1. A method of tracking a non-rigid object in a plurality of images including: generating a first set of parameters for a parameterised shape model by fitting the parameterised shape model to an object in a first image of the plurality of images, wherein the parameterised shape model is fitted to the object in the first image according to one or more constraints defined independently of the object; and generating a second set of parameters for the parameterised shape model by fitting the parameterised shape model to the object in a second image of the plurality of images, wherein the parameterised shape model is fitted to the object in the second image according to: the one or more constraints defined independently of the object; and the first set of parameters; wherein the first and second sets of parameters define a tracking of the non-rigid object between the first and second images.