Figure 4.

Schematic diagram showing the expansion operation on a 2D grid. Expansion of SE₀ yields SE₁, and expansion of SE₁ yields SE₂. The widths of the boxes are twice as long in each dimension after expansion. The grids are shown to relative scale, so SE₁ contains the volume covered by SE₀ but extends it. Likewise, SE₂ extends the area covered by SE₁. This is all done to ensure that any atom in space has a nonzero derivative with respect to the objective function (which is blurred onto this expandable grid as illustrated in Fig. 5).