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 [7]. 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 epsilon > 0 FlexDraw is NP-complete for instances with O(n(epsilon)) inflexible edges with pairwise distance Omega(n(1-epsilon)) (including the case where they induce a matching). On the other hand, we give an FPT-algorithm with running time O(2(k) . n . T-flow(n)), where T-flow(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.