Boost C++ Libraries: Ticket #7813: Bug in boost::geometry::centroid with trivially small polygons

status changed; resolution set

Barend Gehrels — Wed, 19 Dec 2012 21:29:51 GMT

status new → closed
resolution → wontfix

According to the docs: "Note that the centroid might be located in a hole or outside a polygon, easily."

The centroid is the centre of mass, and can be inside or outside the polygon (also outside of larger polygons).

wuxiaqing2000@… — Wed, 19 Dec 2012 21:49:19 GMT

if you visualize the polygon and point, there is no way that the centroid computed is correct. How can a convex shape without hole have its centroid outside of itself? And it is far away from the shape itself.

status changed; resolution deleted

Barend Gehrels — Wed, 19 Dec 2012 22:22:11 GMT

status closed → reopened
resolution wontfix

I will look to the specific example. The original message was that it was outside, which might happen. Being far outside a convex polygon may not happen.

status changed; resolution set

Barend Gehrels — Sat, 22 Dec 2012 18:02:35 GMT

status reopened → closed
resolution → wontfix

I reproduced the problem. The polygon is indeed very small, near the limits of floating point resolution. The division (used for centroid) by a similar small value results in the centroid being outside the polygon.

If you really want to use results for this resolution, please use ttmath_big. The problem is than not there.

Summary:

centroid (double) is: POINT(139.6527006820726300 35.8585406437098440)

centroid (ttmath) is: POINT(139.64437785388127853881278548429355447 35.85640358447488584474885847218077831)

centroid (sql server) is: POINT (139.64437785388145 35.856403584474876)

So the double-version is indeed a bit off, and indeed outside the convex polygon.