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Soon, however, users looked for the ability to model curved surfaces as well. Implementors responded in two different ways. One was to represent simple surfaces exactly and the other was to represent surfaces by interpolating between points. These two quite different approaches are, in general, both required although each is best suited to particular applications. We have found that restricting the available surface types to the four natural quadrics (planes, cylinders, spheres and cones) has benefits which outweigh the loss of versatility.