The surface patches that make up the parametric curve surfaces, GM_ParametricCurveSurface (Figure 23), are all continuous families of curves, given by a constructive function of the form:<br/>surface(s,t): [a,b]´[c,d] ®DirectPosition<br/><br/>By fixing the value of either parameter, we have a one-parameter family of curves.<br/>ct(s) = cs(t) = surface(s,t)<br/>