Surfaces are generally approximated independently of each other and small cracks may be visible between them, especially if the approximation is coarse. It may be desirable to use a smaller tolerance for the trimming curves than for the surfaces themselves. If an object is well-modeled, if surfaces meet smoothly along their trimming curves and if the curves are approximated to a high accuracy, the gaps between surfaces become invisible. The ideal solution, however, is to triangulate surfaces consistently along shared edges.
mental ray provides the connect construct for specifying connectivity between surfaces. The two surfaces are named, along with the two trimming curves and the parameter ranges along which they meet. Along such a connection the surfaces will be triangulated consistently resulting in a seamless join.
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