CGAL 6.2 - CGAL and the Boost Graph Library

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Euler Operations
Reference Manual

##

We call high-level operations that maintain the validity of a halfedge graph Euler Operations.

template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::join_vertex (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| joins the two vertices incident to `h`, (that is `source(h, g)` and `target(h, g)`) and removes `source(h,g)`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::split_vertex (typename boost::graph_traits< Graph >::halfedge_descriptor h1, typename boost::graph_traits< Graph >::halfedge_descriptor h2, Graph &g)
| splits the target vertex `v` of `h1` and `h2`, and connects the new vertex and `v` with a new edge.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::split_edge (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| splits the halfedge `h` into two halfedges inserting a new vertex that is a copy of `vertex(opposite(h,g),g)`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::join_face (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| joins the two faces incident to `h` and `opposite(h,g)`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::split_face (typename boost::graph_traits< Graph >::halfedge_descriptor h1, typename boost::graph_traits< Graph >::halfedge_descriptor h2, Graph &g)
| splits the face incident to `h1` and `h2`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::join_loop (typename boost::graph_traits< Graph >::halfedge_descriptor h1, typename boost::graph_traits< Graph >::halfedge_descriptor h2, Graph &g)
| glues the cycle of halfedges of `h1` and `h2` together.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::split_loop (typename boost::graph_traits< Graph >::halfedge_descriptor h1, typename boost::graph_traits< Graph >::halfedge_descriptor h2, typename boost::graph_traits< Graph >::halfedge_descriptor h3, Graph &g)
| cuts the graph along the cycle `(h1,h2,h3)` changing the genus (halfedge `h3` runs on the backside of the three dimensional figure below).
template<typename Graph >
void | CGAL::Euler::remove_face (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| removes the incident face of `h` and changes all halfedges incident to the face into border halfedges or removes them from the graph if they were already border halfedges.
template<typename Graph >
boost::graph_traits< Graph >::edge_descriptor | CGAL::Euler::add_edge (typename boost::graph_traits< Graph >::vertex_descriptor s, typename boost::graph_traits< Graph >::vertex_descriptor t, Graph &g)
| adds and returns the edge `e` connecting `s` and `t` halfedge(e, g) has s as source and t as target
template<typename VertexRange , typename PMesh >
bool | CGAL::Euler::can_add_face (const VertexRange &vrange, const PMesh &sm, const bool verbose=false)
| checks whether a new face defined by a range of vertices (identified by their descriptors, `boost::graph_traits<Graph>::vertex_descriptor`) can be added.
template<typename Graph , typename VertexRange >
boost::graph_traits< Graph >::face_descriptor | CGAL::Euler::add_face (const VertexRange &vr, Graph &g)
| adds a new face defined by a range of vertices (identified by their descriptors, `boost::graph_traits<Graph>::vertex_descriptor`).
template<typename Graph >
void | CGAL::Euler::make_hole (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| removes the incident face of `h` and changes all halfedges incident to the face into border halfedges.
template<typename Graph >
void | CGAL::Euler::fill_hole (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| fills the hole incident to `h`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::add_center_vertex (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| creates a barycentric triangulation of the face incident to `h`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::remove_center_vertex (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| removes the vertex `target(h, g)` and all incident halfedges thereby merging all incident faces.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::add_vertex_and_face_to_border (typename boost::graph_traits< Graph >::halfedge_descriptor h1, typename boost::graph_traits< Graph >::halfedge_descriptor h2, Graph &g)
| appends a new face to the border halfedge `h2` by connecting the tip of `h2` with the tip of `h1` with two new halfedges and a new vertex and creating a new face that is incident to `h2`.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::add_face_to_border (typename boost::graph_traits< Graph >::halfedge_descriptor h1, typename boost::graph_traits< Graph >::halfedge_descriptor h2, Graph &g)
| appends a new face incident to the border halfedge `h1` and `h2` by connecting the vertex `target(h2,g)` and the vertex `target(h1,g)` with a new halfedge, and filling this separated part of the hole with a new face, such that the new face is incident to `h2`.
template<typename Graph >
bool | CGAL::Euler::does_satisfy_link_condition (typename boost::graph_traits< Graph >::edge_descriptor e, const Graph &g)
template<typename Graph >
boost::graph_traits< Graph >::vertex_descriptor | CGAL::Euler::collapse_edge (typename boost::graph_traits< Graph >::edge_descriptor e, Graph &g)
| collapses an edge in a graph.
template<typename Graph , typename EdgeIsConstrainedMap >
boost::graph_traits< Graph >::vertex_descriptor | CGAL::Euler::collapse_edge (typename boost::graph_traits< Graph >::edge_descriptor e, Graph &g, EdgeIsConstrainedMap Edge_is_constrained_map)
| collapses an edge in a graph having non-collapsible edges.
template<typename Graph >
void | CGAL::Euler::flip_edge (typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| performs an edge flip, rotating the edge pointed by `h` by one vertex in the direction of the face orientation.
template<typename Graph >
boost::graph_traits< Graph >::halfedge_descriptor | CGAL::Euler::remove_degree_2_vertex (const typename boost::graph_traits< Graph >::halfedge_descriptor h, Graph &g)
| removes the target vertex of `h`, merging its incident edges into a single edge linking the two vertices adjacent to the vertex being removed.

## Function Documentation

## ◆ add_center_vertex()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::add_center_vertex | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

creates a barycentric triangulation of the face incident to `h`.

Creates a new vertex and connects it to each vertex incident to `h` and splits `face(h, g)` into triangular faces. `h` remains incident to the original face. The time complexity is linear in the size of the face.

Returns
  the halfedge `next(h, g)` after the operation, i.e., a halfedge pointing to the new vertex.

Note that `add_center_vertex()` does not deal with properties of new vertices, halfedges, and faces.

Precondition
  `h` is not a border halfedge.

Parameters

  g | the graph
  h | halfedge descriptor

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

See also
  `remove_center_vertex()`

## ◆ add_face()

template<typename Graph , typename VertexRange >

boost::graph_traits< Graph >::face_descriptor CGAL::Euler::add_face | ( | const VertexRange & | vr,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

adds a new face defined by a range of vertices (identified by their descriptors, `boost::graph_traits<Graph>::vertex_descriptor`).

For each pair of consecutive vertices, the corresponding halfedge is added in `g` if new, and its connectivity is updated otherwise. The face can be added only at the boundary of `g`, or as a new connected component.

Precondition
  `vr` contains at least 3 vertices

Returns
  the added face descriptor, or `boost::graph_traits<Graph>::null_face()` if the face could not be added.

## ◆ add_face_to_border()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::add_face_to_border | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h1,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h2,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

appends a new face incident to the border halfedge `h1` and `h2` by connecting the vertex `target(h2,g)` and the vertex `target(h1,g)` with a new halfedge, and filling this separated part of the hole with a new face, such that the new face is incident to `h2`.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Returns
  the halfedge of the new edge that is incident to the new face.

Precondition
  `h1` and `h2` are border halfedges,
  `h1 != h2`,
  `next(h1,g) != h2`,
  `h1` and `h2` are on the same border.

## ◆ add_vertex_and_face_to_border()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::add_vertex_and_face_to_border | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h1,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h2,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

appends a new face to the border halfedge `h2` by connecting the tip of `h2` with the tip of `h1` with two new halfedges and a new vertex and creating a new face that is incident to `h2`.

Note that `add_vertex_and_face_to_border()` does not deal with properties of new vertices, halfedges, and faces.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Returns
  the halfedge of the new edge that is incident to the new face and the new vertex.

Precondition
  `h1` and `h2` are border halfedges
  `h1 != h2`,
  `h1` and `h2` are on the same border.

## ◆ collapse_edge() [1/2]

template<typename Graph >

boost::graph_traits< Graph >::vertex_descriptor CGAL::Euler::collapse_edge | ( | typename boost::graph_traits< Graph >::edge_descriptor | e,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

collapses an edge in a graph.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Let `h` be the halfedge of `e`, and let `v0` and `v1` be the source and target vertices of `h`. Let `p_h` and `p_o_h` be respectively the edges of `prev(h,g)` and `prev(opposite(h, g), g)`. Let `o_n_h` and `o_n_o_h` be respectively the edges of `opposite(next(h,g))` and `opposite(next(opposite(h, g), g))`.

After the collapse of edge `e` the following holds:

- The edge `e` is no longer in `g`.
- The triangle faces incident to edge `e` are no longer in `g`.
- `v0` is no longer in `g`.
- If `h` is part of a triangle face, `p_h` is no longer in `g` and is replaced by `o_n_h`.
- If the opposite of `h` is part of a triangle face, `p_o_h` is no longer in `g` and is replaced by `o_n_o_h`.
- The halfedges kept in `g` that had `v0` as target and source now have `v1` as target and source, respectively.
- No other incidence information is changed in `g`.

Returns
  vertex `v1`.

Precondition
  g must be a triangulated graph
  `does_satisfy_link_condition`(e,g) == `true`.

## ◆ collapse_edge() [2/2]

template<typename Graph , typename EdgeIsConstrainedMap >

boost::graph_traits< Graph >::vertex_descriptor CGAL::Euler::collapse_edge | ( | typename boost::graph_traits< Graph >::edge_descriptor | e,
|  | Graph & | g,
|  | EdgeIsConstrainedMap | Edge_is_constrained_map
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

collapses an edge in a graph having non-collapsible edges.

Let `h` be the halfedge of `e`, and let `v0` and `v1` be the source and target vertices of `h`. Collapses the edge `e` replacing it with `v1`, as described in the other overload and guarantees that an edge `e2`, for which `get(edge_is_constrained_map, e2)==true`, is not removed after the collapse.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`
  EdgeIsConstrainedMap | mut be a model of `ReadablePropertyMap` with the edge descriptor of `Graph` as key type and a Boolean as value type. It indicates if an edge is constrained or not.

Returns
  vertex `v1`.

Precondition
  This function requires `g` to be an oriented 2-manifold with or without boundaries. Furthermore, the edge `e` must satisfy the link condition, which guarantees that the surface mesh is also 2-manifold after the edge collapse.
  `get(edge_is_constrained_map, e) == false`.
  `v0` and `v1` are not both incident to a constrained edge.

## ◆ does_satisfy_link_condition()

template<typename Graph >

bool CGAL::Euler::does_satisfy_link_condition | ( | typename boost::graph_traits< Graph >::edge_descriptor | e,
|  | const Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

Returns
  `true` if `e` satisfies the link condition [2], which guarantees that the surface is also 2-manifold after the edge collapse.

## ◆ fill_hole()

template<typename Graph >

void CGAL::Euler::fill_hole | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

fills the hole incident to `h`.

Precondition
  `h` must be a border halfedge

## ◆ flip_edge()

template<typename Graph >

void CGAL::Euler::flip_edge | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

performs an edge flip, rotating the edge pointed by `h` by one vertex in the direction of the face orientation.

Precondition
  Both faces incident to `h` are triangles.

## ◆ join_face()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::join_face | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

joins the two faces incident to `h` and `opposite(h,g)`.

The faces may be holes.

If `Graph` is a model of `MutableFaceGraph` the face incident to `opposite(h,g)` is removed.

`join_face()` and `split_face()` are inverse operations, that is `join_face(split_face(h,g),g)` returns `h`.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`.

Parameters

  g | the graph
  h | the halfedge incident to one of the faces to be joined.

Returns
  `prev(h,g)`

Precondition
  `out_degree(source(h,g)), g)) >= 3`
  `out_degree(target(h,g)) >= 3`

See also
  `split_face()`

## ◆ join_loop()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::join_loop | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h1,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h2,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

glues the cycle of halfedges of `h1` and `h2` together.

The vertices in the cycle of `h2` get removed. If `h1` or `h2` are not border halfedges their faces get removed. The vertices on the face cycle of `h1` get removed. The invariant `join_loop(h1, split_loop(h1,h2,h3,g), g)` returns `h1` and keeps the graph unchanged.

Template Parameters

  Graph | must be a `MutableFaceGraph`

Returns
  `h1`.

Precondition
  The faces incident to `h` and `g` are different and have equal number of edges.

## ◆ join_vertex()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::join_vertex | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

joins the two vertices incident to `h`, (that is `source(h, g)` and `target(h, g)`) and removes `source(h,g)`.

Returns the predecessor of `h` around the vertex, i.e., `prev(opposite(h,g))`. The invariant `join_vertex(split_vertex(h,g),g)` returns `h`. The time complexity is linear in the degree of the vertex removed.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Parameters

  g | the graph
  h | the halfedge which incident vertices are joint

Returns
  `prev(opposite(h,g))`

Precondition
  The size of the faces incident to `h` and `opposite(h,g)` is at least 4.

Postcondition
  `source(h, g)` is invalidated
  `h` is invalidated

See also
  `split_vertex()`

## ◆ make_hole()

template<typename Graph >

void CGAL::Euler::make_hole | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

removes the incident face of `h` and changes all halfedges incident to the face into border halfedges.

See `remove_face(g,h)` for a more generalized variant.

Precondition
  None of the incident edges of the face is a border edge.

## ◆ remove_center_vertex()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::remove_center_vertex | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

removes the vertex `target(h, g)` and all incident halfedges thereby merging all incident faces.

The resulting face may not be triangulated. This function is the inverse operation of `add_center_vertex()`. The invariant `h == remove_center_vertex(add_center_vertex(h,g),g)` holds, if `h` is not a border halfedge.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Parameters

  g | the graph
  h | halfedge descriptor

Returns
  `prev(h, g)`

Precondition
  None of the incident faces of `target(h,g)` is a hole. There are at least two distinct faces incident to the faces that are incident to `target(h,g)`. (This prevents the operation from collapsing a volume into two faces glued together with opposite orientations, such as would happen with any vertex of a tetrahedron.)

See also
  `add_center_vertex()`

## ◆ remove_degree_2_vertex()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::remove_degree_2_vertex | ( | const typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

removes the target vertex of `h`, merging its incident edges into a single edge linking the two vertices adjacent to the vertex being removed.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Parameters

  h | halfedge descriptor
  g | the graph

Returns
  a halfedge linking the two vertices adjacent to the vertex being removed.

Precondition
  `degree(target(h, g), g) == 2`.

See also
  `remove_center_vertex()`

## ◆ remove_face()

template<typename Graph >

void CGAL::Euler::remove_face | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

removes the incident face of `h` and changes all halfedges incident to the face into border halfedges or removes them from the graph if they were already border halfedges.

If this creates isolated vertices they get removed as well.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Precondition
  `h` is not a border halfedge

See also
  `make_hole()` for a more specialized variant.

## ◆ split_edge()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::split_edge | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

splits the halfedge `h` into two halfedges inserting a new vertex that is a copy of `vertex(opposite(h,g),g)`.

Is equivalent to `opposite(split_vertex( prev(h,g), opposite(h,g),g), g)`.

Returns
  the new halfedge `hnew` pointing to the inserted vertex. The new halfedge is followed by the old halfedge, i.e., `next(hnew,g) == h`.

## ◆ split_face()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::split_face | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h1,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h2,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

splits the face incident to `h1` and `h2`.

Creates the opposite halfedges `h3` and `h4`, such that `next(h1,g) == h3` and `next(h2,g) == h4`. Performs the inverse operation to `join_face()`. The new face is incident to `h4`.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Parameters

  g | the graph
  h1 |
  h2 |

Returns
  `h3`

Precondition
  `h1` and `h2` are incident to the same face
  `h1 != h2`
  `next(h1,g) != h2` and `next(h2,g) != h1` (no loop)

## ◆ split_loop()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::split_loop | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h1,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h2,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h3,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

cuts the graph along the cycle `(h1,h2,h3)` changing the genus (halfedge `h3` runs on the backside of the three dimensional figure below).

Three new vertices, three new pairs of halfedges, and two new triangular faces are created.

`h1`, `h2`, and `h3` will be incident to the first new face.

Note that `split_loop()` does not deal with properties of new vertices, halfedges, and faces.

Template Parameters

  Graph | must be a `MutableFaceGraph`

Returns
  the halfedge incident to the second new face.

Precondition
  `h1`, `h2`, and `h3` denote distinct, consecutive halfedges of the graph and form a cycle: i.e., `target(h1) == target(opposite(h2,g),g)`, … , `target(h3,g) == target(opposite(h1,g),g)`.
  The six faces incident to `h1`, `h2`, and `h3` are all distinct.

## ◆ split_vertex()

template<typename Graph >

boost::graph_traits< Graph >::halfedge_descriptor CGAL::Euler::split_vertex | ( | typename boost::graph_traits< Graph >::halfedge_descriptor | h1,
|  | typename boost::graph_traits< Graph >::halfedge_descriptor | h2,
|  | Graph & | g
| ) |  |

`#include <CGAL/boost/graph/Euler_operations.h>`

splits the target vertex `v` of `h1` and `h2`, and connects the new vertex and `v` with a new edge.

Let `hnew` be `opposite(next(h1, g), g)` after the split. The split regroups the halfedges around the two vertices. The edge sequence `hnew`, `opposite(next(h2, g), g)`, ..., `h1` remains around the old vertex, while the halfedge sequence `opposite(hnew, g)`, `opposite(next(h1, g), g)` (before the split), ..., `h2` is regrouped around the new vertex. The split returns `hnew`, i.e., the new edge incident to vertex `v`. The time is proportional to the distance from `h1` to `h2` around the vertex.

Template Parameters

  Graph | must be a model of `MutableFaceGraph`

Parameters

  g | the graph
  h1 | halfedge descriptor
  h2 | halfedge descriptor

Returns
  `hnew`

Precondition
  `target(h1, g) == target(h2, g)`, that is `h1` and `h2` are incident to the same vertex
  `h1 != h2`, that is no antennas

See also
  `join_vertex()`