Geometry Boost List: Re: [geometry] convex_hull - 3D possible?

Subject: Re: [geometry] convex_hull - 3D possible?
From: Menelaos Karavelas (menelaos.karavelas_at_[hidden])
Date: 2014-06-05 05:24:56

Hi Oliver.

On 05/06/2014 12:18 μμ, Oliver Weinheimer wrote:
> I am trying to use boost::geometry::convex_hull in 3D.
> The convex hull of a cuboid should be the cuboid itself.
> But i get just 5 instead of 8 points for the convex hull in my example code.
> What is wrong with the code?

I think that convex hull is implemented only for 2D geometries.

> Code:
>
> 	typedef boost::geometry::model::point<double, 3,
> boost::geometry::cs::cartesian> point3D;
> 	typedef boost::geometry::model::multi_point<point3d> points3d;
>
> 	points3d all_points;
> 	boost::geometry::append(all_points,point3d(0,0,0));
> 	boost::geometry::append(all_points,point3d(1,0,0));
> 	boost::geometry::append(all_points,point3d(1,1,0));
> 	boost::geometry::append(all_points,point3d(0,1,0));
> 	boost::geometry::append(all_points,point3d(0,1,1));
> 	boost::geometry::append(all_points,point3d(1,1,1));
> 	boost::geometry::append(all_points,point3d(1,0,1));
> 	boost::geometry::append(all_points,point3d(0,0,1));
>
> 	points3d hull3d;
> 	boost::geometry::convex_hull(all_points, hull3d);
>
> 	using boost::geometry::dsv;
> 	std::cout
> 		<< "all_points: " << dsv(all_points) << std::endl
> 		<< "hull: " << dsv(hull3d) << std::endl;
>
> 	std::cout << "size of all points: " << all_points.size() << std::endl;
> 	std::cout << "size of hull3d: " << hull3d.size() << std::endl;
>
> Output:
>
> 	all_points: ((0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0), (0, 1, 1), (1, 1,
> 1), (1, 0, 1), (0, 0, 1))
> 	hull: ((0, 0, 0), (0, 1, 1), (1, 1, 1), (1, 0, 1), (0, 0, 0))

The 3rd coordinate is basically thrown away. What you get is the convex 
hull of the projections of the points on the xy-plane, represented 
though with your original 3D points.

- m.

>
>
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