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 at least one of rotation and scaling of said image in said spatial domain representation into translation to form a transformed image; and forming a spatial domain representation of said transformed image that is substantially invariant to translation of said transformed image, said spatial domain representation of said transformed image being said matching key from said image, wherein the step of forming said spatial domain representation of said image comprises the sub-steps of: performing a Fourier transform of said image to form a Fourier transformed image; performing a function on the magnitude component of said Fourier transformed image to form an altered Fourier transformed image, said function being commutative within a constant to rotation and scale; and performing an inverse Fourier transform on said altered Fourier transformed image to form said spatial domain representation.