As described in Section 2.3.3 the particle transport can be neglected for some models, if the surface reaction is not limited by the amount of reactants arriving at the surface. However, it is necessary to check if a surface point is connected to the source, because otherwise no reaction would occur on the surface. Obviously, the surface of voids within a structure is not connected to the outside, and hence the surface velocity must be set equal to zero. Therefore, it is necessary to find all existing voids. Since voids can be produced during deposition or disappear during the etching processes, the geometry must be tested for voids at every time step.
One way to check the geometry for voids is to extract an explicit surface from the implicit surface representation using a technique such as the marching cubes algorithm  and to check the triangulated version of the surface for connectivity. However, since this method is computationally very expensive, it would be convenient to have an efficient method to detect voids directly using the implicit LS surface representation.