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## CGAL::convex_hull_3

### Definition

The function convex_hull_3 computes the convex hull of a given set of three-dimensional points Two versions of this function are available. The first can be used when it is known that the result will be a polyhedron and the second when a degenerate hull may also be possible.

#include <CGAL/convex_hull_3.h>

template <class InputIterator, class Polyhedron_3, class Traits>
void |
convex_hull_3 ( | InputIterator first, InputIterator last, Polyhedron_3& P, Traits ch_traits = Default_traits)
|  | computes the convex hull of the set of points in the range [first, last). The polyhedron P is cleared, then the convex hull is stored in P and the plane equations of each face are computed. Precondition: : There are at least four points in the range [first, last) not all of which are collinear.
template <class InputIterator, class Polyhedron_3, class Traits>
void |
convex_hull_3 ( | InputIterator first, InputIterator last, Object& ch_object, Traits ch_traits = Default_traits)
|  | computes the convex hull of the set of points in the range [first, last). The result, which may be a point, a segment, a triangle, or a polyhedron, is stored in ch_object. When the result is a polyhedron, the plane equations of each face are computed.

### Requirements

Both functions require the following:

1. InputIterator::value_type is equivalent to Traits::Point_3.
2. Traits is a model of the concept ConvexHullTraits_3 . When it is known that the input points are not all coplanar, the types Traits_xy, Traits_yx, and Traits_yz need not be provided. For the purposes of checking the postcondition that the convex hull is valid, Traits should also be a model of the concept IsStronglyConvexTraits_3.

Both functions have an additional requirement for the polyhedron that is to be constructed. For the first version this is that:

- Polyhedron_3 is a model of ConvexHullPolyhedron_3,

and for the second, it is required that

- Traits defines a type Polyhedron_3 that is a model of ConvexHullPolyhedron_3.

The default traits class for both versions of convex_hull_3 is Convex_hull_traits_3<R>,with the representation R determined by InputIterator::value_type.

### See Also

CGAL::convex_hull_incremental_3
CGAL::ch_eddy
CGAL::convex_hull_2

### Implementation

The algorithm implemented by these functions is the quickhull algorithm of Barnard et al. [BDH96].

### Example

The following program computes the convex hull of a set of 250 random points chosen from a sphere of radius 100. It then determines if the resulting hull is a segment or a polyhedron. Notice that the traits class is not necessary in the call to convex_hull_3 but is used in the definition of Polyhedron_3.

    // file: examples/Convex_hull_3/ch_quickhull_3_ex.C

    #include <CGAL/Homogeneous.h>
    #include <CGAL/point_generators_3.h>
    #include <CGAL/copy_n.h>
    #include <CGAL/Convex_hull_traits_3.h>
    #include <CGAL/convex_hull_3.h>
    #include <vector>

    #ifdef CGAL_USE_GMP
    #include <CGAL/Gmpz.h>
    typedef CGAL::Gmpz RT;
    #else
    #include <CGAL/MP_Float.h>
    typedef CGAL::MP_Float RT;
    #endif

    typedef CGAL::Homogeneous<RT>                     K;
    typedef CGAL::Convex_hull_traits_3<K>             Traits;
    typedef Traits::Polyhedron_3                      Polyhedron_3;
    typedef K::Segment_3                              Segment_3;

    // define point creator
    typedef K::Point_3                                Point_3;
    typedef CGAL::Creator_uniform_3<double, Point_3>  PointCreator;

    int main()
    {
      CGAL::Random_points_in_sphere_3<Point_3, PointCreator> gen(100.0);

      // generate 250 points randomly on a sphere of radius 100.0
      // and copy them to a vector
      std::vector<Point_3> points;
      CGAL::copy_n( gen, 250, std::back_inserter(points) );

      // define object to hold convex hull
      CGAL::Object ch_object;

      // compute convex hull
      CGAL::convex_hull_3(points.begin(), points.end(), ch_object);

      // determine what kind of object it is
      Segment_3 segment;
      Polyhedron_3 polyhedron;
      if ( CGAL::assign(segment, ch_object) )
         std::cout << "convex hull is a segment " << std::endl;
      else if ( CGAL::assign (polyhedron, ch_object) )
         std::cout << "convex hull is a polyhedron " << std::endl;
      else
         std::cout << "convex hull error!" << std::endl;

      return 0;
    }

Next: convex_hull_incremental_3

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The CGAL Project . Tue, December 21, 2004 .