(a)–(b) The scheme of an omnidirectional hexagonal cloak consisting of homogeneous materials. The cloak is divided into several segments, grouped into two types (region I and region II), and in each segment a linear coordinate transformation is applied along axis of its own coordinates. (c) The scheme of a multi-directional polarization-sensitive hexagonal cloak [68]. (d) The scheme of a multi-directional polarization-insensitive hexagonal cloak [71]. (e) A multi-directional, broadband, polarization-sensitive, and large scale practical cloak [68]. (f)–(i) A multi-directional, broadband, polarization-insensitive, and large scale practical cloak [71].