Re: [boost] Possible Bug in Boost Geometry's Huiller Algorithm

Subject: Re: [boost] Possible Bug in Boost Geometry's Huiller Algorithm
From: Barend Gehrels (barend_at_[hidden])
Date: 2012-02-29 17:19:25

Hi Brian,

>
>   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
>
>   In the apply() function, we have
> if(lon2<  lon1){ E =-E;}
>   which I believe should instead be
>   calculation_type lon;if( lon2<  lon1 ) lon = lon1 - lon2;else lon = lon1 - lon2 + two_pi;if(2* half_pi<  lon ) E =-E;
>   according to Graphics Gems 5, pg 45 - 46. 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.
>   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.
>
>   Can anybody else confirm that this is an issue? Any suggestions for cleanly overriding boost's implementation until this is fixed?
>

Thanks for your report. I will have a look at this and correct it.

Regards, Barend