Orthogonal Graph Drawing with Inflexible Edges

We consider the problem of creating plane orthogonal drawings of 4-planar graphs (planar graphs with maximum degree 4) with constraints on the number of bends per edge. More precisely, we have a flexibility function assigning to each edge e a natural number flex(e), its flexibility. The problem FLEXDRAW asks whether there exists an orthogonal drawing such that each edge e has at most flex(e) bends. It is known that FLEXDRAW is NP-hard if flex(e) = 0 for every edge e [8]. On the other hand, FLEXDRAW can be solved efficiently if flex(e)≥ 1 [2] and is trivial if flex(e)≥ 2 [1] for every edge e. To close the gap between the NP-hardness for flex(e) = 0 and the efficient algorithm for flex(e) ≥ 1, we investigate the computational complexity of FLEXDRAW in case only few edges are inflexible (i.e., have flexibility 0). We show that for any ε > 0 FLEXDRAW is NP-complete for instances with O(nε ) inflexible edges with pairwise distance Ω(n1−ε ) (including the case where they induce a matching). On the other hand, we give an FPT-algorithm with running time O(2k · n ·Tflow(n)), where Tflow(n) is the time necessary to compute a maximum flow in a planar flow network with multiple sources and sinks, and k is the number of inflexible edges having at least one endpoint of degree 4.