**Reference**:- Oswin Aichholzer, Carmen Cortés, Erik D. Demaine, Vida Dujmović, Jeff Erickson, Henk Meijer, Mark Overmars, Belén Palop, Suneeta Ramaswami, and Godfried T. Toussaint, “Flipturning Polygons”,
*Discrete & Computational Geometry*, volume 28, number 2, August 2002, pages 231–253. **Abstract**:-
A
*flipturn*is an operation that transforms a nonconvex simple polygon into another simple polygon, by rotating a concavity 180 degrees around the midpoint of its bounding convex hull edge. Joss and Shannon proved in 1973 that a sequence of flipturns eventually transforms any simple polygon into a convex polygon. This paper describes several new results about such flipturn sequences. We show that any orthogonal polygon is convexified after at most*n*− 5 arbitrary flipturns, or at most 5 (*n*− 4)/6 well-chosen flipturns, improving the previously best upper bound of (*n*− 1)!/2. We also show that any simple polygon can be convexified by at most*n*^{2}− 4*n*+ 1 flipturns, generalizing earlier results of Ahn*et al.*These bounds depend critically on how degenerate cases are handled; we carefully explore several possibilities. We describe how to maintain both a simple polygon and its convex hull in O(log^{4}*n*) time per flipturn, using a data structure of size O(*n*). We show that although flipturn sequences for the same polygon can have very different lengths, the shape and position of the final convex polygon is the same for all sequences and can be computed in O(*n*log*n*) time. Finally, we demonstrate that finding the longest convexifying flipturn sequence of a simple polygon is NP-hard. **Comments**:- This paper is also available from SpringerLink. An older but longer version is also available as arXiv:cs.CG/0008010 of the Computing Research Repository (CoRR).
**Length**:- The paper is 22 pages.
**Availability**:- The paper is available in PostScript (1788k), gzipped PostScript (276k), and PDF (356k).
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**Related papers**:- JCDCG2000a (Flipturning Polygons)

See also other papers by Erik Demaine.

Last updated January 4, 2022 by Erik Demaine.