CGAL 5.3 - 2D Boolean Operations on Nef Polygons
CGAL::Nef_polyhedron_2< T > Class Template Reference

#include <CGAL/Nef_polyhedron_2.h>

## Definition

An instance of data type Nef_polyhedron_2<T> is a subset of the plane that is the result of forming complements and intersections starting from a finite set H of halfspaces.

Nef_polyhedron_2 is closed under all binary set operations intersection, union, difference, complement and under the topological operations boundary, closure, and interior.

The template parameter T is specified via an extended kernel concept. T must be a model of the concept ExtendedKernelTraits_2.

Exploration - Point location - Ray shooting

As Nef polyhedra are the result of forming complements and intersections starting from a set H of halfspaces that are defined by oriented lines in the plane, they can be represented by an attributed plane map $$M = (V,E,F)$$. For topological queries within M the following types and operations allow exploration access to this structure.

Implementation

Nef polyhedra are implemented on top of a halfedge data structure and use linear space in the number of vertices, edges and facets. Operations like empty take constant time. The operations clear, complement, interior, closure, boundary, regularization, input and output take linear time. All binary set operations and comparison operations take time $$O(n \log n)$$ where $$n$$ is the size of the output plus the size of the input.

The point location and ray shooting operations are implemented in two flavors. The NAIVE operations run in linear query time without any preprocessing, the DEFAULT operations (equals LMWT) run in sub-linear query time, but preprocessing is triggered with the first operation. Preprocessing takes time $$O(N^2)$$, the sub-linear point location time is either logarithmic when LEDA's persistent dictionaries are present or if not then the point location time is worst-case linear, but experiments show often sublinear runtimes. Ray shooting equals point location plus a walk in the constrained triangulation overlayed on the plane map representation. The cost of the walk is proportional to the number of triangles passed in direction d until an obstacle is met. In a minimum weight triangulation of the obstacles (the plane map representing the polyhedron) the theory provides a $$O(\sqrt{n})$$ bound for the number of steps. Our locally minimum weight triangulation approximates the minimum weight triangulation only heuristically (the calculation of the minimum weight triangulation is conjectured to be NP hard). Thus we have no runtime guarantee but a strong experimental motivation for its approximation.

Example

Nef polyhedra are parameterized by a so-called extended geometric kernel. There are three kernels, one based on a homogeneous representation of extended points called Extended_homogeneous<RT> where RT is a ring type providing additionally a gcd operation, one based on a Cartesian representation of extended points called Extended_cartesian<NT> where NT is a field type, and finally Filtered_extended_homogeneous<RT> (an optimized version of the first). The following example uses the filtered homogeneous kernel to construct the intersection of two halfspaces.

#include <CGAL/Filtered_extended_homogeneous.h>
#include <CGAL/Nef_polyhedron_2.h>
typedef Nef_polyhedron::Line Line;
int main()
{
Nef_polyhedron N1(Line(1,0,0));
Nef_polyhedron N2(Line(0,1,0), Nef_polyhedron::EXCLUDED);
Nef_polyhedron N3 = N1 * N2; // line (*)
return 0;
}

After line (*) N3 is the intersection of N1 and N2. The member types of Nef_polyhedron_2< Extended_homogeneous<NT> > map to corresponding types of the standard CGAL geometry kernel (type equality in pseudo-code notation):

The same holds for the types Line and Direction in the local scope of Nef_polyhedron_2<...>.

Examples:
Nef_2/nef_2_construction.cpp, Nef_2/nef_2_exploration.cpp, Nef_2/nef_2_intersection.cpp, and Nef_2/nef_2_polylines.cpp.

## Classes

class  Explorer
a decorator to examine the underlying plane map. More...

class  Topological_explorer
An instance D of the data type Topological_explorer is a decorator for interfacing the topological structure of a plane map P (read-only). More...

## Types

enum  Boundary { EXCLUDED, INCLUDED }
construction selection. More...

enum  Content { EMPTY, COMPLETE }
construction selection More...

enum  Location_mode { DEFAULT, NAIVE, LMWT }
selectionflag for the point location mode. More...

typedef unspecified_type Line
the oriented lines modeling halfplanes.

typedef unspecified_type Point
the affine points of the plane.

typedef unspecified_type Direction
directions in our plane.

typedef unspecified_type Polygons_tag
tag for calling polygon constructor.

typedef unspecified_type Polylines_tag
tag for calling polyline constructor.

typedef unspecified_type Object_handle
a generic handle to an object of the underlying plane map. More...

## Creation

Nef_polyhedron_2 (Content plane=EMPTY)
creates an instance N of type Nef_polyhedron_2<T> and initializes it to the empty set if plane == EMPTY and to the whole plane if plane == COMPLETE.

Nef_polyhedron_2 (Line l, Boundary line=INCLUDED)
creates a Nef polyhedron N containing the halfplane left of l including l if line==INCLUDED, excluding l if line==EXCLUDED.

template<class Forward_iterator >
Nef_polyhedron_2 (Forward_iterator it, Forward_iterator end, Boundary b=INCLUDED)
creates a Nef polyhedron N from the simple polygon P spanned by the list of points in the iterator range [it,end) and including its boundary if b = INCLUDED excluding the boundary otherwise. More...

template<class Forward_iterator >
Nef_polyhedron_2 (Forward_iterator it, Forward_iterator end, Polygons_tag)
The iterator range [it, end) defines a set point ranges, each of which defines the boundary of simple polygon.

template<class Forward_iterator >
Nef_polyhedron_2 (Forward_iterator it, Forward_iterator end, Polylines_tag)
The iterator range [it, end) defines a set point ranges, each of which defines a polyline.

## Operations

void clear (Content plane=EMPTY)
makes N the empty set if plane == EMPTY and the full plane if plane == COMPLETE.

bool is_empty ()
returns true if N is empty, false otherwise.

bool is_plane ()
returns true if N is the whole plane, false otherwise.

bool contains (Object_handle h)
returns true iff the object h is contained in the set represented by N.

bool contained_in_boundary (Object_handle h)
returns true iff the object h is contained in the $$1$$-skeleton of N.

Object_handle locate (const Point &p, Location_mode m=DEFAULT)
returns a generic handle h to an object (face, halfedge, vertex) of the underlying plane map that contains the point p in its relative interior. More...

Object_handle ray_shoot (const Point &p, const Direction &d, Location_mode m=DEFAULT)
returns a handle h with N.contains(h), that can be converted to a Vertex_/Halfedge_/Face_const_handle as described above. More...

Object_handle ray_shoot_to_boundary (const Point &p, const Direction &d, Location_mode m=DEFAULT)
returns a handle h, that can be converted to a Vertex_/Halfedge_const_handle as described above. More...

Explorer explorer ()
returns a decorator object that allows read-only access of the underlying plane map. More...

## Constructive Operations

Additionally there are operators *,+,-,^,! which implement the binary operations intersection, join, difference, symmetric difference, and the unary operation complement, respectively.

There are also the corresponding modification operations There are also comparison operations like <,<=,>,>=,==,!= which implement the relations subset, subset or equal, superset, superset or equal, equality, inequality, respectively.

Nef_polyhedron_2< T > complement ()
returns the complement of N in the plane.

Nef_polyhedron_2< T > interior ()
returns the interior of N.

Nef_polyhedron_2< T > closure ()
returns the closure of N.

Nef_polyhedron_2< T > boundary ()
returns the boundary of N.

Nef_polyhedron_2< T > regularization ()
returns the regularized polyhedron (closure of interior).

Nef_polyhedron_2< T > intersection (const Nef_polyhedron_2< T > &N1)
returns N $$\cap$$ N1.

Nef_polyhedron_2< T > join (const Nef_polyhedron_2< T > &N1)
returns N $$\cup$$ N1. More...

Nef_polyhedron_2< T > difference (const Nef_polyhedron_2< T > &N1)
returns N $$-$$ N1.

Nef_polyhedron_2< T > symmetric_difference (const Nef_polyhedron_2< T > &N1)
returns the symmectric difference N - T $$\cup$$ T - N.

## ◆ Object_handle

template<typename T>
 typedef unspecified_type CGAL::Nef_polyhedron_2< T >::Object_handle

a generic handle to an object of the underlying plane map.

The kind of object (vertex, halfedge, face) can be determined and the object can be assigned to a corresponding handle by the three functions:

bool assign(Vertex_const_handle& h, Object_handle)

bool assign(Halfedge_const_handle& h, Object_handle)

bool assign(Face_const_handle& h, Object_handle)

where each function returns true iff the assignment to h was done.

## ◆ Boundary

template<typename T>

construction selection.

Enumerator
EXCLUDED
INCLUDED

## ◆ Content

template<typename T>

construction selection

Enumerator
EMPTY
COMPLETE

## ◆ Location_mode

template<typename T>

selectionflag for the point location mode.

LMWT stands for Locally Minimum Weight Triangulation, a locally optimized constrained triangulation where the weight corresponds to the length of the edges of the triangulation.

Enumerator
DEFAULT
NAIVE
LMWT

## ◆ Nef_polyhedron_2()

template<typename T>
template<class Forward_iterator >
 CGAL::Nef_polyhedron_2< T >::Nef_polyhedron_2 ( Forward_iterator it, Forward_iterator end, Boundary b = INCLUDED )

creates a Nef polyhedron N from the simple polygon P spanned by the list of points in the iterator range [it,end) and including its boundary if b = INCLUDED excluding the boundary otherwise.

Forward_iterator has to be an iterator with value type Point. This construction expects that P is simple. The degenerate cases where P contains no point, one point or spans just one segment (two points) are correctly handled. In all degenerate cases there's only one unbounded face adjacent to the degenerate polygon. If b == INCLUDED then N is just the boundary. If b == EXCLUDED then N is the whole plane without the boundary.

## ◆ explorer()

template<typename T>
 Explorer CGAL::Nef_polyhedron_2< T >::explorer ( )

returns a decorator object that allows read-only access of the underlying plane map.

See the manual page Explorer for its usage.

Examples:
Nef_2/nef_2_exploration.cpp.

## ◆ join()

template<typename T>
 Nef_polyhedron_2 CGAL::Nef_polyhedron_2< T >::join ( const Nef_polyhedron_2< T > & N1 )

returns N $$\cup$$ N1.

Note that "union" is a keyword of C++ and cannot be used for this operation.

## ◆ locate()

template<typename T>
 Object_handle CGAL::Nef_polyhedron_2< T >::locate ( const Point & p, Location_mode m = DEFAULT )

returns a generic handle h to an object (face, halfedge, vertex) of the underlying plane map that contains the point p in its relative interior.

The point p is contained in the set represented by N if N.contains(h) is true. The location mode flag m allows one to choose between different point location strategies.

## ◆ ray_shoot()

template<typename T>
 Object_handle CGAL::Nef_polyhedron_2< T >::ray_shoot ( const Point & p, const Direction & d, Location_mode m = DEFAULT )

returns a handle h with N.contains(h), that can be converted to a Vertex_/Halfedge_/Face_const_handle as described above.

The object returned is intersected by the ray starting in p with direction d and has minimal distance to p. The operation returns an empty Object_handle if the ray shoot along d does not hit any object h of N with N.contains(h). The location mode flag m allows one to choose between different point location strategies.

## ◆ ray_shoot_to_boundary()

template<typename T>
 Object_handle CGAL::Nef_polyhedron_2< T >::ray_shoot_to_boundary ( const Point & p, const Direction & d, Location_mode m = DEFAULT )

returns a handle h, that can be converted to a Vertex_/Halfedge_const_handle as described above.

The object returned is part of the $$1$$-skeleton of N, intersected by the ray starting in p with direction d and has minimal distance to p. The operation returns an empty Object_handle if the ray shoot along d does not hit any $$1$$-skeleton object h of N. The location mode flag m allows one to choose between different point location strategies.