Filtering ranges with predicates

This examples shows:

• How to use predicates to filter which elements of a mesh you want iterate over

In the previous tutorial we discussed already that the ranges returned by functions like all_vertices(), voh_range() or outgoing_halfedges() provide a few helpful methods such as avg() or to_vector(). Another interesting method is filtered() which requires as argument something that can be called for an element of the range and returns a bool. The resulting range will then only iterate over elements for which the filter returs true. The filter can be a lambda, a function pointer, a property manager holding a bool property, or a functor object such as the predicates defined in <OpenMesh/Core/Utils/Predicates.hh>. The predefined predicates can check the status of a mesh element and test if they are boundary. With their help you can for example count all boundary vertices:

  // Count boundary vertices
  std::cout << "Mesh contains " << mesh.vertices().count_if(Boundary()) << " boundary vertices";

Predicates can be composed using the operators ||, &&, and !. This enables you to specify precisely which elements you want process in your loop, e.g. inner vertices that are selected:

  // Selected inner vertices
  std::cout << "These are the selected inner vertices: " << std::endl;
  for (auto vh : mesh.vertices().filtered(!Boundary() && Selected()))
    std::cout << vh.idx() << ", ";
  std::cout << std::endl;

As mentioned above, the filter argument can be anything returning a bool for a given input, e.g. a function pointer:

  // Faces whose id is divisible by 3
  auto vec = mesh.faces().filtered(is_divisible_by_3).to_vector();
  std::cout << "There are " << vec.size() << " faces whose id is divisible by 3" << std::endl;

However, function pointers, lambdas etc are not composable. Fortunately, you can use make_predicate to turn them into composable predicates:

  // Faces which are tagged or whose id is not divisible by 3
  auto vec2 = mesh.faces().filtered(Tagged() || !make_predicate(is_divisible_by_3)).to_vector();
  std::cout << "There are " << vec2.size() << " faces which are tagged or whose id is not divisible by 3" << std::endl;

Below is the complete source code:

#include <OpenMesh/Core/IO/MeshIO.hh>
#include <OpenMesh/Core/Mesh/DefaultTriMesh.hh>
#include <OpenMesh/Core/Utils/PropertyManager.hh>
#include <OpenMesh/Core/Utils/Predicates.hh>

#include <iostream>
#include <vector>

using MyMesh = OpenMesh::TriMesh;

bool is_divisible_by_3(OpenMesh::FaceHandle vh) { return vh.idx() % 3 == 0; }

int main(int argc, char** argv)
{
  using namespace OpenMesh::Predicates; // for easier access to predicates

  // Read command line options
  MyMesh mesh;
  if (argc != 4) {
    std::cerr << "Usage: " << argv[0] << " infile" << std::endl;
    return 1;
  }
  const std::string infile = argv[1];
  
  // Read mesh file
  if (!OpenMesh::IO::read_mesh(mesh, infile)) {
    std::cerr << "Error: Cannot read mesh from " << infile << std::endl;
    return 1;
  }

  // Count boundary vertices
  std::cout << "Mesh contains " << mesh.vertices().count_if(Boundary()) << " boundary vertices";

  // Selected inner vertices
  std::cout << "These are the selected inner vertices: " << std::endl;
  for (auto vh : mesh.vertices().filtered(!Boundary() && Selected()))
    std::cout << vh.idx() << ", ";
  std::cout << std::endl;

  // Faces whose id is divisible by 3
  auto vec = mesh.faces().filtered(is_divisible_by_3).to_vector();
  std::cout << "There are " << vec.size() << " faces whose id is divisible by 3" << std::endl;

  // Faces which are tagged or whose id is not divisible by 3
  auto vec2 = mesh.faces().filtered(Tagged() || !make_predicate(is_divisible_by_3)).to_vector();
  std::cout << "There are " << vec2.size() << " faces which are tagged or whose id is not divisible by 3" << std::endl;

  // Edges that are longer than 10 or shorter than 2
  OpenMesh::EProp<bool> longer_than_10(mesh);
  for (auto eh : mesh.edges())
    longer_than_10[eh] = mesh.calc_edge_length(eh) > 10;
  std::cout << "There are " <<
    mesh.edges().count_if(make_predicate(longer_than_10) || make_predicate([&](OpenMesh::EdgeHandle eh) { return mesh.calc_edge_length(eh) < 2; })) <<
    " edges which are shorter than 2 or longer than 10" << std::endl;

}

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