// add in specular relectance contribution to color
                if (ks > 0) {
                    float spec = v.dot(2*lambert*n.x - l.x,
                                       2*lambert*n.y - l.y,
                                       2*lambert*n.z - l.z);
                    if (spec > 0) {
                        spec = ks*((float) Math.pow((double) spec, (double) ns));
                        r += spec*light.ir;
                        g += spec*light.ig;
                        b += spec*light.ib;
                    }
                }
            }
        } // else (directional or point light source)
    } // while (loop over lights)