Modified subdivision surfaces with continuous curvature

We present a modification to subdivision surfaces, which guarantees second-order smoothness everywhere in the surface, including extraordinary points. The idea is to blend the limit surface with a low degree polynomial defined over the characteristic map, in the vicinity of each extraordinary point. We demonstrate our method on Catmull-Clark surfaces, but a similar modification can be applied to other schemes as well. The proposed modification to Catmull-Clark is simple to implement and can be applied to quad meshes of arbitrary topological type, even when extraordinary vertices share edges.