1. A method of generating visibility data indicative of the relative visibility of points for at least part of the surface of an object from a viewpoint, said method comprising the steps of: receiving model data defining a plurality of polygons indicative of at least part of the surface of an object for which visibility data is to be determined; generating a representation of said object as viewed from said viewpoint wherein the surface of said object viewed from said viewpoint in said representation is rendered utilizing a determined visibility for the corresponding portion of said object as seen from said viewpoint; and generating visibility data utilizing said generated representation.