#define CGAL_PMP_SMOOTHING_DEBUG 1
#define CGAL_PMP_SMOOTHING_OUTPUT_INTERMEDIARY_STEPS 1

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Surface_mesh.h>
#include <CGAL/draw_surface_mesh.h>
#include <CGAL/Polygon_mesh_processing/smooth_shape.h>
#include <CGAL/Polygon_mesh_processing/IO/polygon_mesh_io.h>
#include <iostream>
#include <set>
#include <string>
typedef CGAL::Exact_predicates_inexact_constructions_kernel   K;
typedef CGAL::Surface_mesh<K::Point_3>                        Mesh;
namespace PMP = CGAL::Polygon_mesh_processing;
int main(int argc, char* argv[])
{
	QCoreApplication::addLibraryPath("C:/Qt/5.15.2/msvc2019_64/plugins");
	const std::string filename = (argc > 1) ? argv[1] : "../meshes/pig.off";
	Mesh mesh;
	if(!PMP::IO::read_polygon_mesh(filename, mesh))
	{
		std::cerr << "Invalid input." << std::endl;
		return 1;
	}
	draw(mesh);
	const unsigned int nb_iterations = (argc > 2) ? std::atoi(argv[2]) : 10;
	const double time = (argc > 3) ? std::atof(argv[3]) : 0.0001;
	std::set<Mesh::Vertex_index> constrained_vertices;
	for(Mesh::Vertex_index v : vertices(mesh))
	{
		if(is_border(v, mesh))
			constrained_vertices.insert(v);
	}
	std::cout << "Constraining: " << constrained_vertices.size() << " border vertices" << std::endl;
	CGAL::Boolean_property_map<std::set<Mesh::Vertex_index> > vcmap(constrained_vertices);


	std::cerr.rdbuf(std::cout.rdbuf());
	std::set<Mesh::Face_index> faces_to_smooth;
	faces_to_smooth.insert(faces(mesh).begin(), faces(mesh).end()); // case 1: all faces -> success
	//faces_to_smooth.insert(faces(mesh).begin(), faces(mesh).begin() + 1); // case 2: 1 face -> fail !!! why ???
	std::cout << "Smoothing shape... (" << nb_iterations << " iterations)" << std::endl;
	PMP::smooth_shape(faces_to_smooth, mesh, time, CGAL::parameters::number_of_iterations(nb_iterations)
												   .vertex_is_constrained_map(vcmap));
	CGAL::IO::write_polygon_mesh("../meshes/mesh_shape_smoothed.off", mesh, CGAL::parameters::stream_precision(17));
	std::cout << "Done!" << std::endl;
	draw(mesh);


	return EXIT_SUCCESS;
}

Using OpenGL context 4.3 GL
Constraining: 55 border vertices
Smoothing shape... (10 iterations)
iteration #0
iteration #1
iteration #2
iteration #3
iteration #4
iteration #5
iteration #6
iteration #7
iteration #8
iteration #9
Done!
Using OpenGL context 4.3 GL

Using OpenGL context 4.3 GL
Constraining: 55 border vertices
Smoothing shape... (10 iterations)
iteration #0
Could not factorize linear system with preconditioner.
Failed to solve system!
Done!
Using OpenGL context 4.3 GL