1. A method of processing image data defining a plurality of images of a subject object recorded at different positions relative thereto and data defining the relative positions of the images, to generate data defining a three-dimensional computer surface shape model of the subject object and texture maps defining texture data for the surface shape model, the method being performed in a processing apparatus and comprising: processing the image data and the data defining the relative positions to generate data defining a polygon mesh representing the surface shape of the subject object; defining a plurality of texture map images, each representing a view of the polygon mesh from a respective viewing direction; processing the polygon mesh and the texture map images to test the visibility of the polygons in the texture map images and to generate, in dependence thereon: data defining the polygons in the mesh for which the visibility in each texture map image is less than a threshold; and data defining texture coordinates in a texture map image for each other polygon in the polygon mesh; generating texture data for each texture map image in dependence upon the image data, and storing the generated data as texture maps for the polygon mesh; and generating texture data for the identified polygons in the mesh for which the visibility in each texture map image is less than the threshold in dependence upon the image data, and storing the generated texture data as at least one further texture map for the polygon mesh, wherein, in the processing to test the visibility of the polygons in the texture map images, each polygon is tested to determine the proportion thereof which is visible in a texture map image, and wherein each polygon is tested to determine the proportion thereof which is visible in a texture map image by; rendering an image of the polygon mesh into the texture map image using Z-buffering to generate pixel values and depth data for each pixel; rendering the polygon to be tested into the texture map image in a first colour without using Z-buffering, thereby changing the existing pixel values but not the existing depth data; and rendering the polygon to be tested into the texture map image in a second colour using Z-buffering, thereby to change the existing pixel values.