CGAL 6.1 - Bounding Volumes

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CGAL::Min_sphere_of_spheres_d< Traits > Class Template Reference
Reference Manual

`#include <CGAL/Min_sphere_of_spheres_d.h>`

## Definition

template<typename Traits>
class CGAL::Min_sphere_of_spheres_d< Traits >

An object of the class `Min_sphere_of_spheres_d` is a data structure that represents the unique sphere of smallest volume enclosing a finite set of spheres in \( d\)-dimensional Euclidean space \( \E^d\).

For a set \( S\) of spheres we denote by \( ms(S)\) the smallest sphere that contains all spheres of \( S\); we call \( ms(S)\) the minsphere of \( S\). \( ms(S)\) can be degenerate, i.e., \( ms(S)=\emptyset\), if \( S=\emptyset\) and \( ms(S)=\{s\}\), if \( S=\{s\}\). Any sphere in \( S\) may be degenerate, too, i.e., any sphere from \( S\) may be a point. Also, \( S\) may contain several copies of the same sphere.

An inclusion-minimal subset \( R\) of \( S\) with \( ms(R)=ms(S)\) is called a support set for \( ms(S)\); the spheres in \( R\) are the support spheres. A support set has size at most \( d+1\), and all its spheres lie on the boundary of \( ms(S)\). (A sphere \( s'\) is said to lie on the boundary of a sphere \( s\), if \( s'\) is contained in \( s\) and if their boundaries intersect.) In general, the support set is not unique.

The algorithm computes the center and the radius of \( ms(S)\), and finds a support set \( R\) (which remains fixed until the next `insert()`, `clear()` or `set()` operation). We also provide a specialization of the algorithm for the case when the center coordinates and radii of the input spheres are floating-point numbers. This specialized algorithm uses floating-point arithmetic only, is very fast and especially tuned for stability and robustness. Still, its output may be incorrect in some (rare) cases; termination is guaranteed.

When default constructed, an instance of type `Min_sphere_of_spheres_d<Traits>` represents the set \( S=\emptyset\), together with its minsphere \( ms(S)=\emptyset\). You can add spheres to the set \( S\) by calling `insert()`. Querying the minsphere is done by calling the routines `is_empty()`, `radius()` and `center_cartesian_begin()`, among others.

In general, the radius and the Euclidean center coordinates of \( ms(S)\) need not be rational. Consequently, the algorithm computing the exact minsphere will have to deal with algebraic numbers. Fortunately, both the radius and the coordinates of the minsphere are numbers of the form \( a_i+b_i\sqrt{t}\), where \( a_i,b_i,t\in \Q\) and where \( t\ge 0\) is the same for all coordinates and the radius. Thus, the exact minsphere can be described by the number \( t\), which is called the sphere's discriminant, and by \( d+1\) pairs \( (a_i,b_i)\in\Q^2\) (one for the radius and \( d\) for the center coordinates).

Note
  This class (almost) replaces `CGAL::Min_sphere_d<Traits>`, which solves the less general problem of finding the smallest enclosing ball of a set of points. `Min_sphere_of_spheres_d` is faster than `CGAL::Min_sphere_d<Traits>`, and in contrast to the latter provides a specialized implementation for floating-point arithmetic which ensures correct results in a large number of cases (including highly degenerate ones). The only advantage of `CGAL::Min_sphere_d<Traits>` over `Min_sphere_of_spheres_d` is that the former can deal with points in homogeneous coordinates, in which case the algorithm is division-free. Thus, `CGAL::Min_sphere_d<Traits>` might still be an option in case your input number type cannot (efficiently) divide.

Template Parameters

  Traits | must be a model of the concept `MinSphereOfSpheresTraits` as its template argument.

See also
  `CGAL::Min_sphere_d<Traits>`
  `CGAL::Min_circle_2<Traits>`

Implementation

We implement two algorithms, the LP-algorithm and a heuristic [11]. As described in the documentation of concept `MinSphereOfSpheresTraits`, each has its advantages and disadvantages: Our implementation of the LP-algorithm has maximal expected running time \(O(2^d n)\), while the heuristic comes without any complexity guarantee. In particular, the LP-algorithm runs in linear time for fixed dimension \( d\). (These running times hold for the arithmetic model, so they count the number of operations on the number type `FT`.)

On the other hand, the LP-algorithm is, for inexact number types `FT`, much worse at handling degeneracies and should therefore not be used in such a case. (For exact number types `FT`, both methods handle all kinds of degeneracies.)

Currently, we require `Traits::FT` to be either an exact number type or `double` or `float`; other inexact number types are not supported at this time. Also, the current implementation only handles spheres with Cartesian coordinates; homogeneous representation is not supported yet.

Example

File Min_sphere_of_spheres_d/min_sphere_of_spheres_d_d.cpp

// Computes the minsphere of some random spheres.
#include <CGAL/Cartesian_d.h>
#include <CGAL/Random.h>
#include <CGAL/Exact_rational.h>
#include <CGAL/Min_sphere_of_spheres_d.h>
#include <vector>
#include <cassert>
const int N = 1000; // number of spheres
const int D = 3; // dimension of points
const int LOW = 0, HIGH = 10000; // range of coordinates and radii
typedef CGAL::Exact_rational FT;
//typedef double FT;
typedef CGAL::Cartesian_d<FT> K;
typedef CGAL::Min_sphere_of_spheres_d_traits_d<K,FT,D> Traits;
typedef CGAL::Min_sphere_of_spheres_d<Traits> Min_sphere;
typedef K::Point_d Point;
typedef Traits::Sphere Sphere;
int main () {
std::vector<Sphere> S; // n spheres
FT 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_int(LOW,HIGH);
Point p(D,coord,coord+D); // random center...
S.push_back(Sphere(p,r.get_int(LOW,HIGH))); // ...and random radius
}
Min_sphere ms(S.begin(),S.end()); // check in the spheres
assert(ms.is_valid());
}
Exact_rational.h
CGAL::Min_sphere_of_spheres_d_traits_d
The class Min_sphere_of_spheres_d_traits_d<K,FT,Dim,UseSqrt,Algorithm> is a model for concept MinSphe...
Definition: Min_sphere_of_spheres_d_traits_d.h:34
CGAL::Min_sphere_of_spheres_d
An object of the class Min_sphere_of_spheres_d is a data structure that represents the unique sphere ...
Definition: Min_sphere_of_spheres_d.h:102
CGAL::Min_sphere_of_spheres_d::Sphere
unspecified_type Sphere
is a typedef to Traits::Sphere.
Definition: Min_sphere_of_spheres_d.h:111
CGAL::Min_sphere_of_spheres_d::FT
unspecified_type FT
is a typedef to Traits::FT.
Definition: Min_sphere_of_spheres_d.h:116
unspecified_type

Examples
  Min_circle_2/min_circle_2.cpp, Min_sphere_d/min_sphere_3.cpp, Min_sphere_of_spheres_d/benchmark.cpp, Min_sphere_of_spheres_d/min_sphere_of_spheres_d_2.cpp, Min_sphere_of_spheres_d/min_sphere_of_spheres_d_3.cpp, and Min_sphere_of_spheres_d/min_sphere_of_spheres_d_d.cpp.

## Types
typedef unspecified_type | Sphere
| is a typedef to `Traits::Sphere`.
typedef unspecified_type | FT
| is a typedef to `Traits::FT`.
typedef unspecified_type | Result
| is the type of the radius and of the center coordinates of the computed minsphere: When `FT` is an inexact number type (`double`, for instance), then `Result` is simply `FT`.
typedef unspecified_type | Algorithm
| is either `CGAL::LP_algorithm` or `CGAL::Farthest_first_heuristic`.
typedef unspecified_type | Support_iterator
| non-mutable model of the STL concept `BidirectionalIterator` with value type `Sphere`.
typedef unspecified_type | Cartesian_const_iterator
| non-mutable model of the STL concept `BidirectionalIterator` to access the center coordinates of the minsphere.

## Creation
| Min_sphere_of_spheres_d (const Traits &traits=Traits())
| creates a variable of type `Min_sphere_of_spheres_d` and initializes it to \( ms(\emptyset)\).
template<typename InputIterator >
| Min_sphere_of_spheres_d (InputIterator first, InputIterator last, const Traits &traits=Traits())
| creates a variable `minsphere` of type `Min_sphere_of_spheres_d` and inserts (cf.

## Access Functions
Support_iterator | support_begin () const
| returns an iterator referring to the first support sphere of `minsphere`.
Support_iterator | support_end () const
| returns the corresponding past-the-end iterator.
const FT & | discriminant () const
| returns the discriminant of `minsphere`.
Result | radius () const
| returns the radius of `minsphere`.
Cartesian_const_iterator | center_cartesian_begin () const
| returns a const-iterator to the first of the `Traits::D` center coordinates of `minsphere`.
Cartesian_const_iterator | center_cartesian_end () const
| returns the corresponding past-the-end iterator, i.e.

## Predicates
bool | is_empty () const
| returns `true`, iff `minsphere` is empty, i.e. iff \( ms(S)=\emptyset\).

## Modifiers
void | clear ()
| resets `minsphere` to \( ms(\emptyset)\), with \( S:= \emptyset\).
template<class InputIterator >
void | set (InputIterator first, InputIterator last)
| sets `minsphere` to the \( ms(S)\), where \( S\) is the set of spheres in the range [`first`,`last`).
void | insert (const Sphere &s)
| inserts the sphere `s` into the set \( S\) of instance `minsphere`.
template<class InputIterator >
void | insert (InputIterator first, InputIterator last)
| inserts the spheres in the range [`first`,`last`) into the set \( S\) of instance `minsphere`.

## Validity Check
An object `minsphere` is valid, iff `minsphere` contains all spheres of its defining set \( S\), `minsphere` is the smallest sphere containing its support set \( R\), and \( R\) is minimal, i.e., no support sphere is redundant.
bool | is_valid () const
| returns `true`, iff `minsphere` is valid.

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

## Member Typedef Documentation

## ◆ Algorithm

template<typename Traits >

typedef unspecified_type CGAL::Min_sphere_of_spheres_d< Traits >::Algorithm

is either `CGAL::LP_algorithm` or `CGAL::Farthest_first_heuristic`.

As is described in the documentation of concept `MinSphereOfSpheresTraits`, the type `Algorithm` reflects the method which is used to compute the minsphere. (Normally, `Algorithm` coincides with `Traits::Algorithm`. However, if the method `Traits::Algorithm` should not be supported anymore in a future release, then `Algorithm` will have another type.)

## ◆ Result

template<typename Traits >

typedef unspecified_type CGAL::Min_sphere_of_spheres_d< Traits >::Result

is the type of the radius and of the center coordinates of the computed minsphere: When `FT` is an inexact number type (`double`, for instance), then `Result` is simply `FT`.

However, when `FT` is an exact number type, then `Result` is a typedef to a derived class of `std::pair<FT,FT>`; an instance of this type represents the number \( a+b\sqrt{t}\), where \( a\) is the first and \( b\) the second element of the pair and where the number \( t\) is accessed using the member function `discriminant()` of class `Min_sphere_of_spheres_d<Traits>`.

## ◆ Support_iterator

template<typename Traits >

typedef unspecified_type CGAL::Min_sphere_of_spheres_d< Traits >::Support_iterator

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

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

## Constructor & Destructor Documentation

## ◆ Min_sphere_of_spheres_d() [1/2]

template<typename Traits >

CGAL::Min_sphere_of_spheres_d< Traits >::Min_sphere_of_spheres_d | ( | const Traits & | traits = `Traits()` | ) |

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

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

## ◆ Min_sphere_of_spheres_d() [2/2]

template<typename Traits >
template<typename InputIterator >

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

creates a variable `minsphere` of type `Min_sphere_of_spheres_d` and inserts (cf.

`insert()`) the spheres from the range [`first`,`last`).

Template Parameters

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

## Member Function Documentation

## ◆ center_cartesian_begin()

template<typename Traits >

Cartesian_const_iterator CGAL::Min_sphere_of_spheres_d< Traits >::center_cartesian_begin | ( |  | ) | const

returns a const-iterator to the first of the `Traits::D` center coordinates of `minsphere`.

The iterator returns objects of type `Result`. If `FT` is an exact number type, then a center coordinate is represented by a pair \( (a,b)\) describing the real number \( a+b\sqrt{t}\), where \( t\) is the minsphere's discriminant (cf. `discriminant()`).

Precondition
  `minsphere` is not empty.

## ◆ center_cartesian_end()

template<typename Traits >

Cartesian_const_iterator CGAL::Min_sphere_of_spheres_d< Traits >::center_cartesian_end | ( |  | ) | const

returns the corresponding past-the-end iterator, i.e.

\ `center_cartesian_begin()+Traits::D`.

Precondition
  `minsphere` is not empty.

## ◆ discriminant()

template<typename Traits >

const FT & CGAL::Min_sphere_of_spheres_d< Traits >::discriminant | ( |  | ) | const

returns the discriminant of `minsphere`.

This number is undefined when `FT` is an inexact number type. When `FT` is exact, the center coordinates and the radius of the minsphere are numbers of the form \( a+b\sqrt{t}\), where \( t\) is the discriminant of the minsphere as returned by this function.

Precondition
  `minsphere` is not empty, and `FT` is an exact number type.

## ◆ insert()

template<typename Traits >
template<class InputIterator >

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

inserts the spheres in the range [`first`,`last`) into the set \( S\) of instance `minsphere`.

Template Parameters

  InputIterator | is a model of `InputIterator` with `Sphere` as value type.

## ◆ is_valid()

template<typename Traits >

bool CGAL::Min_sphere_of_spheres_d< Traits >::is_valid | ( |  | ) | const

returns `true`, iff `minsphere` is valid.

When `FT` is inexact, this routine always returns `true`.

## ◆ radius()

template<typename Traits >

Result CGAL::Min_sphere_of_spheres_d< Traits >::radius | ( |  | ) | const

returns the radius of `minsphere`.

If `FT` is an exact number type then the radius of the minsphere is the real number \( a+b\sqrt{t}\), where \( t\) is the minsphere's discriminant, \( a\) is the first and \( b\) the second component of the pair returned by `radius()`.

Precondition
  `minsphere` is not empty.

## ◆ set()

template<typename Traits >
template<class InputIterator >

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

sets `minsphere` to the \( ms(S)\), where \( S\) is the set of spheres in the range [`first`,`last`).

Template Parameters

  InputIterator | is a model of `InputIterator` with `Sphere` as value type.