Jefri Marzal and  Hong Xie and  Chun Che Fung,  Computing Visibility Subsets in an Orthogonal Polyhedron.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    November 2015, vol. 106(10). 643 - 646
    [viewed 14 May 2021]. Available from: https://publications.waset.org/pdf/10003329.