(a) Schematic view of the proposed omnidirectional cloak with extreme parameters. (b) Designing an omnidirectional cloak based on transformation optics. The space between the big square and the small one in virtual space is transformed into corresponding regions in physical space. These regions are divided into eight triangular segments that can be grouped into two groups: Region I (I′) and Region II (II′). The green lines represent trajectories of rays. The blue square is the hidden area.