CGAL users discussion list

[Sebastian Bachmann <>]

Great, thanks for the code, Sebastien! I'll give it a go, but I can not 
promise I get this done too soon :)

Best,
Sebastian

On 10.05.2022 09:25, Sebastien Loriot ( via 
cgal-discuss Mailing List) wrote:
> You need to update the mesh domain you are using.
>
> You create a new one that inherit from the one currently used
> (Mesh_domain_with_polyline_features_3).
>
> Then you internally define
>
> struct My_domain:
>      public Domain
> {
>    My_domain(const Domain& d)
>      : Domain(d)
>    {}
>
>    struct Construct_intersection
>      : public Base_domain::Construct_intersection
>    {
>      typedef Base_domain::Construct_intersection Base_functor;
>      typedef Base_domain::Intersection Intersection;
>
>      Construct_intersection(const Mesh_domain& domain)
>        : Base_functor(domain)
>        , r_domain_(domain)
>      {}
>
>      template <typename Query>
>      Intersection operator()(const Query& query) const
>      {
>        //fill here the special case. Query can be Line_3, Ray_3 or 
> Segment_3
>        // if the cube is not intersected use Base_functor
>
>        return static_cast<const Base_functor&>(*this).operator()(query);
>      }
>
>    };
>
>    Construct_intersection construct_intersection_object() const
>    {
>      return Construct_intersection(*this);
>    }
> };
>
> I'd advise to first make it work the My_domain and operator() of 
> Construct_intersection simply returning the base functor value and
> then start modify that piece of code.
>
> Do not hesitate to come back to us if you have questions (and to share
> the working code :))
>
> Best,
>
> Sebastien.
>
> On 5/9/22 17:04, Sebastian Bachmann ( via cgal-discuss 
> Mailing List) wrote:
> > On 09.05.2022 16:13, Sebastien Loriot ( via 
> > cgal-discuss Mailing List) wrote:
> > > On the attached screenshot, you'll see in read the intersection of
> > > the output mesh with the cube (0 0 0 -> 100 100 100).
> > >
> > > So clearly, the mesh is not exactly on the cube.
> > > The initialization (intersection of the image with the cube) is
> > > done with points exactly on the cube. But then, intersection points
> > > are computed by bisection (it is handled as an image).
> >
> > okay I see. but in that case I should see some points directly on the 
> > image boundary? Or does this not apply to the mesh vertices itself?
> >
> > > Snapping could be done by using a dedicated oracle using a different
> > > intersection computation function when the segment intersect the
> > > boundary cube.
> >
> > Do I understand it correctly, that the bisection is handled here: 
> > https://github.com/CGAL/cgal/blob/acbd1ee182211c6309f355925927802f036fbe13/Mesh_3/include/CGAL/Labeled_mesh_domain_3.h#L713 
> >
> > ?
> >
> > I.e. I would have to change that, such that in the case of the image 
> > boundary, not `mid` is returned but a point exactly on that surface?
> >
> > Best,
> > Sebastian
> >
> >
> >
> > > Best,
> > >
> > > Sebastien.
> > >
> > >
> > > On 5/9/22 10:43, hello ( via cgal-discuss Mailing List) 
> > > wrote:
> > > > Hello,
> > > >
> > > > sure! Attached is an example source code and the resulting file for 
> > > > the 420 example.
> > > >
> > > > I would have expected, that all nodes on the bounding surfaces are 
> > > > either at x/y/z 0 or 100 (as it is for the edges) but instead they 
> > > > appear roughly at 0 or 100 +-0.05.
> > > > It seems to correlate with the relative_error_bound, however I can 
> > > > only make the error smaller but not go away completely. (The 
> > > > attached file is generated with 1e-3)
> > > >
> > > > Best,
> > > > Sebastian
> > > >
> > > >
> > > > On 09.05.22 09:23, Sebastien Loriot ( via 
> > > > cgal-discuss Mailing List) wrote:
> > > > > Hello,
> > > > >
> > > > > could you share a minimal example showing the problem?
> > > > >
> > > > > Thanks,
> > > > >
> > > > > Sebastien.
> > > > >
> > > > > On 5/7/22 13:37, Sebastian Bachmann ( via cgal-discuss 
> > > > > Mailing List) wrote:
> > > > > > Hello,
> > > > > > No one has an idea? Is it not somehow possible to restrict nodes 
> > > > > > onto a surface? I.e. if I have the polyline of the surface's 
> > > > > > boundary, can I specify that all nodes must be on that surface?
> > > > > >
> > > > > > For now I have written a function to go over all nodes and check 
> > > > > > if they are near such a boundary face and then "pull" the nodes 
> > > > > > onto that surface. However, there are certain edge cases were this 
> > > > > > then yields collapsed triangles etc. Thus, solving this during 
> > > > > > meshing would be a great enhancement.
> > > > > >
> > > > > > Best,
> > > > > > Sebastian
> > > > > >
> > > > > > On 25.04.2022 07:16, Sebastian Bachmann ( via 
> > > > > > cgal-discuss Mailing List) wrote:
> > > > > > > Hello,
> > > > > > >
> > > > > > > I created an image meshing tool, similar to the one given here in 
> > > > > > > this example code: 
> > > > > > > https://doc.cgal.org/latest/Mesh_3/Mesh_3_2mesh_3D_image_with_features_8cpp-example.html 
> > > > > > > with the only difference, that I extract the surface elements and 
> > > > > > > not write the full 3D mesh.
> > > > > > > I noticed, that on the image boundary, the vertices would be not 
> > > > > > > exactly on the face but roughly +- the relative error bound given 
> > > > > > > in create_labeled_image_mesh_domain. For examples, nodes on the 
> > > > > > > xy plane at z=0 are not on z=0 but for example -0.01 to 0.01. 
> > > > > > > While the protected polylines are exact and have always z=0.
> > > > > > > I created for example a mesh from the 420.inr example with the 
> > > > > > > same meshing criteria as given in the example and can see that 
> > > > > > > behaviour there.
> > > > > > >
> > > > > > > While you can control the maximal deviation using the relative 
> > > > > > > error bound, there will always be some deviation. And if I set 
> > > > > > > the relative error bound too low, it seems the meshing never 
> > > > > > > finishes.
> > > > > > > I wonder now, if it is possible to constraint the mesh (similar 
> > > > > > > to the 1d polylines), such that the vertices for facets on the 
> > > > > > > image boundary, are always exactly on the image boundary?
> > > > > > >
> > > > > > > Best,
> > > > > > > Sebastian