Surfaces, like planes, cylinders or spheres, are basic primitive surfaces not only for mechanical engineering but also for aesthetic design, world of free-form surfaces, where they are essentially used to answer some functional constraints, like assembling and manufacturing ones, or to achieve specific light effects. The early design steps are characterised by the uncertainty in the definition of the precise geometry and most of the time, product constraints are only partially available. Unfortunately, until now, the insertion of primitive surfaces requires precise curve and surface specifications together with trimming operations, thus imposing that the free-form geometry is recreated each time a modification occurs. In this paper we present a method for the insertion of planar surfaces suitable to handle the uncertainty in the first draft of a product. The approach does not provide effective precise primitive surfaces, but it is able to introduce regions resembling such a behaviour in a free-form surface, without requiring trimming operations, so allowing more efficient shape alternative evaluations.

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