The CGAL Open Source Project is pleased to announce the release 6.0 of CGAL, the Computational Geometry Algorithms Library.

Besides fixes and general enhancement to existing packages, the following has changed since CGAL 5.6:

### General Changes

- CGAL 6.0 is the first release of CGAL that requires a C++ compiler
  with the support of C++17 or later. The new list of supported compilers is:
  - Visual C++ 15.9, 16.10, 17.0 (from Visual Studio 2017, 2019 and 2022) or later
  - Gnu g++ 11.4.0 or later (on Linux or macOS)
  - LLVM Clang version 15.0.7 or later (on Linux)
  - Apple Clang compiler versions 10.0.1, 12.0.5, and 15.0.0 (on macOS)
- The minimal supported version of Boost is now 1.72.0.
- GMP/MPFR are no longer mandatory to use CGAL, [Boost.Multiprecision](https://www.boost.org/doc/libs/1_72_0/libs/multiprecision/doc/html/index.html).
  can be used instead.
- The CGAL `Core` library is no longer based on GMP, but on
  [Boost.Multiprecision](https://www.boost.org/doc/libs/1_72_0/libs/multiprecision/doc/html/index.html).
  Either GMP backend or Boost backend can be used.
- All demos are now based on Qt6.
- **Breaking change**: The CMake file `UseCGAL.cmake` has been removed from CGAL.
  Usages of the CMake variables `${CGAL_USE_FILE}` and `${CGAL_LIBRARIES}` must be replaced
  by a link to the imported target `CGAL::CGAL`, for example:
  `target_link_library(your_target PRIVATE CGAL::CGAL)`.

### [Kinetic Space Partition](https://doc.cgal.org/6.0/Manual/packages.html#PkgKineticSpacePartition) (new package)

-   This package implements kinetic space partition: based on a set of planar input shapes,
    the bounding box of the input data is split into convex volumes. The complexity of the partition
    can be adjusted with a single parameter.

### [Kinetic Surface Reconstruction](https://doc.cgal.org/6.0/Manual/packages.html#PkgKineticSurfaceReconstruction) (new package)

-   The package implements a piece-wise planar surface reconstruction pipeline from point clouds
    combining methods from the [Shape Detection](https://doc.cgal.org/6.0/Manual/packages.html#PkgShapeDetection),
    [Shape Regularization](https://doc.cgal.org/6.0/Manual/packages.html#PkgShapeRegularization)
    and [Kinetic Shape Partition](https://doc.cgal.org/6.0/Manual/packages.html#PkgKineticSpacePartition) packages
    and graph-cut to reconstruct surfaces from point clouds.

### [Basic Viewer](https://doc.cgal.org/6.0/Basic_viewer/index.html#Chapter_Basic_viewer) (new package)

-   The basic viewer package provides interactive visualization for most CGAL packages,
    such as [2D Arrangements](https://doc.cgal.org/6.0/Manual/packages.html#PkgArrangementOnSurface2),
    [2D Regularized Boolean Set-Operations](https://doc.cgal.org/6.0/Manual/packages.html#PkgBooleanSetOperations2),
    [Linear Cell Complex](https://doc.cgal.org/6.0/Manual/packages.html#PkgLinearCellComplex),
    [3D Boolean Operations on Nef Polyhedra](https://doc.cgal.org/6.0/Manual/packages.html#PkgNef3),
    [2D Periodic Triangulations](https://doc.cgal.org/6.0/Manual/packages.html#PkgPeriodic2Triangulation2),
    [3D Point Set](https://doc.cgal.org/6.0/Manual/packages.html#PkgPointSet3),
    [2D Polygons](https://doc.cgal.org/6.0/Manual/packages.html#PkgPolygon2),
    [3D Polyhedral Surface](https://doc.cgal.org/6.0/Manual/packages.html#PkgPolyhedron),
    [2D Straight Skeleton and Polygon Offsetting](https://doc.cgal.org/6.0/Manual/packages.html#PkgStraightSkeleton2),
    [Surface Mesh](https://doc.cgal.org/6.0/Manual/packages.html#PkgSurfaceMesh),
    [2D Triangulations](https://doc.cgal.org/6.0/Manual/packages.html#PkgTriangulation2),
    [3D Triangulations](https://doc.cgal.org/6.0/Manual/packages.html#PkgTriangulation3),
    [2D Voronoi Diagrams](https://doc.cgal.org/6.0/Manual/packages.html#PkgVoronoiDiagram2),
    and more.
    The most simple use case of the basic viewer is the call of the global `CGAL::draw()` function.
    There is one such `draw()` function for each CGAL package that has a basic viewer. Such a call opens
    an interactive window showing the given model and allowing to navigate in the scene,
    show or hide some specific cells, show the interior of the model if any, etc.
    The `Basic_viewer` is based on Qt6.

### [Polygon Repair](https://doc.cgal.org/6.0/Manual/packages.html#PkgPolygonRepair) (new package)

-   This package provides algorithms to repair 2D polygons, polygons with holes,
    and multipolygons with holes, by selecting faces of the arrangement of the input
    using the odd-even heuristic.

### [2D and 3D Linear Geometry Kernel](https://doc.cgal.org/6.0/Manual/packages.html#PkgKernel23)

-   **Breaking change**: Replaced all instances of `boost::variant` with `std::variant`
    in the intersection functions.
-   **Breaking change**: Replaced all instances of `boost::optional` with `std::optional`
    in the intersection functions.

### [3D Polyhedral Surface](https://doc.cgal.org/6.0/Manual/packages.html#PkgPolyhedron)

-   The demo of this package, also known as "Polyhedron Demo" has been renamed "CGAL Lab"
    and moved to its own package ("Lab").

### [2D and 3D Fast Intersection and Distance Computation (AABB Tree)](https://doc.cgal.org/6.0/Manual/packages.html#PkgAABBTree)

- The AABB tree can now be used with 2D or 3D primitives:
  - The concepts `AABBGeomTraits` and `AABBRayIntersectionGeomTraits`
    have been replaced by [`AABBGeomTraits_3`](https://doc.cgal.org/6.0/AABB_tree/classAABBGeomTraits__3.html)
    and by [`AABBRayIntersectionGeomTraits_3`](https://doc.cgal.org/6.0/AABB_tree/classAABBRayIntersectionGeomTraits__3.html),
    respectively.
  - The concepts [`AABBGeomTraits_2`](https://doc.cgal.org/6.0/AABB_tree/classAABBGeomTraits__2.html)
    and [`AABBRayIntersectionGeomTraits_2`](https://doc.cgal.org/6.0/AABB_tree/classAABBRayIntersectionGeomTraits__2.html)
    have been introduced, as the 2D counterparts.
  - The class [`CGAL::AABB_traits`](https://doc.cgal.org/6.0/AABB_tree/group__PkgAABBTreeRef.html#ga764f0fc59c96355877536810aa1aca5b)
    is deprecated and replaced by [`CGAL::AABB_traits_3`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__traits__3.html).
  - The class [`CGAL::AABB_traits_2`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__traits__2.html) is introduced as the 2D counterpart.
  - The class [`CGAL::AABB_segment_primitive`](https://doc.cgal.org/6.0/AABB_tree/group__PkgAABBTreeRef.html#gad0acfd5c4a3c081b7570cc6bd4594c8d)
    has been deprecated and replaced by the class [`CGAL::AABB_segment_primitive_3`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__segment__primitive__3.html).
  - The class [`CGAL::AABB_triangle_primitive`](https://doc.cgal.org/6.0/AABB_tree/group__PkgAABBTreeRef.html#ga54a56f01dc8024624f7d83ee0a01add0)
    has been deprecated and replaced by the class [`CGAL::AABB_triangle_primitive_3`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__triangle__primitive__3.html).
  - The following 2D primitive classes have been added:
    [`CGAL::AABB_segment_primitive_2`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__segment__primitive__2.html),
    [`CGAL::AABB_polyline_segment_primitive_2`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__polyline__segment__primitive__2.html),
    [`CGAL::AABB_triangle_primitive_2`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__triangle__primitive__2.html),
    [`CGAL::AABB_indexed_triangle_primitive_2`](https://doc.cgal.org/6.0/AABB_tree/classCGAL_1_1AABB__indexed__triangle__primitive__2.html).
- **Breaking change**: The concept [`AABBTraits`](https://doc.cgal.org/6.0/AABB_tree/classAABBTraits.html)
    now refines the concept [`SearchTraits`](https://doc.cgal.org/6.0/Spatial_searching/classSearchTraits.html).
- **Breaking change**: Replaced all instances of `boost::optional` with `std::optional`.

### [2D Arrangements](https://doc.cgal.org/6.0/Manual/packages.html#PkgArrangementOnSurface2)

-   **Breaking change**: Replaced all instances of `boost::variant` with `std::variant`.
-   **Breaking change**: The type of the result of point location queries has been changed to
    `std::variant`. Support for the old macro `CGAL_ARR_POINT_LOCATION_VERSION`
    has been removed.
-   **Breaking change**: Eliminated the error-prone C-type casting that was used to define observers.
    In general, backward compatibility was maintained; however, the class template
    [`CGAL::Arr_observer`](https://doc.cgal.org/6.0/Arrangement_on_surface_2/group__PkgArrangementOnSurface2Ref.html#ga8019f986f5469920136c4b92290b7b1b)
    has been replaced by an alias template. (The class `CGAL::Arr_observer`
    was renamed to [`CGAL::Aos_observer`](https://doc.cgal.org/6.0/Arrangement_on_surface_2/classCGAL_1_1Aos__observer.html)).
-   Introduced [`Arr_dcel`](https://doc.cgal.org/6.0/Arrangement_on_surface_2/classCGAL_1_1Arr__dcel.html),
    which essentially replaces the former `CGAL::Arr_default_dcel`.
    Backward compatibility was maintained by the introduction of the alias template
    [`CGAL::Arr_default_dcel`](https://doc.cgal.org/6.0/Arrangement_on_surface_2/group__PkgArrangementOnSurface2DCEL.html#gaf9635869a3794a46d7dcfce63d7de2a6).
    `CGAL::Arr_dcel`, as opposed to the former `CGAL::Arr_default_dcel` is templated
    (in addition to the geometry traits) by `Vertex`, `Halfedge`, and `Face` template parameters,
    and they have default type values. All this enables the layered extension of DCEL records.
-   Fixed a bug in the zone construction code applied to arrangements of geodesic arcs on a sphere,
    when inserting an arc that lies on the identification curve.
-   Introduced a new interactive program that demonstrates 2D arrangements embedded on the sphere
    called `earth`. The program (i) reads a database of all administrative boundaries of the countries
    in the world, (ii) displays the globe with all countries and land covered by water (which is land
    not covered by countries) on a window, and (ii) enables interaction with the user.

### [3D Envelopes](https://doc.cgal.org/6.0/Manual/packages.html#PkgEnvelope3)

-   **Breaking change**: [`Construct_projected_boundary_2`](https://doc.cgal.org/6.0/Envelope_3/classEnvelopeTraits__3.html#ac7b8f72870f0572834a0a3de62c67bc1)
    in [`EnvelopeTraits_3`](https://doc.cgal.org/6.0/Envelope_3/classEnvelopeTraits__3.html)
    now uses `std::variant` instead of [`CGAL::Object`](https://doc.cgal.org/6.0/STL_Extension/classCGAL_1_1Object.html).
-    Passed the base class of [`CGAL::Env_plane_traits_3`](https://doc.cgal.org/6.0/Envelope_3/classCGAL_1_1Env__plane__traits__3.html)
    as a template parameter with a default value (being the 2D arrangement linear traits).
    Similarly, passed the base class of `CGAL::Env_triangle_traits_3` as a template parameter
    with a default value (being the 2D arrangement segment traits).

### [Combinatorial Maps](https://doc.cgal.org/6.0/Manual/packages.html#PkgCombinatorialMaps) and [Generalized Maps](https://doc.cgal.org/6.0/Manual/packages.html#PkgGeneralizedMaps)

-    Added the function [`insert_cell_1_between_two_cells_2()`](https://doc.cgal.org/6.0/Combinatorial_map/classGenericMap.html#aa29570a0812094c7876e24a228373f12)
    to the [`GenericMap`](https://doc.cgal.org/6.0/Combinatorial_map/classGenericMap.html)
    concept, which enables users to insert an edge between two different faces in order to create faces with holes.

### [Quadtrees, Octrees, and Orthtrees](https://doc.cgal.org/6.0/Manual/packages.html#PkgOrthtree)

- **Breaking change**:
  - Node splitting behavior and per-node data are now customizable via the Traits class.
  - Nodes are now stored as a property map, with properties of each node accessed by index.
  - Nearest neighbors functions only work for Orthtrees which provide the necessary functionality.

### [CGAL and the Boost Graph Library (BGL)](https://doc.cgal.org/6.0/Manual/packages.html#PkgBGL)

-   Added the function [`CGAL::remove_all_elements()`](https://doc.cgal.org/6.0/BGL/group__PkgBGLHelperFct.html#gac7e199820c95ed1fc6ab536750749358),
    which removes vertices, halfedges, and faces without collecting garbage and without removing properties.
-   [Dynamic property maps](https://doc.cgal.org/6.0/BGL/group__PkgBGLPropertiesDynamic.html)
    can now have a default value.
-   The class [`CGAL::Face_filtered_graph`](https://doc.cgal.org/6.0/BGL/structCGAL_1_1Face__filtered__graph.html)
    now supports patch IDs of any type and not just `faces_size_type`. The only requirement is that
    the type is hashable.

### [Polygon Mesh Processing](https://doc.cgal.org/6.0/Manual/packages.html#PkgPolygonMeshProcessing)

-   Added the function [`CGAL::Polygon_mesh_processing::autorefine_triangle_soup()`](https://doc.cgal.org/6.0/Polygon_mesh_processing/group__PMP__corefinement__grp.html#gaec85370aa0b2acc0919e5f8406cfb74c),
    which can be used to refine a soup of triangles such that no pair of triangles intersects
    in their interiors. Also added, the function [`CGAL::Polygon_mesh_processing::autorefine()`](https://doc.cgal.org/6.0/Polygon_mesh_processing/group__PMP__corefinement__grp.html#ga3e3a0a82b6c04bdc3a6c070e8da4aed5)
    operating directly on a triangle mesh and updating it using the aforementioned function on a triangle soup.
-   Added the class [`CGAL::Corefinement::Non_manifold_output_visitor`](https://doc.cgal.org/6.0/Polygon_mesh_processing/structCGAL_1_1Polygon__mesh__processing_1_1Corefinement_1_1Non__manifold__output__visitor.html),
    which can be used in corefinement based functions to deal with non-manifold outputs.
-   Added the option to use a variable sizing field for [`CGAL::Polygon_mesh_processing::isotropic_remeshing()`](https://doc.cgal.org/6.0/Polygon_mesh_processing/group__PMP__meshing__grp.html#ga66cb01cf228ed22f0a2a474cfa2aeb3f),
    and a sizing function based on a measure of local curvature for adaptive remeshing.
-   Added the function [`CGAL::Polygon_mesh_processing::interpolated_corrected_curvatures()`](https://doc.cgal.org/6.0/Polygon_mesh_processing/group__PMP__corrected__curvatures__grp.html#ga22665c9ce92aaedab07df1b05f20bdb2)
    which can be used to compute the mean and Gaussian curvatures, as well as the principal curvature
    and directions.
-   Added the function [`CGAL::Polygon_mesh_processing::refine_mesh_at_isolevel()`](https://doc.cgal.org/6.0/Polygon_mesh_processing/group__PkgPolygonMeshProcessingRef.html#ga396505d5a60b5f6d29792b214fa59352)
    which can be used to refine a polygon mesh along an isocurve.
-   Added the function [`CGAL::Polygon_mesh_processing::add_bbox()`](https://doc.cgal.org/6.0/Polygon_mesh_processing/group__PkgPolygonMeshProcessingRef.html#gabaf98d2fd9ae599ff1f3a5a6cde79cf3),
    which enables adding a tight or extended, triangulated or not, bounding box to a face graph.

### [2D Triangulations](https://doc.cgal.org/6.0/Manual/packages.html#PkgTriangulation2)
-   **Breaking change**: the concept [`TriangulationTraits_2`](https://doc.cgal.org/6.0/Triangulation_2/classTriangulationTraits__2.html) now requires an additional functor `Compare_xy_2`.

### [3D Triangulations](https://doc.cgal.org/6.0/Manual/packages.html#PkgTriangulation3)

-   Added three member functions [`vertices()`](https://doc.cgal.org/6.0/Triangulation_3/classCGAL_1_1Triangulation__3.html#a02faf334255e1ca8caa1a6f412533759)
    to the class [`CGAL::Triangulation_3`](https://doc.cgal.org/6.0/Triangulation_3/classCGAL_1_1Triangulation__3.html).
    Each of them returns an array containing the vertices of the given triangulation simplex.

### [dD Triangulations](https://doc.cgal.org/6.0/Manual/packages.html#PkgTriangulations)

-   **Breaking change**: `CGAL::TDS_full_cell_mirror_storage_policy` is now unsupported in dimension larger than 127.
-   **Breaking change**: Inserting multiple unweighted points in the same
    position now keeps the first one, instead of switching to the latest. This
    only affects custom point types where not all points in the same position
    are equivalent.

### [Tetrahedral Remeshing](https://doc.cgal.org/6.0/Manual/packages.html#PkgTetrahedralRemeshing)

-   Added a sizing field as new parameter of [`CGAL::tetrahedral_isotropic_remeshing()`](https://doc.cgal.org/6.0/Tetrahedral_remeshing/group__PkgTetrahedralRemeshingRef.html#ga263775c52eeb483a86a16aeb9eb31af0),
    which can be used to perform non-uniform and adaptive remeshing.
-   **Breaking change**: The template parameters of
    [`CGAL::Tetrahedral_remeshing::Remeshing_cell_base_3`](https://doc.cgal.org/6.0/Tetrahedral_remeshing/classCGAL_1_1Tetrahedral__remeshing_1_1Remeshing__cell__base__3.html)
    have been modified, reverting changes introduced in CGAL 5.6.
-   **Breaking change**: The vertex base of
    [`CGAL::Tetrahedral_remeshing::Remeshing_vertex_base_3`](https://doc.cgal.org/6.0/Tetrahedral_remeshing/classCGAL_1_1Tetrahedral__remeshing_1_1Remeshing__vertex__base__3.html)
    must now be a model of the concept
    [`SimplicialMeshVertexBase_3`](https://doc.cgal.org/6.0/SMDS_3/classSimplicialMeshVertexBase__3.html)
    (and not only [`TriangulationVertexBase_3`](https://doc.cgal.org/6.0/Triangulation_3/classTriangulationVertexBase__3.html)).

### [3D Simplicial Mesh Data Structure](https://doc.cgal.org/6.0/Manual/packages.html#PkgSMDS3)

-   **Breaking change**: The template parameters of
    [`CGAL::Simplicial_mesh_cell_base_3`](https://doc.cgal.org/6.0/SMDS_3/classCGAL_1_1Simplicial__mesh__cell__base__3.html)
    have been modified to enable passing a geometric traits and a custom cell base class.

### [3D Mesh Generation](https://doc.cgal.org/6.0/Manual/packages.html#PkgMesh3)

-   **Breaking change**: Removed the concept `TriangleAccessor`, the template parameter `TriangleAccessor`,
    as well as the class `Triangle_accessor`. These were no longer used for several releases.
-   **Breaking change**: Removed the class templates `CGAL::Gray_image_mesh_domain_3`, `CGAL::Implicit_mesh_domain_3`,
    and `CGAL::Labeled_image_mesh_domain_3`, which were deprecated since CGAL-4.13.
-   Added new meshing criterion `edge_distance`, an upper bound for the distance from the edge to the 1D feature.
- **Breaking change**: the concept `MeshEdgeCriteria_3` was modified to include the new meshing criterion `edge_distance`.


### [3D Surface Mesh Generation](https://doc.cgal.org/6.0/Manual/packages.html#PkgSurfaceMesher3)

-   This package is deprecated and the package [3D Mesh Generation](https://doc.cgal.org/6.0/Manual/packages.html#PkgMesh3) should
    be used instead.

### [Surface Mesh Parameterization](https://doc.cgal.org/6.0/Manual/packages.html#PkgSurfaceMeshParameterization)

-   **Breaking change**: The method [`CGAL::Surface_mesh_parameterization::LSCM_parameterizer_3`](https://doc.cgal.org/6.0/Surface_mesh_parameterization/classCGAL_1_1Surface__mesh__parameterization_1_1LSCM__parameterizer__3.html)
    now requires the Eigen library.
-   **Breaking change**: CGAL no longer ships its own version of OpenNL.

### [Surface Mesh](https://doc.cgal.org/6.0/Manual/packages.html#PkgSurfaceMesh)

-   **Breaking change**: The return type of [`CGAL::Surface_mesh::property_map()`](https://doc.cgal.org/6.0/Surface_mesh/classCGAL_1_1Surface__mesh.html#afc99c7ea179dc1c21a2ab59ed183184a)
    has been changed to `std::optional`.

### [3D Point Set](https://doc.cgal.org/6.0/Manual/packages.html#PkgPointSet3)

-   **Breaking change**: The return type of [`CGAL::Point_set_3::property_map()`](https://doc.cgal.org/6.0/Point_set_3/classCGAL_1_1Point__set__3.html#a571ecc603cd32d78c7effaf86fe120ad)
    has been changed to `std::optional`.

### [Shape Detection](https://doc.cgal.org/6.0/Manual/packages.html#PkgShapeDetection)

-   **Breaking change**: Replaced all instances of `boost::shared_ptr` with `std::shared_ptr`.

### [2D Straight Skeleton and Polygon Offsetting](https://doc.cgal.org/6.0/Manual/packages.html#PkgStraightSkeleton2)

-   **Breaking change**: Replaced all instances of `boost::shared_ptr` with `std::shared_ptr`.
-   **Breaking change**: Replaced all instances of `boost::optional` with `std::optional`.

