1. A method of determining the total ambient occlusion (AO) at a vertex of an object within a frame comprising the steps of: receiving an indicator to produce a computer-generated digital image, the computer-generated digital image to include a first object; determining a location of a vertex of the first object using a computer-generated function; determining a global ambient occlusion AO at the vertex of the first object using a function of a geometry of a set of spheres in a sphere-based model; determining a local ambient occlusion AO at the vertex of the first object using a function of a geometry of a set of triangles in a triangle-based model; determining a total ambient occlusion AO at the vertex of the first object as a function of the local AO and global AO; and, producing the computer-generated image, the shading at the vertex of the first object to be dependent on the total AO.