The model must be watertight, that is closed and labeled with consistently oriented normals, otherwise, the distance field cannot be continuously defined. In this diagram I have colored interior space blue and exterior space white. The sign of the distance field in the green regions is unknown. Fortunately these types of problems are easy to detect with standard surface mesh tools, and often the problems can be solved automatically.

A third restriction on the surface is that it contain no self intersections. Detecting these problems in large models can be very expensive, and these problems are difficult to correct automatically. It is not uncommon for scanned meshes to contain minor self-intersections, so it was necessary to relax this restriction and develop a special distance field cleanup procedure for my initialization algorithm. The details are described in my thesis.