Over the past ten years nonuniform rational B-splines (NURBS) have become the geometry definition of choice for computer aided design and computer aided manufacturing (CAD/CAM) implementers and vendors. Thus NURBS models have become the predominant raw material for gridding practitioners. The attractions to CAD implementers of a relatively simple, unified, homogeneous geometry definition are manifold. Frequently cited is NURBS ability to accurately represent conic sections. This seems to have expanded to a uncritical acceptance of NURBS as an accurate geometric representation.

We demonstrate a number of deficiencies of NURBS. The most significant deficiency is that NURBS is an open system: Most operations on NURBS objects (composition, intersection, projection) do not inherently result in NURBS objects. NURBS implementations rely heavily on fitting and approximation to forcibly close an inherently open system..

DEFICIENCIES OF NURBS IN GEOMETRIC MODELING AND PANELIZATION:

CONCLUSIONS

Difficulties experienced with use of CAD models for volume gridding can be traced to fundamental limitations in geometric modeling with NURBS curves and surfaces. While we realize that gridding practitioners often have no control over the source of their geometry, we feel that it is important that designers at very least be aware of the NURBS' limitations.

ACKNOWLEDGMENT