1. A method of deriving a representation of an image, by processing signals corresponding to the image, comprising: identifying a plurality of feature points in the image, deriving feature descriptors of the feature points, and assigning the feature descriptors to pre-defined centre points, wherein each feature descriptor is assigned to a plurality of centre points, the method further comprising, for each centre point, calculating a vector difference between each feature descriptor assigned to said centre point and said centre point, deriving a value descriptor for each centre point from said calculated vector differences, and deriving the representation of the image from said value descriptors for said centre points.