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CGAL 4.8 - 2D Polygons

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User Manual

Authors
  Geert-Jan Giezeman and Wieger Wesselink

# Introduction

A polygon is a closed chain of edges. Several algorithms are available for polygons. For some of those algorithms, it is necessary that the polygon is simple. A polygon is simple if edges don't intersect, except consecutive edges, which intersect in their common vertex.

The following algorithms are available:

- find the leftmost, rightmost, topmost and bottommost vertex.
- compute the (signed) area.
- check if a polygon is simple.
- check if a polygon is convex.
- find the orientation (clockwise or counterclockwise)
- check if a point lies inside a polygon.

All those operations take two forward iterators as parameters in order to describe the polygon. These parameters have a point type as value type.

The type `Polygon_2` can be used to represent polygons. Polygons are dynamic. Vertices can be modified, inserted and erased. They provide the algorithms described above as member functions. Moreover, they provide ways of iterating over the vertices and edges.

Currently, the `Polygon_2` class is a nice wrapper around a container of points, but little more. Especially, computed values are not cached. That is, when the `Polygon_2::is_simple()` member function is called twice or more, the result is computed each time anew.

# Examples

## Polygon Class

The following example creates a polygon and illustrates the usage of some member functions.

File Polygon/Polygon.cpp

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Polygon_2.h>
#include <iostream>
typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef K::Point_2 Point;
typedef CGAL::Polygon_2<K> Polygon_2;
using std::cout; using std::endl;
int main()
{
Point points[] = { Point(0,0), Point(5.1,0), Point(1,1), Point(0.5,6)};
Polygon_2 pgn(points, points+4);
// check if the polygon is simple.
cout << "The polygon is " <<
(pgn.is_simple() ? "" : "not ") << "simple." << endl;
// check if the polygon is convex
cout << "The polygon is " <<
(pgn.is_convex() ? "" : "not ") << "convex." << endl;
return 0;
}

Figure 15.1 A polygon and some points

## Operating on Sequences of Points

The following example creates a polygon and illustrates the usage of some global functions that operate on sequences of points.

File Polygon/polygon_algorithms.cpp

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Polygon_2_algorithms.h>
#include <iostream>
typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef K::Point_2 Point;
using std::cout; using std::endl;
void check_inside(Point pt, Point *pgn_begin, Point *pgn_end, K traits)
{
cout << "The point " << pt;
switch(CGAL::bounded_side_2(pgn_begin, pgn_end,pt, traits)) {
case CGAL::ON_BOUNDED_SIDE :
cout << " is inside the polygon.\n";
break;
case CGAL::ON_BOUNDARY:
cout << " is on the polygon boundary.\n";
break;
case CGAL::ON_UNBOUNDED_SIDE:
cout << " is outside the polygon.\n";
break;
}
}
int main()
{
Point points[] = { Point(0,0), Point(5.1,0), Point(1,1), Point(0.5,6)};
// check if the polygon is simple.
cout << "The polygon is "
<< (CGAL::is_simple_2(points, points+4, K()) ? "" : "not ")
<< "simple." << endl;
check_inside(Point(0.5, 0.5), points, points+4, K());
check_inside(Point(1.5, 2.5), points, points+4, K());
check_inside(Point(2.5, 0), points, points+4, K());
return 0;
}

## in 3D Space

Sometimes it is useful to run a 2D algorithm on 3D data. Polygons may be contours of a 3D object, where the contours are organized in parallel slices, generated by segmentation of image data from a scanner.

In order to avoid an explixit projection on the `xy` plane, one can use the traits class `Projection_traits_xy_3` which is part of the 2D and 3D Linear Geometric Kernel.

File Polygon/projected_polygon.cpp

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Projection_traits_yz_3.h>
#include <CGAL/Polygon_2_algorithms.h>
#include <iostream>
typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef K::Point_3 Point_3;
int main()
{
Point_3 points[4] = { Point_3(0,1,1), Point_3(0,2,1), Point_3(0,2,2), Point_3(0,1,2) };
bool b = CGAL::is_simple_2(points,
points+4,
CGAL::Projection_traits_yz_3<K>());
if (!b){
std::cerr << "Error polygon is not simple" << std::endl;
return 1;
}
return 0;
}