
722 CHAPTER 23. GRAPH DRAWING AND CARTOGRAPHY
and Sering suggested to use the slope of the maximum-width strip that separates the two
edges in the input drawing. The second trick they applied is to not add all new planarity
constraints before solving the QP, but only in case the solution of the QP actually contains
crossings. For each such crossing, exactly the constraints that forbid it are added to the
QP, and the modified instance is given back to the QP solver. A similar trick was used by
N¨ollenburg and Wolff [NW11] in order to deal with planarity constraints in their MIP for
drawing metro maps, see Section 23.5.2.
Concerning an example, consider ...