Surface mesh approximation error or bug

[KeepFaithMe]

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Issue Details

When I used the function 'VSA::approximate_triangle_mesh' to approximate my triangle_mesh,a error or bug ocurred.The details of this error are as follows: CGAL error: assertion violation! Expression : chord_size > 2 File :..\vge_variational_shape_appraximation.h Line: 1856 I've dealt with other models with this function, and none of them have had this problem.So, I'm confused.

Source Code

The source code is https://doc.cgal.org/latest/Surface_mesh_approximation/Surface_mesh_approximation_2vsa_simple_approximation_example_8cpp-example.html CGAL version:4.14.1 the data is:https://gist.github.com/KeepFaithMe/adf34b1da4defc36fe16fdeb5d9e95c6

Environment

• Operating system (Windows/Mac/Linux, 32/64 bits):
• Compiler:
• Release or debug mode:
• Specific flags used (if any):
• CGAL version:
• Boost version:
• Other libraries versions if used (Eigen, TBB, etc.):

[sgiraudot]

The problem comes from the fact that your input is not 2-manifold: surface mesh approximation only works on 2-manifold meshes as stated at the beginning of the manual.

One way to fix this is to load your input as a polygon soup, then orient it (which will fix non-manifold cases by duplicating vertices) and build the mesh from it. See this example which does exactly this.

This modified code works for your data set:

#include <iostream>
#include <fstream>

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Polygon_mesh_processing/orient_polygon_soup.h>
#include <CGAL/Polygon_mesh_processing/polygon_soup_to_polygon_mesh.h>
#include <CGAL/IO/OFF_reader.h>
#include <CGAL/Surface_mesh.h>
#include <CGAL/Surface_mesh_approximation/approximate_triangle_mesh.h>

typedef CGAL::Exact_predicates_inexact_constructions_kernel Kernel;
typedef CGAL::Surface_mesh<Kernel::Point_3> Mesh;

namespace VSA = CGAL::Surface_mesh_approximation;

int main(int argc, char** argv)
{
  // First read polygon soup
  std::vector<Kernel::Point_3> points;
  std::vector<std::vector<std::size_t> > polygons;
  std::ifstream input(argv[1]);
  if(!input || !CGAL::read_OFF(input, points, polygons) || points.empty())
  {
    std::cerr << "Cannot open file " << std::endl;
    return EXIT_FAILURE;
  }

  // Then orient it
  CGAL::Polygon_mesh_processing::orient_polygon_soup(points, polygons);

  // Then create the mesh from it
  Mesh mesh;
  CGAL::Polygon_mesh_processing::polygon_soup_to_polygon_mesh(points, polygons, mesh);

  // The output will be an indexed triangle mesh
  std::vector<Kernel::Point_3> anchors;
  std::vector<std::array<std::size_t, 3> > triangles;

  // free function interface with named parameters
  VSA::approximate_triangle_mesh(mesh,
    CGAL::parameters::verbose_level(VSA::MAIN_STEPS).
    max_number_of_proxies(200).
    anchors(std::back_inserter(anchors)). // anchor points
    triangles(std::back_inserter(triangles))); // indexed triangles

  std::cout << "#anchor points: " << anchors.size() << std::endl;
  std::cout << "#triangles: " << triangles.size() << std::endl;

  return EXIT_SUCCESS;
}

[KeepFaithMe]

using the code :'CGAL::Polygon_mesh_processing::orient_polygon_soup(points, polygons)' and 'CGAL::Polygon_mesh_processing::polygon_soup_to_polygon_mesh(points, polygons, mesh)',will a new vertex or a new face be introduced?And Can the self-intersection problem in mesh be solved by using the above two pieces of code?

[afabri]

@sgiraudot let'add a precondition in the code.

[KeepFaithMe]

When I used the function 'VSA::approximate_triangle_mesh' to approximate my triangle_mesh, I found that the number of faces in each category varies greatly. Is there any parameter that can be adjusted to make the number of faces between categories more balance.

[afabri]

@sgiraudot As the function takes a FaceGraph as argument it is a combinatorial 2-manifold. Should the precondition be that it is a geometric 2-manifold?