Trimming for subdivision surfaces

Trimming is an important primitive operation in geometric modeling. It is also the root of many numerical and topological problems in modern NURBS based CAGD systems. In this paper we introduce a new method for trimming subdivision surfaces. It is based on the use of combined subdivision schemes to guarantee exact interpolation of trim curves. The latter ensures, for example, that if two surfaces share a trim curve, they will meet exactly at the trim curve. In contrast to traditional approaches to trimming (e.g., for NURBS) we construct a new control mesh with each trim operation. This causes a perturbation of the surface near the trim region, which we control through the use of multiresolution details. These are computed rapidly and at low cost with the help of a novel set of quasi-interpolation operators. We demonstrate our algorithm with a number of examples.