CGAL 5.6.3 - 3D Mesh Generation

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Functions
Mesh Generation Functions
3D Mesh Generation Reference

##

The two main functions to generate a mesh are `make_mesh_3()` and `refine_mesh_3()`.

The other functions allow to optimize an existing mesh.

## Functions
template<typename NamedParameters = CGAL::parameters::Default_named_parameters>
CGAL::Image_3 | CGAL::Mesh_3::generate_label_weights (const CGAL::Image_3 &image, const float &sigma)
| Free function that generates a `CGAL::Image_3` of weights associated to each voxel of `image`, to make the output mesh surfaces smoother.
template<typename C3T3 , typename NamedParameters = CGAL::parameters::Default_named_parameters>
Mesh_optimization_return_code | CGAL::exude_mesh_3 (C3T3 &c3t3, const NamedParameters &np=parameters::default_values())
| The function `exude_mesh_3()` performs a sliver exudation process on a Delaunay mesh.
template<typename C3T3 , typename MeshDomain , typename NamedParameters = CGAL::parameters::Default_named_parameters>
Mesh_optimization_return_code | CGAL::odt_optimize_mesh_3 (C3T3 &c3t3, const MeshDomain &domain, const NamedParameters &np=parameters::default_values())
| The function `odt_optimize_mesh_3()` is a mesh optimization process based on the minimization of a global energy function.
template<typename C3T3 , typename MeshDomain , typename NamedParameters = CGAL::parameters::Default_named_parameters>
Mesh_optimization_return_code | CGAL::lloyd_optimize_mesh_3 (C3T3 &c3t3, const MeshDomain &domain, const NamedParameters &np=parameters::default_values())
| The function `lloyd_optimize_mesh_3()` is a mesh optimization process based on the minimization of a global energy function.
template<typename C3T3 , typename MeshDomain , typename NamedParameters = CGAL::parameters::Default_named_parameters>
Mesh_optimization_return_code | CGAL::perturb_mesh_3 (C3T3 &c3t3, const MeshDomain &domain, const NamedParameters &np=parameters::default_values())
| The function `perturb_mesh_3()` is a mesh optimizer that improves the quality of a Delaunay mesh by changing the positions of some vertices of the mesh.
template<typename C3T3 , typename MeshDomain , typename MeshCriteria , typename NamedParameters = CGAL::parameters::Default_named_parameters>
void | CGAL::refine_mesh_3 (C3T3 &c3t3, const MeshDomain &domain, const MeshCriteria &criteria, const NamedParameters &np=parameters::default_values())
| The function `refine_mesh_3()` is a 3D mesh generator.
template<typename C3T3 , typename MeshDomain , typename MeshCriteria , typename NamedParameters = CGAL::parameters::Default_named_parameters>
C3T3 | CGAL::make_mesh_3 (const MeshDomain &domain, const MeshCriteria &criteria, const NamedParameters &np=parameters::default_values())
| The function `make_mesh_3()` is a 3D mesh generator.

## Function Documentation

## ◆ exude_mesh_3()

template<typename C3T3 , typename NamedParameters = CGAL::parameters::Default_named_parameters>

Mesh_optimization_return_code CGAL::exude_mesh_3 | ( | C3T3 & | c3t3,
|  | const NamedParameters & | np = `parameters::default_values()`
| ) |  |

`#include <CGAL/exude_mesh_3.h>`

The function `exude_mesh_3()` performs a sliver exudation process on a Delaunay mesh.

The sliver exudation process consists in optimizing the weights of vertices of the weighted Delaunay triangulation in such a way that slivers disappear and the quality of the mesh improves.

Warning
  This optimizer modifies the weight of vertices of the triangulation and, if called, must be the last optimizer to be called. If the mesh is refined after this optimization has been performed, all improvements will be lost.

Template Parameters

  C3T3 | a model of the concept `MeshComplex_3InTriangulation_3`.
  NamedParameters | a sequence of Named Parameters

Parameters

  c3t3 | the initial mesh that will be modified by the algorithm to represent the final optimized mesh.
  np | an optional sequence of Named Parameters among the ones listed below:

Optional Named Parameters

  is used to set up, in seconds, a CPU time limit after which the optimization process is stopped. This time is measured using the `Real_timer` class. The default value is 0 and means that there is no time limit. Type: `double` Precondition: `time_limit >= 0` Default: 0 |
  is designed to give, in degrees, a targeted lower bound on dihedral angles of mesh cells. The exudation process considers in turn all the mesh cells that have a smallest dihedral angle less than `sliver_bound` and tries to make them disappear by weighting their vertices. The optimization process stops when every cell in the mesh achieves this quality. The default value is 0 and means that there is no targeted bound: the exuder then runs as long as it can improve the smallest dihedral angles of the set of cells incident to some vertices. Type: `double` Precondition: `0<= sliver_bound <= 180` Default: 0 |

Returns
  a value of type `CGAL::Mesh_optimization_return_code` which is:
  - `CGAL::BOUND_REACHED` when the targeted bound for the smallest dihedral angle in the mesh is reached.
  - `CGAL::TIME_LIMIT_REACHED` when the time limit is reached.
  - `CGAL::CANT_IMPROVE_ANYMORE` when exudation process stops because it can no longer improve the smallest dihedral angle of the set of cells incident to some vertex in the mesh.

Example

// Exude without sliver_bound, using at most 10s CPU time
exude_mesh_3(c3t3,
parameters::time_limit(10));
CGAL::exude_mesh_3
Mesh_optimization_return_code exude_mesh_3(C3T3 &c3t3, const NamedParameters &np=parameters::default_values())
The function exude_mesh_3() performs a sliver exudation process on a Delaunay mesh.
Definition: exude_mesh_3.h:95

See also
  `CGAL::Mesh_optimization_return_code`
  `CGAL::make_mesh_3()`
  `CGAL::refine_mesh_3()`
  `CGAL::perturb_mesh_3()`
  `CGAL::lloyd_optimize_mesh_3()`
  `CGAL::odt_optimize_mesh_3()`

Examples
  Mesh_3/mesh_implicit_domains.cpp, Mesh_3/mesh_implicit_domains_2.cpp, Mesh_3/mesh_implicit_ellipsoid.cpp, and Mesh_3/mesh_optimization_lloyd_example.cpp.

## ◆ generate_label_weights()

template<typename NamedParameters = CGAL::parameters::Default_named_parameters>

CGAL::Image_3 CGAL::Mesh_3::generate_label_weights | ( | const CGAL::Image_3 & | image,
|  | const float & | sigma
| ) |  |

`#include <CGAL/Mesh_3/generate_label_weights.h>`

Free function that generates a `CGAL::Image_3` of weights associated to each voxel of `image`, to make the output mesh surfaces smoother.

The weights image is generated using the algorithm described by Stalling et al in [17]. The Insight toolkit is needed to compile this function.

Parameters

  image | the input labeled image from which the weights image is computed. Both will then be used to construct a `Labeled_mesh_domain_3`.
  sigma | the standard deviation parameter of the internal Gaussian filter, measured in real-world distances. The size of a voxel (e.g. shortest length or longest length) usually is a good value for this parameter. Note that if `sigma` is too small, the "stair-effect" of meshing from a voxel image can appear. On the other side, if `sigma` is too large, thin volumes (basically one voxel thick) may be lost in the meshing process because the computed weights are too blurry.

Returns
  a `CGAL::Image_3` of weights used to build a quality `Labeled_mesh_domain_3`, with the same dimensions as `image`

Examples
  Mesh_3/mesh_3D_weighted_image.cpp.

## ◆ lloyd_optimize_mesh_3()

template<typename C3T3 , typename MeshDomain , typename NamedParameters = CGAL::parameters::Default_named_parameters>

Mesh_optimization_return_code CGAL::lloyd_optimize_mesh_3 | ( | C3T3 & | c3t3,
|  | const MeshDomain & | domain,
|  | const NamedParameters & | np = `parameters::default_values()`
| ) |  |

`#include <CGAL/lloyd_optimize_mesh_3.h>`

The function `lloyd_optimize_mesh_3()` is a mesh optimization process based on the minimization of a global energy function.

In `lloyd_optimize_mesh_3()`, the minimized global energy may be interpreted as the \( L^1\)-norm of the error achieved when the function \( x^2\) is interpolated on the mesh domain using a piecewise linear function which is linear in each cell of the Voronoi diagram of the mesh vertices.

The optimizer `lloyd_optimize_mesh_3()` works in iterative steps. At each iteration, mesh vertices are moved into positions that bring to zero the energy gradient and the Delaunay triangulation is updated. Vertices on the mesh boundaries are handled in a special way so as to preserve an accurate representation of the domain boundaries.

Template Parameters

  C3T3 | a model of the concept `MeshComplex_3InTriangulation_3`.
  MD | a model of the concept `MeshDomain_3`.
  NamedParameters | a sequence of Named Parameters

Parameters

  c3t3 | the initial mesh that will be modified by the algorithm to represent the final optimized mesh.
  domain | the domain used to create the `c3t3` parameter
  np | an optional sequence of Named Parameters among the ones listed below:

Optional Named Parameters

  to set up, in seconds, a CPU time limit after which the optimization process is stopped. This time is measured using `CGAL::Real_timer`. 0 means that there is no time limit. Type: `double` Precondition: `time_limit >= 0` Default: 0 |
  limit on the number of performed iterations. 0 means that there is no limit on the number of performed iterations. Precondition: `max_iteration_number >=0` Type: `int` Default: 0 |
  designed to reduce running time of each optimization iteration. Any vertex that has a displacement less than a given fraction of the length of its shortest incident edge, is frozen (i.e. is not relocated). The parameter `freeze_bound` gives the threshold ratio. If it is set to 0, freezing of vertices is disabled. Precondition: `0<= freeze_bound <=1` Type: `double` Default: 0.01 |
  threshold ratio of stopping criterion based on convergence: the optimization process is stopped when at the last iteration the displacement of any vertex is less than a given fraction of the length of the shortest edge incident to that vertex. Precondition: `0 <=convergence <= 1` Type: `double` Default: 0.02 |
  completes the `freeze_bound` parameter. If it is set to `true` (default value), frozen vertices will not move anymore in next iterations. Otherwise, at each iteration, any vertex that moves, unfreezes all its incident vertices. Type: `bool` Default: true |

Returns
  a value of type `CGAL::Mesh_optimization_return_code` which is:
  - `CGAL::TIME_LIMIT_REACHED` when the time limit is reached.
  - `CGAL::MAX_ITERATION_NUMBER_REACHED` when `lloyd_optimize_mesh_3()` stops because it has performed `max_iteration_number` iterations.
  - `CGAL::CONVERGENCE_REACHED` when `lloyd_optimize_mesh_3()` stops because the convergence criterion is achieved.
  - `CGAL::ALL_VERTICES_FROZEN` when all vertices have been frozen, when the `do_freeze` parameter is set to true.
  - `CGAL::CANT_IMPROVE_ANYMORE` when `lloyd_optimize_mesh_3()` stops because most vertices have been frozen, and no better convergence can be reached.

Example

// Lloyd-smoothing until convergence reaches 0.01, freezing vertices which
// move less than 0.001*shortest_incident_edge_length
lloyd_optimize_mesh_3(c3t3,
domain,
parameters::convergence(0.01).
parameters::freeze_bound(0.001).
parameters::do_freeze(true));
CGAL::lloyd_optimize_mesh_3
Mesh_optimization_return_code lloyd_optimize_mesh_3(C3T3 &c3t3, const MeshDomain &domain, const NamedParameters &np=parameters::default_values())
The function lloyd_optimize_mesh_3() is a mesh optimization process based on the minimization of a gl...
Definition: lloyd_optimize_mesh_3.h:138

See also
  `CGAL::Mesh_optimization_return_code`
  `CGAL::make_mesh_3()`
  `CGAL::refine_mesh_3()`
  `CGAL::exude_mesh_3()`
  `CGAL::perturb_mesh_3()`
  `CGAL::odt_optimize_mesh_3()`

Note
  This function requires the Eigen library.

Examples
  Mesh_3/mesh_optimization_lloyd_example.cpp.

## ◆ make_mesh_3()

template<typename C3T3 , typename MeshDomain , typename MeshCriteria , typename NamedParameters = CGAL::parameters::Default_named_parameters>

C3T3 CGAL::make_mesh_3 | ( | const MeshDomain & | domain,
|  | const MeshCriteria & | criteria,
|  | const NamedParameters & | np = `parameters::default_values()`
| ) |  |

`#include <CGAL/make_mesh_3.h>`

The function `make_mesh_3()` is a 3D mesh generator.

It produces simplicial meshes which discretize 3D domains.

The mesh generation algorithm is a Delaunay refinement process followed by an optimization phase. The criteria driving the Delaunay refinement process may be tuned to achieve the user needs with respect to the size of mesh elements, the accuracy of boundaries approximation, etc.

The optimization phase is a sequence of optimization processes, amongst the following available optimizers: an ODT-smoothing, a Lloyd-smoothing, a sliver perturber, and a sliver exuder. Each optimization process can be activated or not, according to the user requirements and available time. By default, only the perturber and the exuder are activated. Note that the benefits of the exuder will be lost if the mesh is further refined afterward, and that ODT-smoothing, Lloyd-smoothing, and sliver perturber should never be called after the sliver exuder. In the case of further refinement, only the sliver exuder can be used.

The function outputs the mesh to an object which provides iterators to traverse the resulting mesh data structure or can be written to a file (see Examples ).

Template Parameters

  C3T3 | either a model of the concept `MeshComplex_3InTriangulation_3` or of `MeshComplexWithFeatures_3InTriangulation_3` if `MD` is a model of `MeshDomainWithFeatures_3`. The type `C3T3` is in particular required to provide a nested type `C3T3::Triangulation` for the 3D triangulation embedding the mesh. The vertex and cell base classes of the triangulation `C3T3::Triangulation` are required to be models of the concepts `MeshVertexBase_3` and `MeshCellBase_3` respectively.
  MD | either a model of the concept `MeshDomain_3` or of `MeshDomainWithFeatures_3` if 0 and 1-dimensional features of the input complex have to be accurately represented in the mesh.
  MC | either a model of the concept `MeshCriteria_3` or a model of `MeshCriteriaWithFeatures_3` if the domain has exposed features.
  NamedParameters | a sequence of Named Parameters

Parameters

  domain | the domain used to create the `c3t3` parameter. It is the sole link through which the domain to be discretized is known by the mesh generation algorithm.
  criteria | specifies the size and shape requirements for mesh tetrahedra and surface facets. These criteria form the rules which drive the refinement process. All mesh elements satisfy those criteria at the end of the refinement process. In addition, if the domain has features, the argument `criteria` provides a sizing field to guide the discretization of 1-dimensional exposed features.
  np | an optional sequence of Named Parameters among the ones listed below:

Optional Named Parameters

  If the domain is a model of `MeshDomainWithFeatures_3`, 0 and 1-dimensional features can be taken into account while generating the mesh. The following two named parameters control this option: `parameters::features(domain)` `parameters::no_features()` Default: `parameters::features(domain)` |
  In order to drive the meshing algorithm and ensure that the output mesh follows a desired topological criterion, three named parameters control this option: `parameters::manifold()` `parameters::manifold_with_boundary()` `parameters::non_manifold()` Note that the meshing algorithm cannot generate a manifold surface if the input surface is not manifold. Default: `parameters::non_manifold()` |
  `lloyd_optimize_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_lloyd()` `parameters::lloyd_optimize_mesh_3()` Default: `parameters::no_lloyd()` |
  `odt_optimize_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_odt()` `parameters::odt()` Default: `parameters::no_odt()` |
  `perturb_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_perturb()` `parameters::perturb()` Default: `parameters::perturb()` |
  `exude_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_exude()` `parameters::exude()` Default: `parameters::exude()` |

Note that regardless of which optimization processes are activated, they are always launched in the order that is a suborder of the following (see user manual for further details): ODT-smoother, Lloyd-smoother, perturber, and exuder.

Beware that optimization of the mesh is obtained by perturbing mesh vertices and modifying the mesh connectivity and that this has an impact on the strict compliance to the refinement criteria. Though a strict compliance to mesh criteria is guaranteed at the end of the Delaunay refinement, this may no longer be true after some optimization processes. Also beware that the default behavior does involve some optimization processes.

See also
  `refine_mesh_3()`
  `exude_mesh_3()`
  `perturb_mesh_3()`
  `lloyd_optimize_mesh_3()`
  `odt_optimize_mesh_3()`

## ◆ odt_optimize_mesh_3()

template<typename C3T3 , typename MeshDomain , typename NamedParameters = CGAL::parameters::Default_named_parameters>

Mesh_optimization_return_code CGAL::odt_optimize_mesh_3 | ( | C3T3 & | c3t3,
|  | const MeshDomain & | domain,
|  | const NamedParameters & | np = `parameters::default_values()`
| ) |  |

`#include <CGAL/odt_optimize_mesh_3.h>`

The function `odt_optimize_mesh_3()` is a mesh optimization process based on the minimization of a global energy function.

In `odt_optimize_mesh_3()`, the minimized global energy may be interpreted as the \( L^1\)-norm of the error achieved when the function \( x^2\) is interpolated on the mesh domain using a piecewise linear function which is linear in each mesh cell.

The optimizer `odt_optimize_mesh_3()` works in iterative steps. At each iteration, mesh vertices are moved into positions that bring to zero the energy gradient and the Delaunay triangulation is updated. Vertices on the mesh boundaries are handled in a special way so as to preserve an accurate representation of the domain boundaries.

Template Parameters

  C3T3 | a model of the concept `MeshComplex_3InTriangulation_3`
  MeshDomain | a model of the concept `MeshDomain_3`
  NamedParameters | a sequence of Named Parameters

Parameters

  c3t3 | the initial mesh and is modified by the algorithm to represent the final optimized mesh.
  domain | the domain used to create the `c3t3` parameter
  np | an optional sequence of Named Parameters among the ones listed below:

Optional Named Parameters

  is used to set up, in seconds, a CPU time limit after which the optimization process is stopped. This time is measured using `Real_timer`. The default value is 0 and means that there is no time limit. Type: `double` Precondition: `time_limit >= 0` Default: 0 |
  sets a limit on the number of performed iterations. The default value of 0 means that there is no limit on the number of performed iterations. Type: `std::size_t` Default: 0 |
  is a stopping criterion based on convergence: the optimization process is stopped, when at the last iteration, the displacement of any vertex is less than a given percentage of the length the shortest edge incident to that vertex. The parameter `convergence` gives the threshold ratio. Type: `double` Precondition: `0 <= convergence <= 1` Default: 0.02 |
  is designed to reduce running time of each optimization iteration. Any vertex that has a displacement less than a given percentage of the length of its shortest incident edge, is frozen (i.e. is not relocated). The parameter `freeze_bound` gives the threshold ratio. Type: `double` Precondition: `0 <= freeze_bound <= 1` Default: 0.01 |
  completes the `freeze_bound` parameter. If it is set to `true`, frozen vertices will not move anymore in next iterations. Otherwise, at each iteration, any vertex that moves, unfreezes all its incident vertices. Type: `bool` Default: true |

Returns
  a value of type `CGAL::Mesh_optimization_return_code` which is:
  - `CGAL::TIME_LIMIT_REACHED` when the time limit is reached.
  - `CGAL::MAX_ITERATION_NUMBER_REACHED` when `odt_optimize_mesh_3()` stops because it has performed `max_iteration_number` iterations.
  - `CGAL::CONVERGENCE_REACHED` when `odt_optimize_mesh_3()` stops because the convergence criterion is achieved.
  - `CGAL::ALL_VERTICES_FROZEN` when all vertices have been frozen, when the `do_freeze` parameter is set to true.
  - `CGAL::CANT_IMPROVE_ANYMORE` when `odt_optimize_mesh_3()` stops because most vertices have been frozen, and no better convergence can be reached.

Example

// 100 iterations of ODT-smoothing
odt_optimize_mesh_3(c3t3,
domain,
parameters::max_iteration_number = 100,
parameters::convergence = 0);
CGAL::odt_optimize_mesh_3
Mesh_optimization_return_code odt_optimize_mesh_3(C3T3 &c3t3, const MeshDomain &domain, const NamedParameters &np=parameters::default_values())
The function odt_optimize_mesh_3() is a mesh optimization process based on the minimization of a glob...
Definition: odt_optimize_mesh_3.h:133

See also
  `CGAL::Mesh_optimization_return_code`
  `CGAL::make_mesh_3()`
  `CGAL::refine_mesh_3()`
  `CGAL::exude_mesh_3()`
  `CGAL::perturb_mesh_3()`
  `CGAL::lloyd_optimize_mesh_3()`

## ◆ perturb_mesh_3()

template<typename C3T3 , typename MeshDomain , typename NamedParameters = CGAL::parameters::Default_named_parameters>

Mesh_optimization_return_code CGAL::perturb_mesh_3 | ( | C3T3 & | c3t3,
|  | const MeshDomain & | domain,
|  | const NamedParameters & | np = `parameters::default_values()`
| ) |  |

`#include <CGAL/perturb_mesh_3.h>`

The function `perturb_mesh_3()` is a mesh optimizer that improves the quality of a Delaunay mesh by changing the positions of some vertices of the mesh.

The perturber tries to improve the dihedral angles of the worst cells in the mesh degree by degree: the step number `n` is considered as successful if after this step the worst tetrahedron of the mesh has a minimal dihedral angle larger than `n` degrees. The perturber exits if this is not the case.

Template Parameters

  C3T3 | a model of the concept `MeshComplex_3InTriangulation_3`
  MD | a model of the concept `MeshDomain_3`
  NamedParameters | a sequence of Named Parameters

Parameters

  c3t3 | the initial mesh, modified by the algorithm to represent the final optimized mesh
  domain | the domain used to create the `c3t3` parameter
  np | an optional sequence of Named Parameters among the ones listed below:

Optional Named Parameters

  is used to set up, in seconds, a CPU time limit after which the optimization process is stopped. This time is measured using the `Real_timer` class. The default value is 0 and means that there is no time limit. Type: `double` Precondition: `time_limit >= 0` Default: 0 |
  is designed to give, in degrees, a targeted lower bound on dihedral angles of mesh cells. The function `perturb_mesh_3()` runs as long as steps are successful and step number `sliver_bound` (after which the worst tetrahedron in the mesh has a smallest angle larger than `sliver_bound` degrees) has not been reached. The default value is 0 and means that there is no targeted bound: the perturber then runs as long as steps are successful. Type: `double` Precondition: `0 <= sliver_bound <= 180` Default: 0 |

Returns
  a value of type `CGAL::Mesh_optimization_return_code` which is:
  - `CGAL::BOUND_REACHED` when the targeted bound for the smallest dihedral angle in the mesh is reached.
  - `CGAL::TIME_LIMIT_REACHED` when the time limit is reached.
  - `CGAL::CANT_IMPROVE_ANYMORE` when the perturbation process stops because the last step is unsuccessful.

Example

// Perturb until every dihedral angle of the mesh is >= 10 degrees
// No time bound is set
perturb_mesh_3(c3t3,
domain,
parameters::sliver_bound = 10);
CGAL::perturb_mesh_3
Mesh_optimization_return_code perturb_mesh_3(C3T3 &c3t3, const MeshDomain &domain, const NamedParameters &np=parameters::default_values())
The function perturb_mesh_3() is a mesh optimizer that improves the quality of a Delaunay mesh by cha...
Definition: perturb_mesh_3.h:105

See also
  `CGAL::Mesh_optimization_return_code`
  `CGAL::make_mesh_3()`
  `CGAL::refine_mesh_3()`
  `CGAL::exude_mesh_3()`
  `CGAL::lloyd_optimize_mesh_3()`
  `CGAL::odt_optimize_mesh_3()`

Examples
  Mesh_3/mesh_implicit_domains.cpp, Mesh_3/mesh_implicit_domains_2.cpp, Mesh_3/mesh_implicit_ellipsoid.cpp, and Mesh_3/mesh_optimization_example.cpp.

## ◆ refine_mesh_3()

template<typename C3T3 , typename MeshDomain , typename MeshCriteria , typename NamedParameters = CGAL::parameters::Default_named_parameters>

void CGAL::refine_mesh_3 | ( | C3T3 & | c3t3,
|  | const MeshDomain & | domain,
|  | const MeshCriteria & | criteria,
|  | const NamedParameters & | np = `parameters::default_values()`
| ) |  |

`#include <CGAL/refine_mesh_3.h>`

The function `refine_mesh_3()` is a 3D mesh generator.

It produces simplicial meshes which discretize 3D domains.

The mesh generation algorithm is a Delaunay refinement process followed by an optimization phase. The criteria driving the Delaunay refinement process may be tuned to achieve the user needs with respect to the size of mesh elements, the accuracy of boundaries approximation, etc.

The optimization phase is a sequence of optimization processes, amongst the following available optimizers: an ODT smoothing, a Lloyd smoothing, a sliver perturber, and a sliver exuder. Each optimization process can be activated or not, according to the user requirements and available time. By default, only the perturber and the exuder are activated. Note that the benefits of the exuder will be lost if the mesh is further refined afterward.

Attention
  The function template `refine_mesh_3()` may be used to refine a previously computed mesh, e.g.:
  C3T3 c3t3 = CGAL::make_mesh_3<C3T3>(domain,criteria);
  CGAL::refine_mesh_3(c3t3, domain, new_criteria);
  CGAL::refine_mesh_3
  void refine_mesh_3(C3T3 &c3t3, const MeshDomain &domain, const MeshCriteria &criteria, const NamedParameters &np=parameters::default_values())
  The function refine_mesh_3() is a 3D mesh generator.
  Definition: refine_mesh_3.h:264

Please note that we guarantee the result if and only if the domain does not change from one refinement to the next one.

Template Parameters

  C3T3 | either a model of the concept `MeshComplex_3InTriangulation_3` or of `MeshComplexWithFeatures_3InTriangulation_3` if `MD` is a model of `MeshDomainWithFeatures_3`. The type `C3T3` is in particular required to provide a nested type `C3T3::Triangulation` for the 3D triangulation embedding the mesh. The vertex and cell base classes of the triangulation `C3T3::Triangulation` are required to be models of the concepts `MeshVertexBase_3` and `MeshCellBase_3` respectively.
  MD | either a model of the concept `MeshDomain_3` or of `MeshDomainWithFeatures_3` if 0 and 1-dimensional features of the input complex have to be accurately represented in the mesh.
  MC | either a model of the concept `MeshCriteria_3` or a model of `MeshCriteriaWithFeatures_3` if the domain has exposed features.
  NamedParameters | a sequence of Named Parameters

Parameters

  c3t3 | the mesh to be refined that is modified by the refinement process. As the refinement process only adds points to the triangulation, all vertices of the triangulation of `c3t3` remain in the mesh during the refinement process. `c3t3` can be used to insert specific points in the domain to ensure that they will be contained in the final triangulation.
  domain | the domain used to create the `c3t3` parameter. It is the sole link through which the domain to be discretized is known by the mesh generation algorithm.
  criteria | specifies the size and shape requirements for mesh tetrahedra and surface facets. These criteria form the rules which drive the refinement process. All mesh elements satisfy those criteria at the end of the refinement process. In addition, if the domain has features, the argument `criteria` provides a sizing field to guide the discretization of 1-dimensional exposed features.
  np | an optional sequence of Named Parameters among the ones listed below. They control which optimization processes are performed and allow the user to tune the parameters of the optimization processes. Individual optimization parameters are not described here as they are internal types (see instead the documentation page of each optimizer). For each optimization algorithm, there exist two global functions that allow to enable or disable the optimizer.

Optional Named Parameters

  In order to drive the meshing algorithm and ensure that the output mesh follows a desired topological criterion, three named parameters control this option: `parameters::manifold()` `parameters::manifold_with_boundary()` `parameters::non_manifold()` Note that the meshing algorithm cannot generate a manifold surface if the input surface is not manifold. Default: `parameters::non_manifold()` |
  `lloyd_optimize_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_lloyd()` `parameters::lloyd_optimize_mesh_3()` Default: `parameters::no_lloyd()` |
  `odt_optimize_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_odt()` `parameters::odt()` Default: `parameters::no_odt()` |
  `perturb_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::no_perturb()` `parameters::perturb()` Default: `parameters::perturb()` |
  `exude_mesh_3()` can optionally be called after the meshing process. Two named parameters control this behavior: `parameters::exude()` `parameters::no_exude()` Default: `parameters::exude()` |

The optimization parameters can be passed in arbitrary order. If one parameter is not passed, its default value is used. The default values are `no_lloyd()`, `no_odt()`, `perturb()` and `exude()`. Note that regardless of which optimization processes are activated, they are always launched in the order that is a suborder of the following (see user manual for further details): ODT-smoother, Lloyd-smoother, perturber, and exuder.

Beware that optimization of the mesh is obtained by perturbing mesh vertices and modifying the mesh connectivity and that this has an impact on the strict compliance to the refinement criteria. Though a strict compliance to mesh criteria is guaranteed at the end of the Delaunay refinement, this may no longer be true after some optimization processes. Also beware that the default behavior does involve some optimization processes.

See also
  `CGAL::make_mesh_3()`
  `CGAL::parameters::manifold()`
  `CGAL::parameters::manifold_with_boundary()`
  `CGAL::parameters::non_manifold()`
  `CGAL::exude_mesh_3()`
  `CGAL::perturb_mesh_3()`
  `CGAL::lloyd_optimize_mesh_3()`
  `CGAL::odt_optimize_mesh_3()`
  `CGAL::parameters::exude()`
  `CGAL::parameters::no_exude()`
  `CGAL::parameters::perturb()`
  `CGAL::parameters::no_perturb()`
  `CGAL::parameters::lloyd()`
  `CGAL::parameters::no_lloyd()`
  `CGAL::parameters::odt()`
  `CGAL::parameters::no_odt()`

Examples
  Mesh_3/mesh_3D_gray_image_with_custom_initialization.cpp, Mesh_3/mesh_3D_image_with_custom_initialization.cpp, and Mesh_3/mesh_polyhedral_domain.cpp.