Curved Objects:

The similar rules and guidelines for b-rep of non-polyhedral objects as cylinders and spheres apply as for polyhedral objects along with the major difference that two types of objects result if closed curved edges or faces exist. As illustrated in Figure, a closed cylindrical face contains one vertex and no edges. The boundary model of a cylinder has three faces (top, bottom and cylindrical face, itself), two vertices, and three edges associating the two vertices. The other "edges" are for visualization purposes, and are called limbs, virtual edges, or silhouette edges. The boundary model of a sphere has one face, one vertex, and no edges. Both of the models satisfy Euler law: F - E + V = 2 for simple polyhedra.

There are direct and indirect schemes for the representation of curved edges. In direct schemes, an edge is shown by a curve equation and ordered end points. In indirect schemes the edge is shown by the intersection of two surfaces, and is preferred.