CGAL users discussion list

[Christopher Mitchell <>]

I believe the problem may be that your second polygon is oriented CW instead of CCW.

Sincerely,

Christopher

On Fri, Feb 4, 2022 at 8:44 AM Bob Bill <> wrote:

Dear Sebastien,

thanks for the quick reply. I changed the code as you asked. However the result is still the same. Does it work properly on your machine?

Best,

Bob

Il venerdì 4 febbraio 2022, 14:29:17 CET, Sebastien Loriot" ( via cgal-discuss Mailing List) <> ha scritto:

Replace

typedef CGAL::Simple_cartesian<double> K;

by

#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
typedef CGAL::Exact_predicates_exact_constructions_kernel K;

More details about why here:
https://www.cgal.org/FAQ.html#inexact_NT

Best,

Sebastien.


On 2/4/22 2:26 PM, Bob Bill ( via cgal-discuss Mailing
List) wrote:
> I'm trying to find the area of the intersection between a square and
> another polygon. My code so far is the following (adapted from here
> <https://stackoverflow.com/questions/10502672/compute-intersection-area-of-two-polygons-with-cgal>,

> I just changed the points):
>
> #include<CGAL/Boolean_set_operations_2.h>
> #include<CGAL/Polygon_2.h>
> #include<CGAL/Polygon_2_algorithms.h>
> #include<CGAL/Polygon_with_holes_2.h>
> #include<CGAL/Simple_cartesian.h>
> #include<iostream>
>
> typedef CGAL::Simple_cartesian<double> K;
> typedef K::Point_2 Point;
> typedef CGAL::Polygon_2<K> Polygon_2;
> typedef CGAL::Polygon_with_holes_2<K> Polygon_with_holes_2;
>
> int main() {
> const std::vector<Point> pts{{-1, -1}, {1, -1}, {1, 1}, {-1, 1}};
> const std::vector<Point> pts2{{0, 1}, {0, 2}, {2, 1}, {1, 0}};
>
> Polygon_2 poly1(pts.begin(), pts.end());
> Polygon_2 poly2(pts2.begin(), pts2.end());
> std::list<Polygon_with_holes_2> polyI;
>
> CGAL::intersection(poly1, poly2, std::back_inserter(polyI));
>
> double totalArea = 0;
> typedef std::list<Polygon_with_holes_2>::iterator LIT;
> for (LIT lit = polyI.begin(); lit != polyI.end(); lit++) {
> std::cout << *lit << '\n';
> totalArea += lit->outer_boundary().area();
> }
> std::cout << "TotalArea::" << totalArea;
> }
>
> It compiles just fine, but then when I run it, I got the following output:
> *3 0 1 1 1 0 1  0 *
> *TotalArea::0*
>
> meaning that two points are collinear and hence the area is 0. What is
> my mistake here? I'm honestly a bit puzzled by the result and I don't
> know how I am supposed to fix this.
>
>
> Best,
> Bob
>
>    
>
>    
>
>
>    Compute intersection area of two polygons with cgal
>
> Given the vertices of two convex polygons, what is the simplest way of
> computing the area of their intersection ...

>
> <https://stackoverflow.com/questions/10502672/compute-intersection-area-of-two-polygons-with-cgal>
>
>
>
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