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# 1.3 Edge Merging and Adjacency Detection

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.

If the system generating the input for mental ray cannot determine such connectivity, adjacency detection may be used to discover connectivity automatically. One may give a `merge` epsilon within a geometry group which will cause all surfaces in that group to be examined. If any two surfaces approach each other along a trimming curve (or the surface boundary, if the surface is not trimmed) to within the given epsilon, they will be considered adjacent and an appropriate connection will be generated.

Essential to the fast and correct determination of adjacency is the gathering of surfaces into appropriate groups. Obviously, the door of a car should not be considered connected to the body no matter how close the two surfaces approach each other. Moreover, the larger the groups, the more time will be required for adjacency detection.

Next: 1.4 Special Points and Up: 1. Functionality Previous: 1.2 Free-Form Surfaces