We prove the tight lower bound 4n - 4 on the size of tangent visibility graphs on n pairwise disjoint bounded obstacles in the euclidean plane, and we give a simple description of the configurations of convex obstacles which realize this lower bound.
Pocchiola, M., & Vegter, G. (1996). Minimal tangent visibility graphs. Computational Geometry: Theory and Applications, 6(5), 303–314. https://doi.org/10.1016/0925-7721(95)00016-X