How can i detect if a point is in a polygon with Boost Within

Question

I'm trying to detect every coordinates possible for my polygon. (it's not everytime a triangle, it's for that).But when i'm trying to check every x & y possible, my program is not working at all.

I'm using https://www.boost.org/doc/libs/1_62_0/libs/geometry/doc/html/geometry/reference/algorithms/within/within_2.html

Here is my program

int main()
    {
    
        int x = 0;
        int y = 0;
        typedef boost::geometry::model::d2::point_xy<double> point_type;
        boost::geometry::model::polygon<point_type> poly2;
        boost::geometry::read_wkt("POLYGON((375 200,700 900,1100 190))", poly2);
        for (x = 0; x < 1200; x++)
        {
            if (x > 1150)
            {
                x = 0;
                y++;
            }
            else if (y > 1000)
                exit(0);
            else
            {
                point_type p(x, y);
                bool check_covered = boost::geometry::within(p, poly2);
                if (check_covered)
                {
                  std::cout << "in" << std::endl;
                }
            }
        }
        return 0;
    }

But i don't have output. Basically, they're never entering in my "if (check_covered)"

I don't understand why. When i'm using a draw to check if the polygon is working, i have my triangle

sehe · Accepted Answer · 2021-05-29 11:53:02Z

2

Geometry of polygon is not valid.

Model polygon reference

Template parameter(s)
bool Closed
Default true

You can check if a geometry is valid by boost::geometry::is_valid, for your polygon it returns false.

You may add the last point:

"POLYGON((375 200,700 900,1100 190,375 200))"

or call boost::geometry::correct(poly2) (it will add a missing point) before checking by within.

Here's a live demo, also making the point iteration a bit more effective:

Live On Coliru

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/polygon.hpp>
#include <iostream>

namespace bg  = boost::geometry;
namespace bgm = bg::model;
using Point   = bgm::d2::point_xy<double>;

int main()
{
    bgm::polygon<Point> poly;
    bgm::box<Point>     box;
    bg::read_wkt("POLYGON((375 200,700 900,1100 190,375 200))", poly);

    bg::envelope(poly, box);

    for (int x = box.min_corner().x(); x < box.max_corner().x(); ++x)
        for (int y = box.min_corner().y(); y < box.max_corner().y(); ++y)
            if (Point p(x,y); bg::within(p, poly)) {
                std::cout << "in: " << bg::wkt(p) << std::endl;
            }
}

Prints

in: POINT(376 200)
in: POINT(376 201)
in: POINT(376 202)
in: POINT(377 200)
in: POINT(377 201)
in: POINT(377 202)
in: POINT(377 203)
in: POINT(377 204)
...

etc.

edited May 29, 2021 at 11:53

sehe

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answered May 29, 2021 at 10:39

rafix07

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sehe Over a year ago

＋1, added a live demo On Coliru

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