Surface boundaries and discontinuities are view-independent and only need to be calculated once per model. Boundaries can be found easily from the surface implementation. As discussed in Section 3.1 and Figure 3.4, not all boundaries should be drawn. I define unshared boundaries to mean those surface boundaries which are not shared by any other surface [18]. Only ``unshared'' boundaries should be drawn, or in the cases where the joint between two surface boundaries changes abruptly. Discontinuities are due to knot multiplicities and are very simple to extract since they fall along isolines.