Exact and approximate orientation selective fixed points of OPM optimization models. (a) Pinwheel-free OS pattern. Diagram shows the position of the wave vector in Fourier space. (b) rPWC with four nonzero wave vectors. (c) Essentially complex planforms (ECPs). The index n indicates the number of nonzero wave vectors. The index i enumerates nonequivalent configurations of wave vectors with the same n, starting with i = 0 for the most anisotropic planform. For n = 3, 5, and 15, there are 2, 4, and 612 different ECPs, respectively. OPM layouts become more irregular with increasing n.