CGAL 6.2 - 3D Polyhedral Surface

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CGAL::Polyhedron_3< Traits >::Halfedge Class Reference
Reference Manual

`#include <CGAL/Polyhedron_3.h>`

## Definition

template<typename Traits>
class CGAL::Polyhedron_3< Traits >::Halfedge

A halfedge is an oriented edge between two vertices.

It is always paired with a halfedge pointing in the opposite direction. The `Halfedge::opposite()` member function returns this halfedge of opposite orientation. If a halfedge is incident to a facet the `Halfedge::next()` member function points to the successor halfedge around this facet. For border edges the `Halfedge::next()` member function points to the successor halfedge along the hole. For more than two border edges at a vertex, the next halfedge along a hole is not uniquely defined, but a consistent assignment of the next halfedge will be maintained in the data structure. An invariant is that successive assignments of the form `h = h->next()` cycle counterclockwise around the facet (or hole) and traverse all halfedges incident to this facet (or hole). A similar invariant is that successive assignments of the form `h = h->next()->opposite()` cycle clockwise around the vertex and traverse all halfedges incident to this vertex. Two circulators are provided for these circular orders.

The three classes `Vertex`, `Halfedge`, and `Facet` of the polyhedral surface. Member functions with shaded background are mandatory. The others are optionally supported.

The incidences encoded in `Halfedge::opposite()` and `Halfedge::next()` are available for each instantiation of polyhedral surfaces. The other incidences are optionally available as indicated with type tags. The `Halfedge::prev()` member function points to the preceding halfedge around the same facet. It is always available, though it might perform a search around the facet using the `Halfedge::next()` member function to find the previous halfedge if the underlying halfedge data structure does not provide an efficient `Halfedge::prev()` member function for halfedges. Handles to the incident vertex and facet are optionally stored.

The circulators are assignable to the `Halfedge_handle`. The circulators are bidirectional if the halfedge provided to the polyhedron with the `Items` template argument provides a member function `Halfedge::prev()`, otherwise they are of the forward category.

See also
  `CGAL::Polyhedron_3::Vertex`
  `CGAL::Polyhedron_3::Facet`
  `CGAL::Polyhedron_3<Traits>`

Implementation

The member functions `Halfedge::prev()` and `Halfedge::prev_on_vertex()` work in constant time if `Supports_halfedge_prev` \( \equiv\) `CGAL::Tag_true`. Otherwise both methods search for the previous halfedge around the incident facet.

## Types
typedef unspecified_type | Vertex
| type of incident vertices.
typedef unspecified_type | Facet
| type of incident facets.
typedef unspecified_type | Vertex_handle
| handle to vertex.
typedef unspecified_type | Halfedge_handle
| handle to halfedge.
typedef unspecified_type | Facet_handle
| handle to facet.
typedef unspecified_type | Halfedge_around_vertex_circulator
| circulator of halfedges around a vertex.
typedef unspecified_type | Halfedge_around_facet_circulator
| circulator of halfedges around a facet.
typedef unspecified_type | Vertex_const_handle
typedef unspecified_type | Halfedge_const_handle
typedef unspecified_type | Facet_const_handle
typedef unspecified_type | Halfedge_around_vertex_const_circulator
typedef unspecified_type | Halfedge_around_facet_const_circulator
typedef unspecified_type | Supports_halfedge_prev
| \( \equiv\) `CGAL::Tag_true` or `CGAL::Tag_false`.
typedef unspecified_type | Supports_halfedge_vertex
| \( \equiv\) `CGAL::Tag_true` or `CGAL::Tag_false`.
typedef unspecified_type | Supports_halfedge_face
| \( \equiv\) `CGAL::Tag_true` or `CGAL::Tag_false`.

## Creation
| Halfedge ()
| default constructor.

## Operations
Halfedge_handle | opposite ()
Halfedge_const_handle | opposite () const
| the opposite halfedge.
Halfedge_handle | next ()
Halfedge_const_handle | next () const
| the next halfedge around the facet.
Halfedge_handle | prev ()
Halfedge_const_handle | prev () const
| the previous halfedge around the facet.
Halfedge_handle | next_on_vertex ()
Halfedge_const_handle | next_on_vertex () const
| the next halfedge around the vertex (clockwise).
Halfedge_handle | prev_on_vertex ()
Halfedge_const_handle | prev_on_vertex () const
| the previous halfedge around the vertex (counterclockwise).
bool | is_border () const
| is true if `h` is a border halfedge.
bool | is_border_edge () const
| is true if this or the opposite halfedge is a border halfedge.
Halfedge_around_vertex_circulator | vertex_begin ()
Halfedge_around_vertex_const_circulator | vertex_begin () const
| circulator of halfedges around the vertex (clockwise), the vertex of the halfedges being `*this`.
Halfedge_around_facet_circulator | facet_begin ()
Halfedge_around_facet_const_circulator | facet_begin () const
| circulator of halfedges around the facet (counterclockwise).
std::size_t | vertex_degree () const
| the degree of the incident vertex, i.e., number of edges emanating from this vertex.
bool | is_bivalent () const
| returns `true` if the incident vertex has exactly two incident edges.
bool | is_trivalent () const
| returns `true` if the incident vertex has exactly three incident edges.
std::size_t | facet_degree () const
| the degree of the incident facet, i.e., number of edges on the boundary of this facet.
bool | is_triangle () const
| returns `true` if the incident facet is a triangle.
bool | is_quad () const
| returns `true` if the incident facet is a quadrilateral.

## Operations available if Supports_halfedge_vertex is CGAL::Tag_true
Vertex_handle | vertex ()
Vertex_const_handle | vertex () const
| the incident vertex of the halfedge.

## Operations available if Supports_halfedge_facet is CGAL::Tag_true
Facet_handle | facet ()
Facet_const_handle | facet () const
| the incident facet of the halfedge.

## Member Function Documentation

## ◆ facet()

template<typename Traits >

Facet_const_handle CGAL::Polyhedron_3< Traits >::Halfedge::facet | ( |  | ) | const

the incident facet of the halfedge.

If the hafedge is a border halfedge the result is default construction of the handle.

## ◆ next_on_vertex()

template<typename Traits >

Halfedge_const_handle CGAL::Polyhedron_3< Traits >::Halfedge::next_on_vertex | ( |  | ) | const

the next halfedge around the vertex (clockwise).

Is equal to `h.next()->opposite()`.

## ◆ prev_on_vertex()

template<typename Traits >

Halfedge_const_handle CGAL::Polyhedron_3< Traits >::Halfedge::prev_on_vertex | ( |  | ) | const

the previous halfedge around the vertex (counterclockwise).

Is equal to `h.opposite()->prev()`.