1. A method of processing image data defining images of a subject object to generate data defining a three-dimensional computer model of the subject object, comprising: calculating the imaging positions and orientations of at least some of the images to generate a plurality of respective registered sets of images, at least one of the sets containing a plurality of images and each other set containing at least one image, and the images within each respective set which contains more than one image having imaging positions and orientations registered to each other; generating a preliminary three-dimensional computer model of the subject object using at least some of the images from a first registered set containing a plurality of images; registering all of the respective registered sets of images by registering the preliminary three-dimensional computer model with each registered set of images other than the first set, the registration with each set being carried out by projecting the preliminary three-dimensional computer model into at least one image in the set; and generating a refined three-dimensional computer model of the subject object using the calculated registration of the different sets.