CGAL 6.1 - 2D Visibility

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Visibility_2 Concept Reference
Reference Manual » Concepts

## Definition

A model of the `Visibility_2` concept can be attached to an `Arrangement_2` instance to answer visibility queries within the faces of this arrangement.

Has models
  `CGAL::Simple_polygon_visibility_2`
  `CGAL::Rotational_sweep_visibility_2`
  `CGAL::Triangular_expansion_visibility_2`

## Types
typedef unspecified_type | Arrangement_2
| The arrangement type of the input.
typedef Arrangement_2::Point_2 | Point_2
| The 2D point type used for the queries.
typedef Arrangement_2::Face_const_handle | Face_const_handle
| The face handle type of the input arrangement.
typedef Arrangement_2::Halfedge_const_handle | Halfedge_const_handle
| The halfedge handle type of the input arrangement.

## Tags
typedef unspecified_type | Regularization_category
| identifies whether the regularized visibility area is computed (either `#Tag_true` or `#Tag_false`).
typedef unspecified_type | Supports_general_polygon_category
| identifies whether general polygons (with holes) are supported (either `#Tag_true` or `#Tag_false`).
typedef unspecified_type | Supports_simple_polygon_category
| identifies whether simple polygons are supported (either `#Tag_true` or `#Tag_false`).

## Constructors
| Visibility_2 ()
| Default constructor creates an empty `Visibility_2` object that is not attached to any arrangement yet.
| Visibility_2 (const Arrangement_2 &arr)
| Constructs a `Visibility_2` object that is attached to `arr`.

## Functions
bool | is_attached () const
| Returns whether an arrangement is attached to the visibility object.
void | attach (const Arrangement_2 &arr)
| Attaches the given arrangement `arr` to the visibility object.
void | detach ()
| Detaches the arrangement from the visibility object it is currently attached to.
const Arrangement_2 & | arrangement_2 () const
| Access to the attached arrangement.
template<typename VisibilityArrangement_2 >
Visibility_arrangement_2::Face_handle | compute_visibility (const Point_2 &q, const Face_const_handle f, VisibilityArrangement_2 &out_arr) const
| Computes the visibility region for the given query point `q` in the face \( f \) of the arrangement that is attached to the visibility object.
template<typename VisibilityArrangement_2 >
Visibility_arrangement_2::Face_handle | compute_visibility (const Point_2 &q, const Halfedge_const_handle e, Visibility_arrangement_2 &out_arr) const
| Computes the visibility region in `e->face()` for the given query point `q` which must be located on `e`.

## Member Typedef Documentation

## ◆ Supports_general_polygon_category

typedef unspecified_type Visibility_2::Supports_general_polygon_category

identifies whether general polygons (with holes) are supported (either `#Tag_true` or `#Tag_false`).

A general polygon is represented by a face \( f \) with no isolated vertex and any edge of \( f \) separates \( f \) from another face. Note that this allows \( f \) to touch a vertex from several sides.

## ◆ Supports_simple_polygon_category

typedef unspecified_type Visibility_2::Supports_simple_polygon_category

identifies whether simple polygons are supported (either `#Tag_true` or `#Tag_false`).

A simple polygon is represented by a face \( f \) with no holes, no isolated vertex and any edge of \( f \) separates \( f \) from another face. Note that this allows \( f \) to touch a vertex from several sides.

## Member Function Documentation

## ◆ attach()

void Visibility_2::attach | ( | const Arrangement_2 & | arr | ) |

Attaches the given arrangement `arr` to the visibility object.

In case the object is already attached to another arrangement, the visibility object gets detached before being attached to `arr`.

## ◆ compute_visibility() [1/2]

template<typename VisibilityArrangement_2 >

Visibility_arrangement_2::Face_handle Visibility_2::compute_visibility | ( | const Point_2 & | q,
|  | const Face_const_handle | f,
|  | VisibilityArrangement_2 & | out_arr
| ) |  | const

Computes the visibility region for the given query point `q` in the face \( f \) of the arrangement that is attached to the visibility object.

The visibility region of `q` is stored in `out_arr`, that is, all features but the unbounded face of `out_arr` represent the visibility region.

Template Parameters

  VisibilityArrangement_2 | is the type of the output arrangement representing the visibility polygon. It must be an instance of `CGAL::Arrangement_2`, where its `CGAL::Arrangement_2::Traits_2` must be mutual convertible to `VisibilityArrangement_2::Traits_2`.

Parameters

  q | is the query point
  f | is the face of the arrangement in which the visibility region is computed
  out_arr | is the output arrangement

Precondition
  `f` is a face of `this->arrangement_2()`
  `q` is in the interior of the given face `f`

Returns
  the face handle to the face in `out_arr` that represents interior of the visibility region

## ◆ compute_visibility() [2/2]

template<typename VisibilityArrangement_2 >

Visibility_arrangement_2::Face_handle Visibility_2::compute_visibility | ( | const Point_2 & | q,
|  | const Halfedge_const_handle | e,
|  | Visibility_arrangement_2 & | out_arr
| ) |  | const

Computes the visibility region in `e->face()` for the given query point `q` which must be located on `e`.

If `q` is an interior point of `e`, the computed visibility region is restricted to the side indicated by the halfedge `e`. If `q` is an endpoint of `e`, the visibility region is restricted by `e` and `e->next()`. The visibility region of `q` is stored in `out_arr`, that is, all features but the unbounded face of `out_arr` represent the visibility region.

Template Parameters

  VisibilityArrangement_2 | is the type of the output arrangement representing the visibility polygon. It must be an instance of `CGAL::Arrangement_2`, where its `CGAL::Arrangement_2::Traits_2` must be mutual convertible to `VisibilityArrangement_2::Traits_2`.

Parameters

  q | is the query point
  e | the halfedge on which `q` is located
  out_arr | is the output arrangement

Precondition
  `e` is a halfedge of `this->arrangement_2()`
  `q` is on `e`
  `q` equals to `e->target()->point()` if `q` is an endpoint of `e`

Returns
  a handle to the face in `out_arr` that represents the interior of the visibility region