1. A method of generating a matching key for an image with said matching key being substantially invariant to rotation, scale and translation, said method comprising the steps of: forming a spatial domain representation of said image that is substantially invariant to translation of said image; transforming rotation and/or scaling in said spatial domain representation into translation to form a transformed image; and forming a representation of said transformed image that is substantially invariant to translation of said transformed image, said representation of said transformed image being said matching key for said image.