CGAL 6.1.1 - 2D Apollonius Graphs (Delaunay Graphs of Disks)

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ApolloniusGraphDataStructure_2 Concept Reference
Reference Manual » Concepts

## Definition

The concept `ApolloniusGraphDataStructure_2` refines the concept `TriangulationDataStructure_2`. In addition it provides two methods for the insertion and removal of a degree 2 vertex in the data structure. The insertion method adds a new vertex to the specified edge, thus creating two new edges. Moreover, it creates two new faces that have the two newly created edges in common (see figure below). The removal method performs the reverse operation.

Insertion and removal of degree 2 vertices. Left to right: The edge `(f,i)` is replaced by two edges by means of inserting a vertex `v` on the edge. The faces \( f_1\) and \( f_2\) are created. Right to left: the faces \( f_1\) and \( f_2\) are destroyed. The vertex `v` is deleted and its two adjacent edges are merged.

We only describe the additional requirements with respect to the `TriangulationDataStructure_2` concept.

Refines
  `TriangulationDataStructure_2`

Has models
  `CGAL::Triangulation_data_structure_2<Vb,Fb>`

See also
  `TriangulationDataStructure_2`
  `ApolloniusGraphVertexBase_2`
  `TriangulationFaceBase_2`

## Insertion
Vertex_handle | insert_degree_2 (Face_handle f, int i)
| inserts a degree two vertex and two faces adjacent to it that have two common edges.

## Removal
void | remove_degree_2 (Vertex_handle v)
| Removes a degree 2 vertex and the two faces adjacent to it.

## Member Function Documentation

## ◆ insert_degree_2()

Vertex_handle ApolloniusGraphDataStructure_2::insert_degree_2 | ( | Face_handle | f,
|  | int | i
| ) |  |

inserts a degree two vertex and two faces adjacent to it that have two common edges.

The edge defined by the face handle `f` and the integer `i` is duplicated. It returns a handle to the vertex created.

## ◆ remove_degree_2()

void ApolloniusGraphDataStructure_2::remove_degree_2 | ( | Vertex_handle | v | ) |

Removes a degree 2 vertex and the two faces adjacent to it.

The two edges of the star of `v` that are not incident to it are collapsed.

Precondition
  The degree of `v` must be equal to 2.