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[Claus Volko <>]

I've got it now. It is perhaps not the most elegant piece of code but it seems to work fine:

  std::list<int> listRegions = {};

  for (std::size_t i = 0; i < regions.size(); i++)
  {
      Region_growing::Primitive_and_region region = regions[i];
      Line_3* l = new Line_3(Point_3(0, -1, 0), Point_3(0, +1, 0));
      auto intersection = boost::get<Point_3>(&*CGAL::intersection(region.first, *l));

      if (intersection && region.first.has_on(*intersection))
      {
          auto x11 = region.second.front()->halfedge()->vertex()->point().x();
          auto z11 = region.second.front()->halfedge()->vertex()->point().z();
          auto x12 = region.second.front()->halfedge()->next()->vertex()->point().x();
          auto z12 = region.second.front()->halfedge()->next()->vertex()->point().z();
          auto x13 = region.second.front()->halfedge()->next()->next()->vertex()->point().x();
          auto z13 = region.second.front()->halfedge()->next()->next()->vertex()->point().z();
          auto x1a = x11 < x12 ? x11 < x13 ? x11 : x13 : x12 < x13 ? x12 : x13;
          auto z1a = z11 < z12 ? z11 < z13 ? z11 : z13 : z12 < z13 ? z12 : z13;
          auto x1b = x11 > x12 ? x11 > x13 ? x11 : x13 : x12 > x13 ? x12 : x13;
          auto z1b = z11 > z12 ? z11 > z13 ? z11 : z13 : z12 > z13 ? z12 : z13;

          if (intersection->x() > x1a && intersection->x() < x1b && intersection->z() > z1a && intersection->z() < z1b)
          {
              std::cout << "Region " << i << " has point on it : " << region.first << " at " << *intersection << std::endl;
              listRegions.push_back(i);
          }
      }
  }

Am Do., 14. Sept. 2023 um 07:28 Uhr schrieb Claus Volko <>:

I tried coloring the regions with intersections red. The result looked strange. That's why I don't think the code I posted does its job.

Am Mi., 13. Sept. 2023 um 12:41 Uhr schrieb Claus Volko <>:

I'm now trying this:

  for (std::size_t i = 0; i < regions.size(); i++)
  {
      Region_growing::Primitive_and_region region = regions[i];
      Line_3* l = new Line_3(Point_3(0, -1, 0), Point_3(0, +1, 0));
      auto intersection = boost::get<Point_3>(&*CGAL::intersection(region.first, *l));

      if (intersection && region.first.has_on(*intersection)) std::cout << "Region has point on it: " << region.first << " at" << *intersection << std::endl;
  }

It returns fewer incidences of intersection, but I'm still not sure if it does what it should.

Am Mi., 13. Sept. 2023 um 11:39 Uhr schrieb Claus Volko <>:

I've realized the way I did it was not correct because it searched for the intersection of the plane with the line, which always returns a point. But I actually want to check for intersection with the finite region.

However, I've not understood how region_map() would help me in my endeavour. Could you perhaps point me to an example?

Thank you!

Am Mi., 13. Sept. 2023 um 11:03 Uhr schrieb Sebastien Loriot <>:

region_map()

https://doc.cgal.org/latest/Shape_detection/classCGAL_1_1Shape__detection_1_1Region__growing.html#a60adecff409f4025585c64cd3bd70110

Provides a property map associating indicating the region index for each
input element. The key of the map depends on your instantiation. But
everything should be documented in the documentation of your model.

Best,

Sebastien.

On 9/13/23 09:56, Claus Volko ( via cgal-discuss
Mailing List) wrote:
> OK, it seems I've found it out myself by now:
>
>    for (Region_growing::Primitive_and_region region : regions)
>    {
>        Line_3* l = new Line_3(Point_3(0, -10, 0), Point_3(0, +10, 0));
>
>        if (CGAL::intersection(region.first, *l))
>        {
>            std::cout << "Region has point on it" << std::endl;
>            break;
>        }
>    }
>
> Am Mi., 13. Sept. 2023 um 09:39 Uhr schrieb Claus Volko
> < <mailto:>>:
>
>     OK, I've realized by now that the code I posted was not wrong but
>     that what I really need is to check if a line intersects with a
>     region, as I don't know the y coordinate of the point I'm looking
>     for. How can that be done?
>
>     Am Mi., 13. Sept. 2023 um 07:04 Uhr schrieb Claus Volko
>     < <mailto:>>:
>
>         I would like to find the region that contains a given point
>         after using the region growing algorithm for shape detection. I
>         tried this code but it does not seem to work:
>
>            for (Region_growing::Primitive_and_region region : regions)
>            {
>                Point_3 *point = new Point_3(0, 0, 0);
>                if (region.first.has_on(*point))
>                    std::cout << "Region has zero point on it" << std::endl;
>            }
>
>         Also, if I found a region, how can I access the information
>         about the boundaries of the region?
>
>         --
>         Dipl.-Ing. Dr. Claus D. Volko, BSc
>         http://www.cdvolko.net/ <http://www.cdvolko.net/>
>
>
>
>
>
>     --
>     Dipl.-Ing. Dr. Claus D. Volko, BSc
>     http://www.cdvolko.net/ <http://www.cdvolko.net/>
>
>
>
>
>
> --
> Dipl.-Ing. Dr. Claus D. Volko, BSc
> http://www.cdvolko.net/ <http://www.cdvolko.net/>
>
>
>
>
> --
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Dipl.-Ing. Dr. Claus D. Volko, BSc
http://www.cdvolko.net/

--

Dipl.-Ing. Dr. Claus D. Volko, BSc
http://www.cdvolko.net/

--

Dipl.-Ing. Dr. Claus D. Volko, BSc
http://www.cdvolko.net/

--

Dipl.-Ing. Dr. Claus D. Volko, BSc
http://www.cdvolko.net/