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Subject: Re: [boost] Possible Bug in Boost Geometry's Huiller Algorithm
From: Barend Gehrels (barend_at_[hidden])
Date: 2012-03-03 10:07:51

Hi Brian,

On 28-2-2012 15:48, Brian Ahr wrote:
> Hi Folks.
> I was looking at the implementation of the huiller algorithm in boost, and it seems to be partially incorrect.
> See boost/geometry/strategies/spherical/area_huiller.hpp
> (...)The comments at the top of the C++ file state that the code was derived from code in Graphics Gems 4, though the code was later corrected in Graphics Gems 5.

The comments also say: "The version in Gems didn't account for polygons
crossing the 180 meridian. This version works for (...) for 180 meridian
crossing polygons ... "

So the same bug is apparently corrected (but probably differently).

> I noticed this when trying to calculate areas of polygons on a sphere, where one of the points was (0, 0). In that case, it would always return a negative area, even though the polygon was defined counter-clockwise. Using a point other than (0,0) seemed to alleviate the problem, which got me wondering what was going on.

I cannot reproduce your problem. When I'm doing what you describe, also
using (0,0), I get the correct results. Can you send me a
problem-causing polygon? (preferrably in WKT format).

Thanks, Barend

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