Boost C++ Libraries: Ticket #6443: Convex hull does not cover polygon with zero area

attachment set

cooperch@… — Tue, 24 Jan 2012 17:01:23 GMT

attachment → boost_geom_test_no_multipoint.cpp

c++ code to reproduce problem

component changed; owner set

viboes — Mon, 28 May 2012 17:36:14 GMT

owner set to Barend Gehrels
component None → geometry

Please set the component in the future if you want it to be analyzed.

Barend Gehrels — Fri, 01 Jun 2012 19:39:53 GMT

Thanks for the report - because the component was not set, it did not reach my inbox. However, will now have a look at the problem soon.

Tue, 03 Jul 2012 10:26:51 GMT

Hi. This seems to me to be more of a question on convention followed, than a bug. If a point set S has only two elements (p1 and p2, say) Should the Convex_hull(S) have two edges or one? One edge would mean that the Convex_hull is not a 'closed' polygon. Two edges, on the other hand, would mean that the perimeter calculation is correct.

On a side note, I had to include the #include<boost/geometry/geometries/point_xy.hpp> line to avoid compilation errors in the attached file.

Thu, 05 Dec 2013 18:52:38 GMT

I think this IS a bug according to the mathematical definition of a convex hull of a set of points, c.f. http://en.wikipedia.org/wiki/Convex_hull. The convex hull one point is the point itself; the convex hull of two distinct points are the line segment between the two points.

status changed; resolution set

awulkiew — Sun, 04 Jan 2015 16:12:59 GMT

status new → closed
resolution → fixed

This was fixed in 1.57 as a side effect of fixing https://svn.boost.org/trac/boost/ticket/10562. Since 1.57 a Polygon containing the minimal correct number of Points is returned (3 for open and 4 for closed Polygons) so "correct" area and perimeter can be calculated. Note that according to OGC specification Polygons with area = 0 are invalid.