The connectivity information can be used to ensure that voids do not change over time. If the LS values of all active grid points which do not belong to and are not connected to any neighbor grid point belonging to the region above the surface, are not changed during time integration, the shapes of all voids are maintained. Depending on the orientation of the surface, the region above the surface is represented by the connected component which contains the first or the last grid point in the H-RLE data structure.