how can i do offset meshing with a negative offset value using coding as same as with the polyhedron_3.exe ?

[liu411417492]

Issue Details

when I use the polyhedron_3.exe, I found this demo can do offset meshing with a negative offset value (although the tips said you should input the positive value), and the demo gave me the negative offset mesh as a result。 so my question is how can I do the same thing with code? which API should I call?

I noticed that alpha_wrap_3 can not generate the result when the offset parameter is negative.

Source Code

Mesh sm_extend;

CGAL::alpha_wrap_3(all_teeth_R, 3, 3, sm_extend);

namespace params = CGAL::parameters;

CGAL::Subdivision_method_3::Loop_subdivision(sm_extend, params::number_of_iterations(2));

CGAL::IO::write_polygon_mesh("alpha_out_1121R_before.obj", sm_extend);

Mesh sm_test;

CGAL::alpha_wrap_3(sm_extend, 3, -3, sm_test);

CGAL::IO::write_polygon_mesh("alpha_out_1121R_compare.obj", sm_test);

Environment

• Operating system (Windows/Mac/Linux, 32/64 bits): Win 64
• Compiler: visual studio 2019
• Release or debug mode: release
• CGAL version: 5.5.3
• Boost version: 1.80
• Other libraries versions if used (Eigen, TBB, etc.):

[MaelRL]

Here is the code for the offset plugin of the demo.

3D alpha wrapping by default wraps from outside. You can also wrap from inside if you set up seeds, see this example.

[liu411417492]

Here is the code for the offset plugin of the demo.

3D alpha wrapping by default wraps from outside. You can also wrap from inside if you set up seeds, see this example.

the code for the offset plugin of the demo is too complicated for me ，but I try the example. ，I change the seed to be a point（0，0，0） which is out of input model，but the result did not changed ， so how should I modify the example code to achieve my goal

[MaelRL]

The seed point tags the region you want to carve: if you put the point outside, it'll carve outside, if you put it inside, it'll carve inside.

[liu411417492]

The seed point tags the region you want to carve: if you put the point outside, it'll carve outside, if you put it inside, it'll carve inside.

int testNAmain()

{

	namespace PMP = CGAL::Polygon_mesh_processing;

	using K = CGAL::Exact_predicates_inexact_constructions_kernel;
	using Point_3 = K::Point_3;

	using Mesh = CGAL::Surface_mesh<Point_3>;
	// Read the input
	const std::string filename = CGAL::data_file_path("armadillo.off");
	std::cout << "Reading " << filename << "..." << std::endl;

	Mesh input;
	if (!PMP::IO::read_polygon_mesh(filename, input))
	{
		std::cerr << "Invalid input." << std::endl;
		return EXIT_FAILURE;
	}

	std::cout << "Input: " << num_vertices(input) << " vertices, " << num_faces(input) << " faces" << std::endl;

	const double relative_alpha = 30.;
	const double relative_offset = 600.;

	// Compute the alpha and offset values
	CGAL::Bbox_3 bbox = CGAL::Polygon_mesh_processing::bbox(input);
	const double diag_length = std::sqrt(CGAL::square(bbox.xmax() - bbox.xmin()) +
		CGAL::square(bbox.ymax() - bbox.ymin()) +
		CGAL::square(bbox.zmax() - bbox.zmin()));

	const double alpha = diag_length / relative_alpha;
	const double offset = diag_length / relative_offset;

	// Construct the wrap
	CGAL::Real_timer t;
	t.start();

	// There is no limit on how many seeds can be used.
	// However, the algorithm automatically determines whether a seed can be used
	// to initialize the refinement based on a few conditions (distance to the offset, value of alpha, etc.)
	// See internal function Alpha_wrap_3::initialize_from_cavities() for more information
	std::vector<Point_3> seeds =
	{
	  Point_3(0, 0, 0) // a point within the armadillo surface
	};

	Mesh wrap;
	alpha_wrap_3(input, alpha, offset * -1, wrap, CGAL::parameters::seed_points(std::ref(seeds)));

	t.stop();
	std::cout << "Result: " << num_vertices(wrap) << " vertices, " << num_faces(wrap) << " faces" << std::endl;
	std::cout << "Took " << t.time() << " s." << std::endl;

	// Save the result
	std::string input_name = std::string(filename);
	input_name = input_name.substr(input_name.find_last_of("/") + 1, input_name.length() - 1);
	input_name = input_name.substr(0, input_name.find_last_of("."));
	std::string output_name = input_name + "_cavity_" + std::to_string(static_cast<int>(relative_alpha))
		+ "_" + std::to_string(static_cast<int>(relative_offset)) + ".off";
	std::cout << "Writing to " << output_name << std::endl;
	CGAL::IO::write_polygon_mesh(output_name, wrap, CGAL::parameters::stream_precision(17));

	return EXIT_SUCCESS;
}

Actually， I changed the example you gave for only the seed point, now the seed point (0,0,0) is on the different side compared with the demo, however, the result did not make any difference

[MaelRL]

Can you please share your input data and complete code?

[liu411417492]

Can you please share your input data and complete code?

sure！ Here's my code， actually， I just want to offset meshing with a negative offset value.

int testNAmain()
{

	namespace PMP = CGAL::Polygon_mesh_processing;

	using K = CGAL::Exact_predicates_inexact_constructions_kernel;
	using Point_3 = K::Point_3;

	using Mesh = CGAL::Surface_mesh<Point_3>;
	// Read the input
	const std::string filename = CGAL::data_file_path("armadillo.off");
	std::cout << "Reading " << filename << "..." << std::endl;

	Mesh input;
	if (!PMP::IO::read_polygon_mesh(filename, input))
	{
		std::cerr << "Invalid input." << std::endl;
		return EXIT_FAILURE;
	}

	std::cout << "Input: " << num_vertices(input) << " vertices, " << num_faces(input) << " faces" << std::endl;

	const double relative_alpha = 30.;
	const double relative_offset = 600.;

	// Compute the alpha and offset values
	CGAL::Bbox_3 bbox = CGAL::Polygon_mesh_processing::bbox(input);
	const double diag_length = std::sqrt(CGAL::square(bbox.xmax() - bbox.xmin()) +
		CGAL::square(bbox.ymax() - bbox.ymin()) +
		CGAL::square(bbox.zmax() - bbox.zmin()));

	const double alpha = diag_length / relative_alpha;
	const double offset = diag_length / relative_offset;

	// Construct the wrap
	CGAL::Real_timer t;
	t.start();

	// There is no limit on how many seeds can be used.
	// However, the algorithm automatically determines whether a seed can be used
	// to initialize the refinement based on a few conditions (distance to the offset, value of alpha, etc.)
	// See internal function Alpha_wrap_3::initialize_from_cavities() for more information
	std::vector<Point_3> seeds =
	{
	  Point_3(0, 0, 0) // a point within the armadillo surface
	};

	Mesh wrap;
	alpha_wrap_3(input, alpha, offset * -1, wrap, CGAL::parameters::seed_points(std::ref(seeds)));

	t.stop();
	std::cout << "Result: " << num_vertices(wrap) << " vertices, " << num_faces(wrap) << " faces" << std::endl;
	std::cout << "Took " << t.time() << " s." << std::endl;

	// Save the result
	std::string input_name = std::string(filename);
	input_name = input_name.substr(input_name.find_last_of("/") + 1, input_name.length() - 1);
	input_name = input_name.substr(0, input_name.find_last_of("."));
	std::string output_name = input_name + "_cavity_" + std::to_string(static_cast<int>(relative_alpha))
		+ "_" + std::to_string(static_cast<int>(relative_offset)) + ".off";
	std::cout << "Writing to " << output_name << std::endl;
	CGAL::IO::write_polygon_mesh(output_name, wrap, CGAL::parameters::stream_precision(17));

	return EXIT_SUCCESS;
}

int main(){
  testNAmain();
  return 1;
}

Here is the input model armadillo.off armadillo.zip

[MaelRL]

As documented, the offset parameter must be (strictly) positive.

Also, since you are interested in the inward offset, you should use a seed within the armadillo, for example -6.656980991, 25.0099659, 10.5044879. Here is what you get (note that since the alpha is large, you don't get the offset mesh in the tight parts):