where the reverse condition x ∈ N (n) (y) has been included to ensure that the graph is undirected. Examples of nearest neighbour graphs N 2 and N 3 are shown in Fig. for a 2D grid, where we can see they suffer from several shortcomings. In particular, they are not necessarily connected or move-preserving which leads to the introduction of fictitious local maxima and local minima as can be seen in Fig. .