CGAL 6.1 - Bounding Volumes

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

`#include <CGAL/Approximate_min_ellipsoid_d.h>`

## Definition

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

An object of class `Approximate_min_ellipsoid_d` is an approximation to the ellipsoid of smallest volume enclosing a finite multiset of points in \( d\)-dimensional Euclidean space \( \E^d\), \( d\ge 2\).

An ellipsoid in \( \E^d\) is a Cartesian pointset of the form \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq 0 \}\), where \( E\) is some positive definite matrix from the set \( \mathbb{R}^{d\times d}\), \( e\) is some real \( d\)-vector, and \( \eta\in\mathbb{R}\). A pointset \( P\subseteq \E^d\) is called full-dimensional if its affine hull has dimension \( d\). For a finite, full-dimensional pointset \( P\) we denote by \( (P)\) the smallest ellipsoid that contains all points of \( P\); this ellipsoid exists and is unique.

For a given finite and full-dimensional pointset \( P\subset \E^d\) and a real number \( \epsilon\ge 0\), we say that an ellipsoid \( {\cal E}\subset\E^d\) is an \( (1+\epsilon)\)-appoximation to \( (P)\) if \( P\subset {\cal E}\) and \( ({\cal E}) \leq (1+\epsilon) ((P))\). In other words, an \( (1+\epsilon)\)-approximation to \( (P)\) is an enclosing ellipsoid whose volume is by at most a factor of \( 1+\epsilon\) larger than the volume of the smallest enclosing ellipsoid of \( P\).

Given this notation, an object of class `Approximate_min_ellipsoid_d` represents an \( (1+\epsilon)\)-approximation to \( (P)\) for a given finite and full-dimensional multiset of points \( P\subset\E^d\) and a real constant \( \epsilon>0\).A multiset is a set where elements may have multiplicity greater than \( 1\). When an `Approximate_min_ellipsoid_d<Traits>` object is constructed, an iterator over the points \( P\) and the number \( \epsilon\) have to be specified; the number \( \epsilon\) defines the desired approximation ratio \( 1+\epsilon\). The underlying algorithm will then try to compute an \( (1+\epsilon)\)-approximation to \( (P)\), and one of the following two cases takes place.

- The algorithm determines that \( P\) is not full-dimensional (see `is_full_dimensional()` below).

  Important note: due to rounding errors, the algorithm cannot in all cases decide correctly whether \( P\) is full-dimensional or not. If `is_full_dimensional()` returns `false`, the points lie in such a "thin" subspace of \( \E^d\) that the algorithm is incapable of computing an approximation to \( (P)\). More precisely, if `is_full_dimensional()` returns `false`, there exist two parallel hyperplanes in \( \E^d\) with the points \( P\) in between so that the distance \( \delta\) between the hyperplanes is very small, possible zero. (If \( \delta=0\) then \( P\) is not full-dimensional.)

  If \( P\) is not full-dimensional, linear algebra techniques should be used to determine an affine subspace \( S\) of \( \E^d\) that contains the points \( P\) as a (w.r.t. \( S\)) full-dimensional pointset; once \( S\) is determined, the algorithm can be invoked again to compute an approximation to (the lower-dimensional) \( (P)\) in \( S\). Since `is_full_dimensional()` might (due to rounding errors, see above) return `false` even though \( P\) is full-dimensional, the lower-dimensional subspace \( S\) containing \( P\) need not exist. Therefore, it might be more advisable to fit a hyperplane \( H\) through the pointset \( P\), project \( P\) onto this affine subspace \( H\), and compute an approximation to the minimum-volume enclosing ellipsoid of the projected points within \( H\); the fitting can be done for instance using the `linear_least_squares_fitting()` function from the CGAL package `Principal_component_analysis`.

- The algorithm determines that \( P\) is full-dimensional. In this case, it provides an approximation \( {\cal E}\) to \( (P)\), but depending on the input problem (i.e., on the pair \( (P,\epsilon)\)), it may not have achieved the desired approximation ratio but merely some worse approximation ratio \( 1+\epsilon'>1+\epsilon\). The achieved approximation ratio \( 1+\epsilon'\) can be queried using `achieved_epsilon()`, which returns \( \epsilon'\). The ellipsoid \( {\cal E}\) itself can be queried via the methods `defining_matrix()`, `defining_vector()`, and `defining_scalar()`.

The ellipsoid \( {\cal E}\) computed by the algorithm satisfies the inclusions

\[ \frac{1}{(1+\epsilon')d} {\cal E} \subseteq \mathop{\rm conv}\nolimits(P) \subseteq {\cal E} \]

where \( f {\cal E}\) denotes the ellipsoid \( {\cal E}\) scaled by the factor \( f\in\mathbb{R}^+\) with respect to its center, and where \( \mathop{\rm conv}\nolimits(A)\) denotes the convex hull of a pointset \( A\subset \E^d\).

The underlying algorithm can cope with all kinds of inputs (multisets \( P\), \( \epsilon\in[0,\infty)\)) and terminates in all cases. There is, however, no guarantee that any desired approximation ratio is actually achieved; the performance of the algorithm in this respect highly depends on the input pointset. Values of at least \( 0.01\) for \( \epsilon\) are usually handled without problems.

Internally, the algorithm represents the input points' Cartesian coordinates as `double`'s. For this conversion to work, the input point coordinates must be convertible to `double`. Also, in order to compute the achieved epsilon \( \epsilon'\) mentioned above, the algorithm requires a number type `ET` that provides exact arithmetic. (Both these aspects are discussed in the documentation of the concept `ApproximateMinEllipsoid_d_Traits_d`.)

Template Parameters

  Traits | must be a model for `ApproximateMinEllipsoid_d_Traits_d`.

We provide the model `CGAL::Approximate_min_ellipsoid_d_traits_d<K>` using the \( d\)-dimensional CGAL kernel; the models `CGAL::Approximate_min_ellipsoid_d_traits_2<K>` and `CGAL::Approximate_min_ellipsoid_d_traits_3<K>` are for use with the \( 2\)- and \( 3\)-dimensional CGAL kernel, respectively.

See also
  `CGAL::Min_ellipse_2<Traits>`

Implementation

We implement Khachyian's algorithm for rounding polytopes [10]. Internally, we use `double`-arithmetic and (initially a single) Cholesky-decomposition. The algorithm's running time is \(O(nd^2(\epsilon^{-1}+\ln d + \ln\ln(n)))\), where \( n=|P|\) and \( 1+\epsilon\) is the desired approximation ratio.

Example

To illustrate the usage of `Approximate_min_ellipsoid_d` we give two examples in 2D. The first program generates a random set \( P\subset\E^2\) and outputs the points and a \( 1.01\)-approximation of \( (P)\) as an EPS-file, which you can view using `gv`, for instance. (In both examples you can change the variables `n` and `d` to experiment with the code.)

File Approximate_min_ellipsoid_d/ellipsoid.cpp

#include <CGAL/Cartesian_d.h>
#include <CGAL/MP_Float.h>
#include <CGAL/point_generators_d.h>
#include <CGAL/Approximate_min_ellipsoid_d.h>
#include <CGAL/Approximate_min_ellipsoid_d_traits_d.h>
#include <vector>
#include <iostream>
typedef CGAL::Cartesian_d<double> Kernel;
typedef CGAL::MP_Float ET;
typedef CGAL::Approximate_min_ellipsoid_d_traits_d<Kernel, ET> Traits;
typedef Traits::Point Point;
typedef std::vector<Point> Point_list;
typedef CGAL::Approximate_min_ellipsoid_d<Traits> AME;
int main()
{
const int n = 1000; // number of points
const int d = 2; // dimension
const double eps = 0.01; // approximation ratio is (1+eps)
// create a set of random points:
Point_list P;
CGAL::Random_points_in_cube_d<Point> rpg(d,100.0);
for (int i = 0; i < n; ++i) {
P.push_back(*rpg);
++rpg;
}
// compute approximation:
Traits traits;
AME ame(eps, P.begin(), P.end(), traits);
// write EPS file:
if (ame.is_full_dimensional() && d == 2)
ame.write_eps("example.eps");
// output center coordinates:
std::cout << "Cartesian center coordinates: ";
for (AME::Center_coordinate_iterator c_it = ame.center_cartesian_begin();
c_it != ame.center_cartesian_end();
++c_it)
std::cout << *c_it << ' ';
std::cout << ".\n";
if (d == 2 || d == 3) {
// output axes:
AME::Axes_lengths_iterator axes = ame.axes_lengths_begin();
for (int i = 0; i < d; ++i) {
std::cout << "Semiaxis " << i << " has length " << *axes++ << "\n"
<< "and Cartesian coordinates ";
for (AME::Axes_direction_coordinate_iterator
d_it = ame.axis_direction_cartesian_begin(i);
d_it != ame.axis_direction_cartesian_end(i); ++d_it)
std::cout << *d_it << ' ';
std::cout << ".\n";
}
}
}
MP_Float.h
CGAL::Approximate_min_ellipsoid_d
An object of class Approximate_min_ellipsoid_d is an approximation to the ellipsoid of smallest volum...
Definition: Approximate_min_ellipsoid_d.h:144
CGAL::Approximate_min_ellipsoid_d::Point
unspecified_type Point
typedef Traits::Point Point
Definition: Approximate_min_ellipsoid_d.h:164
CGAL::Approximate_min_ellipsoid_d::traits
const Traits & traits() const
returns a const reference to the traits class object.
CGAL::Approximate_min_ellipsoid_d::ET
unspecified_type ET
typedef Traits::ET ET (which is an exact number type used for exact computation like for example in a...
Definition: Approximate_min_ellipsoid_d.h:159
CGAL::MP_Float
Kernel
CGAL::Approximate_min_ellipsoid_d_traits_d
The class Approximate_min_ellipsoid_d_traits_d is a traits class for CGAL::Approximate_min_ellipsoid_...
Definition: Approximate_min_ellipsoid_d_traits_d.h:29

The second program outputs the approximation in a format suitable for display in Maplesoft's Maple.

File Approximate_min_ellipsoid_d/ellipsoid_for_maple.cpp

// Usage: ./maple_example > maple.text
// Then enter in Maple 'read "maple.text";'.
#include <CGAL/Cartesian_d.h>
#include <CGAL/MP_Float.h>
#include <CGAL/point_generators_d.h>
#include <CGAL/Approximate_min_ellipsoid_d.h>
#include <CGAL/Approximate_min_ellipsoid_d_traits_d.h>
#include <vector>
#include <iostream>
#include <iomanip>
typedef CGAL::Cartesian_d<double> Kernel;
typedef CGAL::MP_Float ET;
typedef CGAL::Approximate_min_ellipsoid_d_traits_d<Kernel, ET> Traits;
typedef Traits::Point Point;
typedef std::vector<Point> Point_list;
typedef CGAL::Approximate_min_ellipsoid_d<Traits> AME;
int main()
{
const int n = 100; // number of points
const int d = 2; // dimension
const double eps = 0.01; // approximation ratio is (1+eps)
// create a set of random points:
Point_list P;
CGAL::Random_points_in_cube_d<Point> rpg(d,1.0);
for (int i = 0; i < n; ++i) {
P.push_back(*rpg);
++rpg;
}
// compute approximation:
Traits traits;
AME mel(eps, P.begin(), P.end(), traits);
// output for Maple:
if (mel.is_full_dimensional() && d == 2) {
const double alpha = (1+mel.achieved_epsilon())*(d+1);
// output points:
using std::cout;
cout << "restart;\n"
<< "with(LinearAlgebra):\n"
<< "with(plottools):\n"
<< "n:= " << n << ":\n"
<< "P:= Matrix(" << d << "," << n << "):\n";
for (int i=0; i<n; ++i)
for (int j=0; j<d; ++j)
cout << "P[" << j+1 << "," << i+1 << "] := "
<< std::setiosflags(std::ios::scientific)
<< std::setprecision(20) << P[i][j] << ":\n";
cout << "\n";
// output defining equation:
cout << "Mp:= Matrix([\n";
for (int i=0; i<d; ++i) {
cout << " [";
for (int j=0; j<d; ++j) {
cout << mel.defining_matrix(i,j)/alpha;
if (j<d-1)
cout << ",";
}
cout << "]";
if (i<d-1)
cout << ",";
cout << "\n";
}
cout << "]);\n" << "mp:= Vector([";
for (int i=0; i<d; ++i) {
cout << mel.defining_vector(i)/alpha;
if (i<d-1)
cout << ",";
}
cout << "]);\n"
<< "eta:= " << (mel.defining_scalar()/alpha-1.0) << ";\n"
<< "v:= Vector([x,y]):\n"
<< "e:= Transpose(v).Mp.v+Transpose(v).mp+eta;\n"
<< "plots[display]({seq(point([P[1,i],P[2,i]]),i=1..n),\n"
<< " plots[implicitplot](e,x=-5..5,y=-5..5,numpoints=10000)},\n"
<< " scaling=CONSTRAINED);\n";
}
}

Note
  This class requires the Eigen library.

Examples
  Approximate_min_ellipsoid_d/ellipsoid.cpp, and Approximate_min_ellipsoid_d/ellipsoid_for_maple.cpp.

## Types
typedef unspecified_type | FT
| `typedef Traits::FT FT` (which is always a typedef to `double`).
typedef unspecified_type | ET
| `typedef Traits::ET ET` (which is an exact number type used for exact computation like for example in `achieved_epsilon()`).
typedef unspecified_type | Point
| `typedef Traits::Point Point`
typedef unspecified_type | Cartesian_const_iterator
| `typedef Traits::Cartesian_const_iterator Cartesian_const_iterator`
typedef unspecified_type | Center_coordinate_iterator
| A model of STL concept `RandomAccessIterator` with value type `double` that is used to iterate over the Cartesian center coordinates of the computed ellipsoid, see `center_cartesian_begin()`.
typedef unspecified_type | Axes_lengths_iterator
| A model of STL concept `RandomAccessIterator` with value type `double` that is used to iterate over the lengths of the semiaxes of the computed ellipsoid, see `axes_lengths_begin()`.
typedef unspecified_type | Axes_direction_coordinate_iterator
| A model of STL concept `RandomAccessIterator` with value type `double` that is used to iterate over the Cartesian coordinates of the direction of a fixed axis of the computed ellipsoid, see `axis_direction_cartesian_begin()`.

## Creation
An object of type `Approximate_min_ellipsoid_d` can be created from an arbitrary point set \( P\) and some nonnegative `double` value `eps`.
template<class Iterator >
| Approximate_min_ellipsoid_d (double eps, Iterator first, Iterator last, const Traits &traits=Traits())
| initializes `ame` to an \( (1+\epsilon)\)-approximation of \( (P)\) with \( P\) being the set of points in the range [`first`,`last`).

## Access Functions
The following methods can be used to query the achieved approximation ratio \( 1+\epsilon'\) and the computed ellipsoid \( {\cal E} = \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq 0 \}\). The methods `defining_matrix()`, `defining_vector()`, and `defining_scalar()` do not return \( E\), \( e\), and \( \eta\) directly but yield multiples of these quantities that are exactly representable using the `double` type. (This is necessary because the parameters \( E\), \( e\), and \( \eta\) of the computed approximation ellipsoid \( {\cal E}\) might not be exactly representable as `double` numbers.) In order to access the center and semiaxes of the computed approximation ellipsoid, the functions `center_cartesian_begin()`, `axes_lengths_begin()`, and `axis_direction_cartesian_begin()` can be used. In contrast to the above access functions `achieved_epsilon()`, `defining_matrix()`, `defining_vector()`, and `defining_scalar()`, which return the described quantities exactly, the routines below return numerical approximations to the real center and real semiaxes of the computed ellipsoid; the comprised relative error may be larger than zero, and there are no guarantees for the returned quantities.
unsigned int | number_of_points () const
| returns the number of points of `ame`, i.e., \( |P|\).
double | achieved_epsilon () const
| returns a number \( \epsilon'\) such that the computed approximation is (under exact arithmetic) guaranteed to be an \( (1+\epsilon')\)-approximation to \( (P)\).
double | defining_matrix (int i, int j) const
| gives access to the \( (i,j)\)th entry of the matrix \( E\) in the representation \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq0 \}\) of the computed approximation ellipsoid \( {\cal E}\).
double | defining_vector (int i) const
| gives access to the \( i\)th entry of the vector \( e\) in the representation \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq0 \}\) of the computed approximation ellipsoid \( {\cal E}\).
double | defining_scalar () const
| gives access to the scalar \( \eta\) from the representation \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq0 \}\) of the computed approximation ellipsoid \( {\cal E}\).
const Traits & | traits () const
| returns a const reference to the traits class object.
int | dimension () const
| returns the dimension of the ambient space, i.e., the dimension of the points \( P\).
Center_coordinate_iterator | center_cartesian_begin ()
| returns an iterator pointing to the first of the \( d\) Cartesian coordinates of the computed ellipsoid's center.
Center_coordinate_iterator | center_cartesian_end ()
| returns the past-the-end iterator corresponding to `center_cartesian_begin()`.
Axes_lengths_iterator | axes_lengths_begin ()
| returns an iterator pointing to the first of the \( d\) descendantly sorted lengths of the computed ellipsoid's axes.
Axes_lengths_iterator | axes_lengths_end ()
| returns the past-the-end iterator corresponding to `axes_lengths_begin()`.
Axes_direction_coordinate_iterator | axis_direction_cartesian_begin (int i)
| returns an iterator pointing to the first of the \( d\) Cartesian coordinates of the computed ellipsoid's \( i\)th axis direction (i.e., unit vector in direction of the ellipsoid's \( i\)th axis).
Axes_direction_coordinate_iterator | axis_direction_cartesian_end (int i)
| returns the past-the-end iterator corresponding to `axis_direction_cartesian_begin()`.

## Predicates
bool | is_full_dimensional () const
| returns whether \( P\) is full-dimensional or not, i.e., returns `true` if and only if \( P\) is full-dimensional.

## Validity Check
An object `ame` is valid iff `ame` contains all points of its defining set \( P\), `ame` is an \( (1+\epsilon')\)-approximation to the smallest ellipsoid \( (P)\) of \( P\), The ellipsoid represented by `ame` fulfills the inclusion ( eqapproximate_min_ellipsoid_incl ).
bool | is_valid (bool verbose=false) const
| returns `true` iff `ame` is valid according to the above definition.

## Miscellaneous
void | write_eps (const std::string &name) const
| Writes the points \( P\) and the computed approximation to \( (P)\) as an EPS-file under pathname `name`.

## Constructor & Destructor Documentation

## ◆ Approximate_min_ellipsoid_d()

template<typename Traits >
template<class Iterator >

CGAL::Approximate_min_ellipsoid_d< Traits >::Approximate_min_ellipsoid_d | ( | double | eps,
|  | Iterator | first,
|  | Iterator | last,
|  | const Traits & | traits = `Traits()`
| ) |  |

initializes `ame` to an \( (1+\epsilon)\)-approximation of \( (P)\) with \( P\) being the set of points in the range [`first`,`last`).

The number \( \epsilon\) in this will be at most `eps`, if possible. However, due to the limited precision in the algorithm's underlying arithmetic, it can happen that the computed approximation ellipsoid has a worse approximation ratio (and \( \epsilon\) can thus be larger than `eps` in general). In any case, the number \( \epsilon\) (and with this, the achieved approximation \( 1+\epsilon\)) can be queried by calling the routine `achieved_epsilon()` discussed below.

Template Parameters

  Iterator | must be a model of `InputIterator` with `Point` as value type.

Precondition
  The dimension \( d\) of the input points must be at least \( 2\), and \( \epsilon>0\).

## Member Function Documentation

## ◆ achieved_epsilon()

template<typename Traits >

double CGAL::Approximate_min_ellipsoid_d< Traits >::achieved_epsilon | ( |  | ) | const

returns a number \( \epsilon'\) such that the computed approximation is (under exact arithmetic) guaranteed to be an \( (1+\epsilon')\)-approximation to \( (P)\).

Precondition
  `ame.is_full_dimensional() == true`.

Postcondition
  \( \epsilon'>=0\).

## ◆ axes_lengths_begin()

template<typename Traits >

Axes_lengths_iterator CGAL::Approximate_min_ellipsoid_d< Traits >::axes_lengths_begin | ( |  | ) |

returns an iterator pointing to the first of the \( d\) descendantly sorted lengths of the computed ellipsoid's axes.

The \( d\) returned numbers are floating-point approximations to the exact axes-lengths of the computed ellipsoid; no guarantee is given w.r.t. the involved relative error. (See also method `axis_direction_cartesian_begin()`.)

Precondition
  `ame.is_full_dimensional() == true`, and \( d\in\{2,3\}\).

## ◆ axes_lengths_end()

template<typename Traits >

Axes_lengths_iterator CGAL::Approximate_min_ellipsoid_d< Traits >::axes_lengths_end | ( |  | ) |

returns the past-the-end iterator corresponding to `axes_lengths_begin()`.

Precondition
  `ame.is_full_dimensional() == true`, and \( d\in\{2,3\}\).

## ◆ axis_direction_cartesian_begin()

template<typename Traits >

Axes_direction_coordinate_iterator CGAL::Approximate_min_ellipsoid_d< Traits >::axis_direction_cartesian_begin | ( | int | i | ) |

returns an iterator pointing to the first of the \( d\) Cartesian coordinates of the computed ellipsoid's \( i\)th axis direction (i.e., unit vector in direction of the ellipsoid's \( i\)th axis).

The direction described by this iterator is a floating-point approximation to the exact axis direction of the computed ellipsoid; no guarantee is given w.r.t. the involved relative error. An approximation to the length of axis \( i\) is given by the \( i\)th entry of `axes_lengths_begin()`.

Precondition
  `ame.is_full_dimensional() == true`, and \( d\in\{2,3\}\), and \( 0\leq i < d\).

## ◆ axis_direction_cartesian_end()

template<typename Traits >

Axes_direction_coordinate_iterator CGAL::Approximate_min_ellipsoid_d< Traits >::axis_direction_cartesian_end | ( | int | i | ) |

returns the past-the-end iterator corresponding to `axis_direction_cartesian_begin()`.

Precondition
  `ame.is_full_dimensional() == true`, and \( d\in\{2,3\}\), and \( 0\leq i < d\).

## ◆ center_cartesian_begin()

template<typename Traits >

Center_coordinate_iterator CGAL::Approximate_min_ellipsoid_d< Traits >::center_cartesian_begin | ( |  | ) |

returns an iterator pointing to the first of the \( d\) Cartesian coordinates of the computed ellipsoid's center.

The returned point is a floating-point approximation to the ellipsoid's exact center; no guarantee is given w.r.t. the involved relative error.

Precondition
  `ame.is_full_dimensional() == true`.

## ◆ center_cartesian_end()

template<typename Traits >

Center_coordinate_iterator CGAL::Approximate_min_ellipsoid_d< Traits >::center_cartesian_end | ( |  | ) |

returns the past-the-end iterator corresponding to `center_cartesian_begin()`.

Precondition
  `ame.is_full_dimensional() == true`.

## ◆ defining_matrix()

template<typename Traits >

double CGAL::Approximate_min_ellipsoid_d< Traits >::defining_matrix | ( | int | i,
|  | int | j
| ) |  | const

gives access to the \( (i,j)\)th entry of the matrix \( E\) in the representation \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq0 \}\) of the computed approximation ellipsoid \( {\cal E}\).

The number returned by this routine is \( (1+\epsilon')(d+1)\,E_{ij}\), where \( \epsilon'\) is the number returned by `achieved_epsilon()`.

Precondition
  \( 0\leq i,j\leq d\), where \( d\) is the dimension of the points \( P\), and `ame.is_full_dimensional() == true`.

## ◆ defining_scalar()

template<typename Traits >

double CGAL::Approximate_min_ellipsoid_d< Traits >::defining_scalar | ( |  | ) | const

gives access to the scalar \( \eta\) from the representation \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq0 \}\) of the computed approximation ellipsoid \( {\cal E}\).

The number returned by this routine is \( (1+\epsilon')(d+1)\,(\eta+1)\), where \( \epsilon'\) is the number returned by `achieved_epsilon()`.

Precondition
  `ame.is_full_dimensional() == true`.

## ◆ defining_vector()

template<typename Traits >

double CGAL::Approximate_min_ellipsoid_d< Traits >::defining_vector | ( | int | i | ) | const

gives access to the \( i\)th entry of the vector \( e\) in the representation \( \{ x\in\E^d \mid x^T E x + x^T e + \eta\leq0 \}\) of the computed approximation ellipsoid \( {\cal E}\).

The number returned by this routine is \( (1+\epsilon')(d+1)\,e_{i}\), where \( \epsilon'\) is the number returned by `achieved_epsilon()`.

Precondition
  \( 0\leq i\leq d\), where \( d\) is the dimension of the points \( P\), and `ame.is_full_dimensional() == true`.

## ◆ is_full_dimensional()

template<typename Traits >

bool CGAL::Approximate_min_ellipsoid_d< Traits >::is_full_dimensional | ( |  | ) | const

returns whether \( P\) is full-dimensional or not, i.e., returns `true` if and only if \( P\) is full-dimensional.

Note: due to the limited precision in the algorithm's underlying arithmetic, the result of this method is not always correct. Rather, a return value of `false` means that the points \( P\) are contained in a "very thin" linear subspace of \( \E^d\), and as a consequence, the algorithm cannot compute an approximation. More precisely, a return value of `false` means that the points \( P\) are contained between two parallel hyperplanes in \( \E^d\) that are very close to each other (possibly at distance zero) - so close, that the algorithm could not compute an approximation ellipsoid. Similarly, a return value of `true` does not guarantee \( P\) to be full-dimensional; but there exists an input pointset \( P'\) such that the points \( P'\) and \( P\) have almost identical coordinates and \( P'\) is full-dimensional.

## ◆ is_valid()

template<typename Traits >

bool CGAL::Approximate_min_ellipsoid_d< Traits >::is_valid | ( | bool | verbose = `false` | ) | const

returns `true` iff `ame` is valid according to the above definition.

If `verbose` is `true`, some messages concerning the performed checks are written to the standard error stream.

## ◆ write_eps()

template<typename Traits >

void CGAL::Approximate_min_ellipsoid_d< Traits >::write_eps | ( | const std::string & | name | ) | const

Writes the points \( P\) and the computed approximation to \( (P)\) as an EPS-file under pathname `name`.

Precondition
  The dimension of points \( P\) must be \( 2\). Note: this routine is provided as a debugging routine; future version of CGAL might not provide it anymore.
  `ame.is_full_dimensional() == true`.