Checking Boundary Non-Manifoldness of Solid Objects for Steel Construction

One of the major limitations of most of the currently available commercial CAD systems is the fact that they lack the possibility of representing non-manifold solids. This leads to the consequence that big limitations are imposed on the types of solids representable in a system: for example, neither dangling faces are allowed, nor are solids obtained through one-dimensional soldering of steel profiles. This paper presents an algorithm directly derived from the definition of 2-manifoldness which performs local checks on the manifoldness of a solid around one of its vertices. The algorithm builds the face-vertex incidence matrix around a vertex of the solid, and deducts the number of angles topologically equivalent to the open disk incident into the vertex. The algorithm is based on the face-vertex representation of a solid, but can be easily extended to work on traditional edge-based or wing-edge data structures. The method has been developed for GT4Steel, a new CAD system for steel construction which allows to represent and manipulate nonmanifold objects.