Re-entrant polygon

Re-entrant polygon

A re-entrant, or concave polygon is one in which at least one interior angle is more than 180 degrees (i.e. a reflex angle). A polygon is re-entrant or concave if there exist two points within the polygon which cannot be connected by a straight line which lies within the polygon.

Alternative definitions

Some authors use "re-entrant" as a synonym for "concave" [http://www.artwork.com/micronic/dxf2mic/des_rul/des_rul.htm#rent] [http://www.artwork.com/pads/polygon.htm] [http://www.linkcad.com/help/5/source/docs/rules/Overview.htm] All such polygons are concave, but not all concave polygons have a boundary that "touches" itself.Some authors define "re-entrant" polygons as polygons with a boundary that intersects itself. [http://www.maths.abdn.ac.uk/~igc/tch/mx4002/notes/node17.html]

All such polygons are concave, but not all polygons that have a boundary that "touches" itself (much less all concave polygons) have a boundary that intersects itself.


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