1. A computer system configured for determining shadowing cast onto a bump mapped surface by one or more bumps thereon, where the bump mapped surface is represented by a plurality of polygons each having a plurality of vertices, each vertex defining a point in a tangent plane of the bump mapped surface, where a normal vector at each said vertex is perturbed to define a bump map of the bump mapped surface that includes a plurality of bumps on the surface, the computer system comprising: means for defining a horizon map of the surface including, for each of a plurality of radial directions in the tangent plane around each said vertex, the largest angle between the normal vector and any direct ray of light to the vertex; means for storing the horizon map as a set of texture maps corresponding to the surface on the three-dimensional graphical object; and wherein for each said vertex, each said texture map has a plurality of color channels, each said color channel having encoded therein the largest angle between the normal vector and any direct ray of light to the vertex that does not contact any of the bumps on the surface.