Simultaneous Visibility Representations of Undirected Pairs of Graphs

05/02/2020

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by   Ben Chugg, et al.

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We consider the problem of determining if a pair of undirected graphs ⟨ G_𝖵, G_𝖧⟩, which share the same vertex set, has a representation using opaque geometric shapes for vertices, and vertical/horizontal visibility between shapes to determine the edges of G_𝖵/G_𝖧. While such a simultaneous visibility representation of two graphs can be determined efficiently if the direction of the required visibility for each edge is provided (and the vertex shapes are sufficiently simple), it was unclear if edge direction is critical for efficiency. We show that the problem is 𝖭𝖯-complete without that information, even for graphs that are only slightly more complex than paths. In addition, we characterize which pairs of paths have simultaneous visibility representations using fixed orientation L-shapes. This narrows the range of possible graph families for which determining simultaneous visibility representation is non-trivial yet not 𝖭𝖯-hard.