CGAL users discussion list

[Claus Volko <>]

It seems to work better if I skip the step with the has_on check.

Am Do., 14. Sept. 2023 um 11:37 Uhr schrieb Claus Volko <>:

Yes, I am doing region growing on a mesh. In the end I would like to check which region intersects with a given line (orthogonal to the xz plane).

I can send you code that is ready to be built. I've attached a file to this email.

The effect of this code should be a ply file in which all regions are black except the one that contains the given point. This last region will be red.

I've already tested the code a bit and it does seem to work, but my tests are not finished yet.

Am Do., 14. Sept. 2023 um 11:32 Uhr schrieb Sebastien Loriot <>:

It is hard to understand what you are trying to do.
If you want to get help, you should post a minimal example showing what
you want to achieve.

Yesterday I thought you were doing region growing on a point set but now
it seems to be on a mesh.

Best,

Sebastien.

On 9/14/23 08:30, Claus Volko ( via cgal-discuss
Mailing List) wrote:
> 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
> < <mailto:>>:
>
>     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
>     < <mailto:>>:
>
>         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
>         < <mailto:>>:
>
>             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
>             < <mailto:>>:
>
>                 region_map()
>
>                 https://doc.cgal.org/latest/Shape_detection/classCGAL_1_1Shape__detection_1_1Region__growing.html#a60adecff409f4025585c64cd3bd70110 <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 (
>                 <mailto:> 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:>
>                 <mailto: <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:>
>                 <mailto: <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/>
>                 <http://www.cdvolko.net/ <http://www.cdvolko.net/>>
>                  >
>                  >
>                  >
>                  >
>                  >
>                  >     --
>                  >     Dipl.-Ing. Dr. Claus D. Volko, BSc
>                  > http://www.cdvolko.net/ <http://www.cdvolko.net/>
>                 <http://www.cdvolko.net/ <http://www.cdvolko.net/>>
>                  >
>                  >
>                  >
>                  >
>                  >
>                  > --
>                  > Dipl.-Ing. Dr. Claus D. Volko, BSc
>                  > http://www.cdvolko.net/ <http://www.cdvolko.net/>
>                 <http://www.cdvolko.net/ <http://www.cdvolko.net/>>
>                  >
>                  >
>                  >
>                  >
>                  > --
>                  > You are currently subscribed to cgal-discuss.
>                  > To unsubscribe or access the archives, go to
>                  > https://sympa.inria.fr/sympa/info/cgal-discuss
>                 <https://sympa.inria.fr/sympa/info/cgal-discuss>
>                  >
>
>                 --
>                 You are currently subscribed to cgal-discuss.
>                 To unsubscribe or access the archives, go to
>                 https://sympa.inria.fr/sympa/info/cgal-discuss
>                 <https://sympa.inria.fr/sympa/info/cgal-discuss>
>
>
>
>
>             --
>             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/>
>
>
>
>
>
> --
> Dipl.-Ing. Dr. Claus D. Volko, BSc
> http://www.cdvolko.net/ <http://www.cdvolko.net/>
>
>
>
>
> --
> You are currently subscribed to cgal-discuss.
> To unsubscribe or access the archives, go to
> https://sympa.inria.fr/sympa/info/cgal-discuss
>

--
You are currently subscribed to cgal-discuss.
To unsubscribe or access the archives, go to
https://sympa.inria.fr/sympa/info/cgal-discuss

--

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

--

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