We exhibit two squares, (i, j) and ﹩(i,j+1)﹩, which represent cases N_c = 2 and N_c = 6 in Figure 1. By interpolation, we generate the black arrows, which are the vectors ﹩{\boldsymbol{e}}﹩_ij and ﹩{\boldsymbol{e}}﹩_ij+1. The brown arrows denote the unit normals to ﹩{\boldsymbol{e}}﹩_ij and ﹩{\boldsymbol{e}}﹩_ij + 1; the angle between them is ﹩{\beta }_{i}﹩. We note that β_i is directed following the anticlockwise conventions of our method.