CGAL users discussion list

[Bob Bill <>]

Right, now it works! 

One more question: if I need to access the coordinates of the vertices of the resulting polygon, what is the best way to achieve this?

I did this way, but I'd love to hear your take on this:

//here poly_vec is the vector where the intersection is written to. I'm using that instead of a list so I can use []. In this case there the resulting polygon_with_holes is the 0-th entry of that vector

for (auto it = poly_vec[0].holes_begin(); it != poly_vec[0].holes_end(); ++it) { //iterate over holes

for (auto it_in_hole = it->vertices_begin(); it_in_hole != it->vertices_end(); ++it) { //iterate over poly

std::cout << "x-coordinate: " << it_in_hole->x() << '\n'

<< "y-coordinate: " << it_in_hole->y() << '\n';

}

}

Best,

Bob

Il venerdì 4 febbraio 2022, 14:56:25 CET, Sebastien Loriot" ( via cgal-discuss Mailing List) <> ha scritto:

I was about to answer the same thing as Christoper.

Best,

Sebastien

On 2/4/22 2:50 PM, Christopher Mitchell ( via cgal-discuss
Mailing List) wrote:
> 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 <
> <mailto:>> 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"
>    ( <mailto:> via cgal-discuss
>    Mailing List) < <mailto:>>
>    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
>    <https://urldefense.proofpoint.com/v2/url?u=https-3A__www.cgal.org_FAQ.html-23inexact-5FNT&d=DwMFaQ&c=slrrB7dE8n7gBJbeO0g-IQ&r=zsvFlmXfwI9H5xCMhxiHhQ&m=5WA2bcou1ZMQ4DLX2ggiRrrEe5CZNLQ0S7ktABh-owQLas3_p7oGcO_ORRdHWLwr&s=336f97XuAKXVN7JrE3mK_vYHoq6AXsJv3Kg_6UsR6xs&e=>
>
>    Best,
>
>    Sebastien.
>
>
>    On 2/4/22 2:26 PM, Bob Bill (
>    <mailto:> 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
>    <https://urldefense.proofpoint.com/v2/url?u=https-3A__stackoverflow.com_questions_10502672_compute-2Dintersection-2Darea-2Dof-2Dtwo-2Dpolygons-2Dwith-2Dcgal&d=DwMFaQ&c=slrrB7dE8n7gBJbeO0g-IQ&r=zsvFlmXfwI9H5xCMhxiHhQ&m=5WA2bcou1ZMQ4DLX2ggiRrrEe5CZNLQ0S7ktABh-owQLas3_p7oGcO_ORRdHWLwr&s=8-NlTFRUl4zF5FUpGJnJDmI9Ei9qujJ8SYEsZtGaF6g&e=>>,
>
>
>      > 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
>    <https://urldefense.proofpoint.com/v2/url?u=https-3A__stackoverflow.com_questions_10502672_compute-2Dintersection-2Darea-2Dof-2Dtwo-2Dpolygons-2Dwith-2Dcgal&d=DwMFaQ&c=slrrB7dE8n7gBJbeO0g-IQ&r=zsvFlmXfwI9H5xCMhxiHhQ&m=5WA2bcou1ZMQ4DLX2ggiRrrEe5CZNLQ0S7ktABh-owQLas3_p7oGcO_ORRdHWLwr&s=8-NlTFRUl4zF5FUpGJnJDmI9Ei9qujJ8SYEsZtGaF6g&e=>>
>      >
>      >
>      >
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