1. Apparatus for processing image data defining images of a subject object to generate data defining a three-dimensional computer model of the surface shape of the subject object and texture data therefor, the apparatus comprising: a three-dimensional computer model generator operable to process data defining a set of images of a subject object comprising image data defining a plurality of images of the subject object and data defining the imaging positions and directions of the images, to generate data defining a three-dimensional computer model of the surface shape of the subject object having a known position and orientation relative to the images in the set; a model registerer operable to register the three-dimensional computer model of the surface shape of the subject object with a further image of the subject object, the model registerer comprising: a renderer operable to generate image data defining images of the three-dimensional computer model in accordance with a virtual camera; a position controller responsive to user input signals to change the relative position of the virtual camera and three-dimensional computer model; a display data generator operable to generate image data for display by combining at least some of the image data of the three-dimensional computer model generated by the renderer with the image data of the further image to generate composite image data defining images of the three-dimensional computer model overlaid on the further image; and a viewing parameter writer operable to write data to memory defining viewing parameters of the virtual camera as viewing parameters of the further image, thereby defining the relative position and orientation of the three-dimensional computer model and the further image; and a texture data generator operable to generate texture data for the three-dimensional computer model in dependence upon the image data from at least one image in the set and the further image.