1. A face classification system comprising: input means for receiving video data representing a video image of a scene in which a face is present; means for locating and extracting the video data representing the face in said scene; transform means for processing the facial data to derive a feature vector representative of the face and which is rotation, scaling, translation and grey level intensity invariant; said transform means comprising means for fitting an outline to the face, means for locating the mid-point between the eyes, and means for producing a feature vector of the face by performing a Fourier-Mellin transformation on the facial data referenced to the mid-point between the eyes; storage means for storing the feature vector; and comparing means for comparing the feature vector of the face in a present video image with a stored feature vector of a face in a previous video image to determine if the face in the present and previous video images is the same.