CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

Indeed, construction are not exact when using double. So the
intersection point will never be on the plane.

Best,

Sebastien.

On 9/15/23 02:06, Claus Volko ( via cgal-discuss 
Mailing List) wrote:
> 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
>          > < <>
>         < <>>>:
>          >
>          >     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
>  
> <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
>  
> <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
>          >                  > <
>         <> <
>         <>>
>          >                 <
>         <> <
>         <>>>>:
>          >                  >
>          >                  >     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
>          >                  >     <
>         <> <
>         <>>
>          >                 <
>         <> <
>         <>>>>:
>          >                  >
>          >                  >         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/>>
>          >                 <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/>>
>          >                 <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/>>
>          >                 <http://www.cdvolko.net/
>         <http://www.cdvolko.net/> <http://www.cdvolko.net/
>         <http://www.cdvolko.net/>>>
>          >                  >
>          >                  >
>          >                  >
>          >                  >
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>          >             --
>          >             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/>>
>          >
>          >
>          >
>          >
>          >
>          > --
>          > Dipl.-Ing. Dr. Claus D. Volko, BSc
>          > http://www.cdvolko.net/ <http://www.cdvolko.net/>
>         <http://www.cdvolko.net/ <http://www.cdvolko.net/>>
>          >
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>     -- 
>     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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