public Color Shade(Ray ray, Vector lights, Vector objects, Color bgnd) {
        // An object shader doesn't really do too much other than
        // supply a few critical bits of geometric information
        // for a surface shader. It must must compute:
        //
        //   1. the point of intersection (p)
        //   2. a unit-length surface normal (n)
        //   3. a unit-length vector towards the ray's origin (v)
        //
        float px, py, pz;
        px = ray.origin.x + ray.t*ray.direction.x;
        py = ray.origin.y + ray.t*ray.direction.y;
        pz = ray.origin.z + ray.t*ray.direction.z;

        Vector3D p, v, n;
        p = new Vector3D(px, py, pz);
        v = new Vector3D(-ray.direction.x, -ray.direction.y, -ray.direction.z);
        n = new Vector3D(px - center.x, py - center.y, pz - center.z);
        n.normalize();

        // The illumination model is applied by the surface's Shade() method
        return surface.Shade(p, n, v, lights, objects, bgnd);
    }
}