Tetrahedra outside the domain (polyline stitching) in CGAL 3D mesh generation

## Issue Details **Unexpected behavior** <img width="1687" height="252" alt="Image" src="https://github.com/user-attachments/assets/c0cf6517-3999-47fc-9434-8abb0d73ef5e" /> <img width="1504" height="627" alt="Image" src="https://github.com/user-attachments/assets/b12dffef-f19c-433d-ab92-3936c9a41ecd" /> I am generating a 3D volume mesh from a surface mesh using: ``` MeshCriteria( CGAL::parameters::facet_size(longestSurfaceEdge) .cell_size(longestSurfaceEdge) .facet_distance(facetDistance) .edge_min_size(0.1 * minTubeLength) .edge_size(longestSurfaceEdge) .cell_radius_edge_ratio(2) .facet_angle(30) ); using Kernel = CGAL::Exact_predicates_inexact_constructions_kernel; using MeshDomain = CGAL::Polyhedral_mesh_domain_with_features_3<Kernel>; MeshDomain domain(surface); domain.add_features(<"the red polylines seen below">); C3t3 c3t3 = CGAL::make_mesh_3<C3t3>( domain, criteria, CGAL::parameters::no_exude().no_perturb()); struct FacetDistance { FacetDistance(double insideValue, double outsideValue, CGAL::AABB_tree<Traits>* domainAndTubeTree, CGAL::AABB_tree<Traits>* crackTree) : insideValue(insideValue) , outsideValue(outsideValue) , domainAndTubeTree(domainAndTubeTree) , crackTree(crackTree) { } double operator()( const Point_3& p, int dimension, const MeshDomain::Index& /*index*/) const { if (dimension != 2) LOG_ERROR( "Querying facet distance parameter with dimension not equal to 2!"); auto domainDistance = domainAndTubeTree->squared_distance(p); auto tubeCrackDistance = crackTree->squared_distance(p); bool isTubeCrack = domainDistance >= tubeCrackDistance; return isTubeCrack ? insideValue : outsideValue; } private: double insideValue; double outsideValue; CGAL::AABB_tree<Traits>* domainAndTubeTree; CGAL::AABB_tree<Traits>* crackTree; }; ``` **The cells circled in red are the problem. They lie outside the domain. This is caused by adding polylines.** This is the surface mesh of the domain: <img width="1730" height="373" alt="Image" src="https://github.com/user-attachments/assets/ac855a3d-4046-4884-bab2-948d3b269cf4" /> In red you can see the polylines that are added. The two polylines along the crack/slit are just two polylines. Each only has two corners, at the ends of the crack/slit. Other points are added as intermediate points of the polyline. ### Questions 1. How to fix the cells that are outside the domain? I can make the sampling parameter edge_min_size small enough for the cells to disappear, as you can see below. But then I have really small cells, which is not wanted. Is there a better way? They could also be removed in postprocessing. <img width="1735" height="280" alt="Image" src="https://github.com/user-attachments/assets/cab884c0-db9f-4e14-85ac-513624df8c44" /> 2. How to have larger facets inside the crack/slit? - For the crack/slit surfaces to be recognized as separate surfaces, I need to set facet_distance parameter to be small enough on the crack/slit surface patches. This is done with a sizing field which is defined above. - This works, but causes the facets on the surface to be very small, as you can see below (on the transparent plot). I do not want such small facets on the surface as it is flat, so they are unnecessary. I tried setting facet_min_size to be larger, but then the two surfaces were no longer recognized as separate and were meshed over. <img width="1682" height="938" alt="Image" src="https://github.com/user-attachments/assets/aee5967e-a0b9-4efb-8e3a-25206be907a7" /> ## Environment * Operating system: Linux 64 bit * Compiler: g++ (GCC) 14.2.0 * Release or debug mode: Debug * Specific flags used: CGAL::Sequential_tag (for CGAL::Mesh_triangulation_3<>), -DCGAL_DISABLE_GMP=1 * CGAL version: 6.1 * Boost version: 1.89.0 * Other libraries versions if used (Eigen, TBB, etc.): /