a match criterion requiring that the value of each of the first elements is similar to the value of the corresponding second element; and (iv) in the event of a match, updating a similarity score (H) in respect of that root element.