feat!: mesh review follow-ups — constraint preservation, corefine, implicit-cut snapping

.github/workflows/ci.yml

@@ -15,7 +15,8 @@ jobs:
         os: [ubuntu-latest, macos-latest, macos-14]
         python-version: ${{ fromJSON(vars.PYTHON_VERSIONS)}}
     env:
-      MACOSX_DEPLOYMENT_TARGET: '15.0' 
+      MACOSX_DEPLOYMENT_TARGET: '15.0'
+      CGAL_VERSION: '6.0.1'
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
@@ -25,8 +26,17 @@ jobs:
         run: |
           python -m pip install --upgrade pip
       - if: matrix.os == 'ubuntu-latest'
+        # ubuntu-latest is still 24.04, whose libcgal-dev is 5.6. Passing an
+        # edge_is_constrained_map to the mesh-mesh PMP::clip does not compile
+        # there: 5.6's put() for Boolean_property_map takes a non-const
+        # reference and the corefinement internals pass it as const. Fixed in
+        # CGAL 6, so fetch the headers (CGAL is header-only) rather than wait
+        # for the runner image to catch up.
         run: |
-          sudo apt-get install -y libgl1-mesa-dev libeigen3-dev libcgal-dev
+          sudo apt-get install -y libgl1-mesa-dev libeigen3-dev libboost-dev libgmp-dev libmpfr-dev
+          wget -q https://github.com/CGAL/cgal/releases/download/v${CGAL_VERSION}/CGAL-${CGAL_VERSION}.tar.xz
+          tar -xf CGAL-${CGAL_VERSION}.tar.xz
+          echo "CMAKE_ARGS=-DCGAL_DIR=$PWD/CGAL-${CGAL_VERSION}/lib/cmake/CGAL" >> $GITHUB_ENV
       - if: ${{ startsWith(matrix.os, 'macos') }}
         run: |
           brew install cgal

CMakeLists.txt

@@ -10,7 +10,10 @@ endif()
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
-find_package(CGAL REQUIRED)
+# 6.0 minimum: the mesh-mesh PMP::clip with an edge_is_constrained_map does not
+# compile against 5.6 (its put() for Boolean_property_map takes a non-const ref).
+# Stating the floor turns a 200-line template error into one clear message.
+find_package(CGAL 6.0 REQUIRED)
 find_package(pybind11 REQUIRED)
 
 include_directories(${CMAKE_SOURCE_DIR}/src)

loop_cgal/bindings.cpp

@@ -31,6 +31,15 @@ PYBIND11_MODULE(_loop_cgal, m)
               "Clip the mesh with the halfspace ax+by+cz+d <= 0. "
               "Uses PMP::clip(mesh, Plane_3) directly — no corefinement, no skirt construction. "
               "Returns the number of faces removed (0 = no-op).")
+         .def("corefine", &TriMesh::corefine, py::arg("other"),
+              py::arg("use_exact_kernel") = true,
+              "Corefine this mesh with another, inserting the shared intersection "
+              "polyline into BOTH meshes in place. After the call the two meshes "
+              "carry coincident vertices/edges along the intersection curve, so "
+              "patches taken from each side stitch together watertight. "
+              "Returns the number of vertices added to this mesh. "
+              "Only the exact kernel is supported; use_exact_kernel=False raises "
+              "ValueError (inexact-kernel corefinement crashes).")
          .def("cut_with_surface", &TriMesh::cutWithSurface, py::arg("surface"),
               py::arg("preserve_intersection") = false,
               py::arg("preserve_intersection_clipper") = false,
@@ -51,7 +60,10 @@ PYBIND11_MODULE(_loop_cgal, m)
               "Vertex index pairs defining edges to be fixed in mesh when remeshing.")
          .def("cut_with_implicit_function", &TriMesh::cut_with_implicit_function,
               py::arg("property"), py::arg("value"),py::arg("cutmode") = ImplicitCutMode::KEEP_POSITIVE_SIDE,
-              "Cut the mesh with an implicit function defined by vertex properties.")
+              py::arg("snap_tol") = 1e-4,
+              "Cut the mesh with an implicit function defined by vertex properties. "
+              "snap_tol reuses an existing vertex when a crossing lands within that "
+              "fraction of an edge from it, preventing sliver triangles (0 = disable).")
          .def("_cgal_area", &TriMesh::area, "Surface area computed directly from CGAL mesh.")
          .def("_cgal_n_faces", &TriMesh::n_faces, "Number of faces in the CGAL mesh.")
          .def("_cgal_n_vertices", &TriMesh::n_vertices, "Number of vertices in the CGAL mesh.")