TABLE OF CONTENTS This chapter contains practical information for NURBS implementers. It contains the most important data specifications, namely two IGES formats, and several examples of commonly encountered NURBS curves and surfaces. Several illustrations of rational biquadratic representations of quadrics are given in Figures 15.1, 15.2, and 15.3.
One of the big obstacles in the CAD/CAM world has been the diversity of data format used by different systems. Ford used Coons patches, General Motors used Gordon surfaces, and Mercedes used bicubic splines, to name just a few. In recent years, there has been a trend toward unification many systems now produce geometry that adheres to one common data standard: IGES (initial graphics exchange specification). IGES defines most conceivable geometric objects, from points, lines and planes to transformation matrices; here we are only interested in their NURBS formats. More advanced data-exchange formats are presently under way, the most important being STEP, which aims beyond pure geometry: a product is not simply defined by its shape, but also by variables such as material, surface finish, or functionality. STEP integrates such descriptors. Further down the road we should expect specifications that involve variable dimensions, such as the radii of cylinders in a complex assembly.
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