Object class identification, verification or object image synthesis

A method of identifying an object class using a model based upon appearance parameters derived by comparing images of objects of different classes, the model including a representation of a probability density function describing a range over which appearance parameters may vary for a given class of object, the model further including a defined relationship between the appearance parameters and the probability density function, wherein the method comprises generating appearance parameters representative of an unknown object, estimating an appropriate probability density function for the unknown object using the defined relationship between the appearance parameters and the probability density function, then iteratively modifying at least some of the appearance parameters within limits determined using the probability density function to identify the object class.