1. A method for deriving a representation of an image by processing signals corresponding to the image, the method comprising: deriving a scale-space representation of the image; processing the scale-space representation to detect a plurality of feature points having amplitude extrema; for one or more of the detected plurality of feature points, determining a scale-dependent image region associated with the corresponding feature point, and deriving a representation for each determined, scale-dependent image region, the method comprising selecting a predetermined number of feature points from the detected plurality of feature points, and determining a scale-dependent image region associated with each of the selected feature points, wherein the spatial positions of the selected feature points are distributed across the image, and the spatial position of each selected feature point is separated from the other selected feature points by at least a predetermined distance in the image, by selecting feature points by order of strength with the restriction that a feature point is not selected if it is within an exclusion zone around any of the previously selected features.