CGAL 6.1 - 2D and 3D Linear Geometry Kernel

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CGAL::Projection_traits_xy_3< Gt > Class Template Reference
Reference Manual » Kernel Classes

`#include <CGAL/Projection_traits_xy_3.h>`

## Definition

template<typename Gt>
class CGAL::Projection_traits_xy_3< Gt >

The class `Projection_traits_xy_3` is an adapter to apply 2D algorithms to the projections of 3D data on the `xy`-plane.

CGAL provides also predefined geometric traits classes `Projection_traits_yz_3<Gt>` and `Projection_traits_xz_3<Gt>` to deal with projections on the `zx`- and the `zy`-plane, respectively.

Template Parameters

  Gt | must be a model of `ProjectionTraitsGeometricTraits_3`

Note
  Internal constructions (projections) are used in the predicate and construction functors of this class. If `Gt` is a model of `Kernel` providing exact constructions or if `Gt` is a `CGAL::Filtered_kernel` (such as for `CGAL::Exact_predicates_inexact_constructions_kernel`), this class automatically provides exact predicates.

Is model of
  The class is a model of several 2D triangulation traits class concepts, except that it does not provide the type and constructors required to build the dual Voronoi diagram.
  `PolygonTraits_2`
  `ConvexHullTraits_2`
  `TriangulationTraits_2`
  `DelaunayTriangulationTraits_2`
  `ConstrainedTriangulationTraits_2`
  `ConvexHullTraits_2`
  `DelaunayMeshTraits_2`
  `AnalyticWeightTraits_2`
  `Barycentric_coordinates::BarycentricTraits_2`

See also
  `CGAL::Projection_traits_3`

## Types
typedef Point_3< Gt > | Point_2
typedef Segment_3< Gt > | Segment_2
typedef Triangle_3< Gt > | Triangle_2
typedef Line_3< Gt > | Line_2

## Functors
The functors provided by this class are those listed in the concepts, except that it does not provide the type and constructors required to build the dual Voronoi diagram. The functors operate on the 2D projection of their arguments. They come with preconditions that projections of the arguments are non-degenerate, e.g. a line segment does not project on a single point, two points do not project on the same point, etc. In the following, we specify the choice of the `z`-coordinate in case a new point is constructed.
typedef unspecified_type | Intersect_2
| A construction object.

## Creation
| Projection_traits_xy_3 ()
| default constructor.
| Projection_traits_xy_3 (Projection_traits_xy_3 tr)
| Copy constructor.
Projection_traits_xy_3 | operator= (Projection_traits_xy_3 tr)
| Assignment operator.

## Member Typedef Documentation

## ◆ Intersect_2

template<typename Gt >

typedef unspecified_type CGAL::Projection_traits_xy_3< Gt >::Intersect_2

A construction object.

Provides the operator :

`std::optional< std::variant<Point_2,Segment_2> > operator()(Segment_2 s1, Segment_2 s2);` which returns a 3D object whose projection on the xy-plane is the intersection of the projections of `s1` and `s2`. If non empty, the returned object is either a segment or a point. Its embedding in 3D is computed as the interpolation between `s1` and `s2`, meaning that any point `p` of the returned object is the midpoint of segment `p1p2` where `p1` and `p2` are the two points of `s1` and `s2` respectively, both projecting on `p`.

Precondition
  The projection of `s1` and the projection of `s2` are non-degenerate `2D` segments.