Decomposition of two nested grids (top) into three individual grids in the modified convolution method: a fine m = 2 grid (Ω², bottom center), a coarse m = l grid with a central cavity (, bottom right), and a doubled fine grid m = 2 (, bottom left). The density in the latter grid is set to zero everywhere except the original physical size of the fine m = 2 grid (shown with a light blue shade), where the density inherits the values from the fine m = 2 grid, ρ = ρ(Ω²).