#include <CGAL/Min_sphere_d.h>

Definition

An object of the class Min_sphere_d is the unique sphere of smallest volume enclosing a finite (multi)set of points in \( d\)-dimensional Euclidean space \( \E^d\).

For a set \( P\) we denote by \( ms(P)\) the smallest sphere that contains all points of \( P\). \( ms(P)\) can be degenerate, i.e. \( ms(P)=\emptyset\) if \( P=\emptyset\) and \( ms(P)=\{p\}\) if \( P=\{p\}\).

An inclusion-minimal subset \( S\) of \( P\) with \( ms(S)=ms(P)\) is called a support set, the points in \( S\) are the support points. A support set has size at most \( d+1\), and all its points lie on the boundary of \( ms(P)\). In general, neither the support set nor its size are unique.

The algorithm computes a support set \( S\) which remains fixed until the next insert or clear operation.

Please note: This class is (almost) obsolete. The class CGAL::Min_sphere_of_spheres_d<Traits> solves a more general problem and is faster then Min_sphere_d even if used only for points as input. Most importantly, CGAL::Min_sphere_of_spheres_d<Traits> has a specialized implementation for floating-point arithmetic which ensures correct results in a large number of cases (including highly degenerate ones). In contrast, Min_sphere_d is not reliable under floating-point computations. The only advantage of Min_sphere_d over CGAL::Min_sphere_of_spheres_d<Traits> is that the former can deal with points in homogeneous coordinates, in which case the algorithm is division-free. Thus, Min_sphere_d might still be an option in case your input number type cannot (efficiently) divide.

Template Parameters

Traits must be a model of the concept OptimisationDTraits as its template argument.

We provide the models CGAL::Optimisation_d_traits_2, CGAL::Optimisation_d_traits_3 and CGAL::Optimisation_d_traits_d for two-, three-, and \( d\)-dimensional points respectively.

See Also: CGAL::Optimisation_d_traits_2<K,ET,NT>; CGAL::Optimisation_d_traits_3<K,ET,NT>; CGAL::Optimisation_d_traits_d<K,ET,NT>; OptimisationDTraits; CGAL::Min_circle_2<Traits>; CGAL::Min_sphere_of_spheres_d<Traits>; CGAL::Min_annulus_d<Traits>

Implementation

We implement the algorithm of Welzl with move-to-front heuristic [16] for small point sets, combined with a new efficient method for large sets, which is particularly tuned for moderately large dimension ( \( d \leq 20\)) [8]. The creation time is almost always linear in the number of points. Access functions and predicates take constant time, inserting a point might take up to linear time, but substantially less than computing the new smallest enclosing sphere from scratch. The clear operation and the check for validity each take linear time.

Example

File Min_sphere_d/min_sphere_d.cpp

#include <CGAL/Cartesian_d.h>
#include <iostream>
#include <cstdlib>
#include <CGAL/Random.h>
#include <CGAL/Min_sphere_annulus_d_traits_d.h>
#include <CGAL/Min_sphere_d.h>
typedef CGAL::Cartesian_d<double>              K;
typedef CGAL::Min_sphere_annulus_d_traits_d<K> Traits;
typedef CGAL::Min_sphere_d<Traits>             Min_sphere;
typedef K::Point_d                             Point;
const int n = 10;                        // number of points
const int d = 5;                         // dimension of points
int main ()
{
    Point         P[n];                  // n points
    double        coord[d];              // d coordinates
    CGAL::Random  r;                     // random number generator
    for (int i=0; i<n; ++i) {
        for (int j=0; j<d; ++j)
            coord[j] = r.get_double();
        P[i] = Point(d, coord, coord+d); // random point
    }
    Min_sphere  ms (P, P+n);             // smallest enclosing sphere
    CGAL::set_pretty_mode (std::cout);
    std::cout << ms;                     // output the sphere
    return 0;
}

Examples:: Min_sphere_d/min_sphere_d.cpp.

Related Functions
(Note that these are not member functions.)
std::ostream &	operator<< (std::ostream &os, const Min_sphere_d< Traits > &min_sphere)
	writes `min_sphere` to output stream `os`. More...

std::istream &	operator>> (std::istream &is, Min_sphere_d< Traits > min_sphere &)
	reads `min_sphere` from input stream `is`. More...

Types
typedef unspecified_type	FT
	typedef to `Traits::FT`.

typedef unspecified_type	Point
	typedef to `Traits::Point`.

typedef unspecified_type	Point_iterator
	non-mutable model of the STL concept `BidirectionalIterator` with value type `Point`. More...

typedef unspecified_type	Support_point_iterator
	non-mutable model of the STL concept `BidirectionalIterator` with value type `Point`. More...

Creation
	Min_sphere_d (const Traits &traits=Traits())
	creates a variable of type `Min_sphere_d` and initializes it to \( ms(\emptyset)\). More...

template<class InputIterator >
	Min_sphere_d (InputIterator first, InputIterator last, const Traits &traits=Traits())
	creates a variable `min_sphere` of type `Min_sphere_d`. More...

int	number_of_points () const
	returns the number of points of `min_sphere`, i.e. \( \|P\|\).

int	number_of_support_points () const
	returns the number of support points of `min_sphere`, i.e. \( \|S\|\).

Point_iterator	points_begin () const
	returns an iterator referring to the first point of `min_sphere`.

Point_iterator	points_end () const
	returns the corresponding past-the-end iterator.

Support_point_iterator	support_points_begin () const
	returns an iterator referring to the first support point of `min_sphere`.

Support_point_iterator	support_points_end () const
	returns the corresponding past-the-end iterator.

int	ambient_dimension () const
	returns the dimension of the points in \( P\). More...

const Point &	center () const
	returns the center of `min_sphere`. More...

FT	squared_radius () const
	returns the squared radius of `min_sphere`. More...

Predicates
By definition, an empty `Min_sphere_d` has no boundary and no bounded side, i.e. its unbounded side equals the whole space \( \E^d\).
Bounded_side	bounded_side (const Point &p) const
	returns `CGAL::ON_BOUNDED_SIDE`, `CGAL::ON_BOUNDARY`, or `CGAL::ON_UNBOUNDED_SIDE` iff `p` lies properly inside, on the boundary, or properly outside of `min_sphere`, resp. More...

bool	has_on_bounded_side (const Point &p) const
	returns `true`, iff `p` lies properly inside `min_sphere`. More...

bool	has_on_boundary (const Point &p) const
	returns `true`, iff `p` lies on the boundary of `min_sphere`. More...

bool	has_on_unbounded_side (const Point &p) const
	returns `true`, iff `p` lies properly outside of `min_sphere`. More...

bool	is_empty () const
	returns `true`, iff `min_sphere` is empty (this implies degeneracy).

bool	is_degenerate () const
	returns `true`, iff `min_sphere` is degenerate, i.e. if `min_sphere` is empty or equal to a single point, equivalently if the number of support points is less than 2.

Modifiers
void	clear ()
	resets `min_sphere` to \( ms(\emptyset)\).

template<class InputIterator >
void	set (InputIterator first, InputIterator last)
	sets `min_sphere` to the \( ms(P)\), where \( P\) is the set of points in the range [`first`,`last`). More...

void	insert (const Point &p)
	inserts `p` into `min_sphere`. More...

template<class InputIterator >
void	insert (InputIterator first, InputIterator last)
	inserts the points in the range [`first`,`last`) into `min_sphere` and recomputes the smallest enclosing sphere, by calling `insert` for all points in the range. More...

Validity Check
An object `min_sphere` is valid, iff `min_sphere` contains all points of its defining set \( P\), `min_sphere` is the smallest sphere containing its support set \( S\), and \( S\) is minimal, i.e. no support point is redundant. Note Under inexact arithmetic, the result of the validation is not realiable, because the checker itself can suffer from numerical problems.
bool	is_valid (bool verbose=false, int level=0) const
	returns `true`, iff `min_sphere` is valid. More...

Miscellaneous
const Traits &	traits () const
	returns a const reference to the traits class object.

Member Typedef Documentation

template<typename Traits >

typedef unspecified_type CGAL::Min_sphere_d< Traits >::Point_iterator

non-mutable model of the STL concept BidirectionalIterator with value type Point.

Used to access the points used to build the smallest enclosing sphere.

template<typename Traits >

typedef unspecified_type CGAL::Min_sphere_d< Traits >::Support_point_iterator

non-mutable model of the STL concept BidirectionalIterator with value type Point.

Used to access the support points defining the smallest enclosing sphere.

Constructor & Destructor Documentation

template<typename Traits >

CGAL::Min_sphere_d< Traits >::Min_sphere_d ( const Traits & traits = Traits())

creates a variable of type Min_sphere_d and initializes it to \( ms(\emptyset)\).

If the traits parameter is not supplied, the class Traits must provide a default constructor.

template<typename Traits >

template<class InputIterator >

CGAL::Min_sphere_d< Traits >::Min_sphere_d	(	InputIterator	first,
		InputIterator	last,
		const Traits &	traits = `Traits()`
	)

creates a variable min_sphere of type Min_sphere_d.

It is initialized to \( ms(P)\) with \( P\) being the set of points in the range [first,last).

Template Parameters

InputIterator is a model of InputIterator with Point as value type. If the traits parameter is not supplied, the class Traits must provide a default constructor.

Precondition: All points have the same dimension.

Member Function Documentation

template<typename Traits >

int CGAL::Min_sphere_d< Traits >::ambient_dimension ( ) const

returns the dimension of the points in \( P\).

If min_sphere is empty, the ambient dimension is \( -1\).

template<typename Traits >

Bounded_side CGAL::Min_sphere_d< Traits >::bounded_side ( const Point & p) const

returns CGAL::ON_BOUNDED_SIDE, CGAL::ON_BOUNDARY, or CGAL::ON_UNBOUNDED_SIDE iff p lies properly inside, on the boundary, or properly outside of min_sphere, resp.

Precondition: If min_sphere is not empty, the dimension of \( p\) equals ambient_dimension().

template<typename Traits >

const Point& CGAL::Min_sphere_d< Traits >::center ( ) const

returns the center of min_sphere.

Precondition: min_sphere is not empty.

template<typename Traits >

bool CGAL::Min_sphere_d< Traits >::has_on_boundary ( const Point & p) const

returns true, iff p lies on the boundary of min_sphere.

Precondition: if min_sphere is not empty, the dimension of \( p\) equals ambient_dimension().

template<typename Traits >

bool CGAL::Min_sphere_d< Traits >::has_on_bounded_side ( const Point & p) const

returns true, iff p lies properly inside min_sphere.

Precondition: If min_sphere is not empty, the dimension of \( p\) equals ambient_dimension().

template<typename Traits >

bool CGAL::Min_sphere_d< Traits >::has_on_unbounded_side ( const Point & p) const

returns true, iff p lies properly outside of min_sphere.

Precondition: If min_sphere is not empty, the dimension of \( p\) equals ambient_dimension().

template<typename Traits >

void CGAL::Min_sphere_d< Traits >::insert ( const Point & p)

inserts p into min_sphere.

If p lies inside the current sphere, this is a constant-time operation, otherwise it might take longer, but usually substantially less than recomputing the smallest enclosing sphere from scratch.

Precondition: The dimension of p equals ambient_dimension() if min_sphere is not empty.

template<typename Traits >

template<class InputIterator >

void CGAL::Min_sphere_d< Traits >::insert	(	InputIterator	first,
		InputIterator	last
	)

inserts the points in the range [first,last) into min_sphere and recomputes the smallest enclosing sphere, by calling insert for all points in the range.

Template Parameters

InputIterator is a model of InputIterator with Point as value type.

Precondition: All points have the same dimension. If min_sphere is not empty, this dimension must be equal to ambient_dimension().

template<typename Traits >

bool CGAL::Min_sphere_d< Traits >::is_valid	(	bool	verbose = `false`,
		int	level = `0`
	)		const

returns true, iff min_sphere is valid.

If verbose is true, some messages concerning the performed checks are written to standard error stream. The second parameter level is not used, we provide it only for consistency with interfaces of other classes.

template<typename Traits >

template<class InputIterator >

void CGAL::Min_sphere_d< Traits >::set	(	InputIterator	first,
		InputIterator	last
	)

sets min_sphere to the \( ms(P)\), where \( P\) is the set of points in the range [first,last).

Template Parameters

InputIterator is a model of InputIterator with Point as value type.

Precondition: All points have the same dimension.

template<typename Traits >

FT CGAL::Min_sphere_d< Traits >::squared_radius ( ) const

returns the squared radius of min_sphere.

Precondition: min_sphere is not empty.

Friends And Related Function Documentation

template<typename Traits >

std::ostream & operator<<	(	std::ostream &	os,
		const Min_sphere_d< Traits > &	min_sphere
	)

An overload of operator<< must be defined for Point.

template<typename Traits >

std::istream & operator>>	(	std::istream &	is,
		Min_sphere_d< Traits > min_sphere &
	)

An overload of operator>> must be defined for Point

Definition

Related Functions

Types

Creation

Predicates

Modifiers

Validity Check

Miscellaneous

Member Typedef Documentation

Constructor & Destructor Documentation

Member Function Documentation

Friends And Related Function Documentation