Untangling a Planar Graph


A straight-line drawing δ of a planar graph G need not be plane but can be made so by untangling it, that is, by moving some of the vertices of G. Let shift(G,δ) denote the minimum number of vertices that need to be moved to untangle δ. We show that shift(G,δ) is NP-hard to compute and to approximate. Our hardness results extend to a version of 1BendPointSetEmbeddability, a well-known graph-drawing problem.

Further we define fix(G,δ)=n−shift(G,δ) to be the maximum number of vertices of a planar n-vertex graph G that can be fixed when untangling δ. We give an algorithm that fixes at least \(\sqrt{((\log n)-1)/\log\log n}\) vertices when untangling a drawing of an n-vertex graph G. If G is outerplanar, the same algorithm fixes at least \(\sqrt{n/2}\) vertices. On the other hand, we construct, for arbitrarily large n, an n-vertex planar graph G and a drawing δ G of G with \(\ensuremath {\mathrm {fix}}(G,\delta_{G})\leq \sqrt{n-2}+1\) and an n-vertex outerplanar graph H and a drawing δ H of H with \(\ensuremath {\mathrm {fix}}(H,\delta_{H})\leq2\sqrt{n-1}+1\) . Thus our algorithm is asymptotically worst-case optimal for outerplanar graphs.


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Corresponding author

Correspondence to Alexander Wolff.

Additional information

This paper is based on two preliminary versions: [7] and [24]. Our work was started at the 9th “Korean” Workshop on Computational Geometry and Geometric Networks organized by A. Wolff and X. Goaoc, July 30–August 4, 2006 in Schloss Dagstuhl, Germany. Further contributions were made at the 2nd Workshop on Graph Drawing and Computational Geometry organized by W. Didimo and G. Liotta, March 11–16, 2007 in Bertinoro, Italy.

J. K. was supported by research project 1M0545 of the Czech Ministry of Education.

Y. O. was supported by GCOE Program “Computationism as a Foundation for the Sciences” and Grant-in-Aid for Scientific Research from Ministry of Education, Science and Culture, Japan, and Japan Society for the Promotion of Science.

C.-S. S. was supported by Research Grant 2008 of the Hankuk University of Foreign Studies.

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Goaoc, X., Kratochvíl, J., Okamoto, Y. et al. Untangling a Planar Graph. Discrete Comput Geom 42, 542–569 (2009). https://doi.org/10.1007/s00454-008-9130-6

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  • Graph drawing
  • Straight-line drawing
  • Planarity
  • NP-hardness
  • Hardness of approximation
  • Moving vertices
  • Untangling
  • Point-set embeddability