CGAL users discussion list

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
#include <vector>
#include <iostream>
#include <CGAL/Mesh_triangulation_3.h>
#include <CGAL/Mesh_complex_3_in_triangulation_3.h>
#include <CGAL/Mesh_criteria_3.h>
#include <CGAL/make_mesh_3.h>
#include <CGAL/Image_3.h>
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Mesh_domain_with_polyline_features_3.h>
#include <CGAL/Polyhedron_3.h>
#include <CGAL/Labeled_mesh_domain_3.h>
typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef CGAL::Labeled_mesh_domain_3<K> Image_domain;
typedef CGAL::Mesh_domain_with_polyline_features_3<Image_domain> Mesh_domain;
#ifdef CGAL_CONCURRENT_MESH_3
typedef CGAL::Parallel_tag Concurrency_tag;
#else
typedef CGAL::Sequential_tag Concurrency_tag;
#endif
// Triangulation
typedef CGAL::Mesh_triangulation_3<Mesh_domain,CGAL::Default,Concurrency_tag>::type Tr;
typedef CGAL::Mesh_complex_3_in_triangulation_3<Tr> C3t3;
// Criteria
typedef CGAL::Mesh_criteria_3<Tr> Mesh_criteria;

typedef K::Point_3 Point_3;
typedef unsigned char Word_type;

using namespace CGAL::parameters;
#include <CGAL/Mesh_3/polylines_to_protect.h> // undocumented header

int main(int argc, char* argv[])
{
  const std::string fname = (argc>1)?argv[1]:CGAL::data_file_path("images/420.inr");
  // Loads image
  CGAL::Image_3 image;
  if(!image.read(fname)){
    std::cerr << "Error: Cannot read file " <<  fname << std::endl;
    return EXIT_FAILURE;
  }
  // Domain
  double tol = (argc>2)?std::stod(argv[2]):1e-3;
  Mesh_domain domain = Mesh_domain::create_labeled_image_mesh_domain(image, relative_error_bound=tol);
  std::vector<std::vector<Point_3> > polylines_on_bbox;
  CGAL::polylines_to_protect<Point_3, Word_type>(image, polylines_on_bbox);
  domain.add_features(polylines_on_bbox.begin(), polylines_on_bbox.end());


  Mesh_criteria criteria(edge_size=6,
                         facet_angle=30, facet_size=6, facet_distance=4,
                         cell_radius_edge_ratio=3, cell_size=8);
  // Meshing
  C3t3 c3t3 = CGAL::make_mesh_3<C3t3>(domain, criteria);

  // Extract surface
  CGAL::Polyhedron_3<K> poly;
  CGAL::facets_in_complex_3_to_triangle_mesh(c3t3, poly);

  std::ofstream mout("420_surface.off");
  mout << poly;
  mout.close();

  return 0;
}

Attachment: 420_surface.off
Description: object/oogl