Michal Meyerovitch, Ron Wein, and Baruch Zukerman
32.1  Introduction  
32.2  The EnvelopeTraits Concept  
32.3  Examples  
32.3.1 Example for Envelope of Triangles  
32.3.2 Example for Envelope of Spheres  
32.3.3 Example for Envelope of Planes 
A continuous surface S in ℝ^{3} is called xymonotone, if every line parallel to the zaxis intersects it at a single point at most. For example, the sphere x^{2} + y^{2} + z^{2} = 1 is not xymonotone as the zaxis intersects it at (0, 0, 1) and at (0, 0, 1); however, if we use the xyplane to split it to an upper hemisphere and a lower hemisphere, these two hemispheres are xymonotone.
An xymonotone surface can therefore be represented as a bivariate function z = S(x,y), defined over some continuous range R_{S} ⊆ ℝ^{2}. Given a set S = { S_{1}, S_{2}, … , S_{n} } of xymonotone surfaces, their lower envelope is defined as the pointwise minimum of all surfaces. Namely, the lower envelope of the set S can be defined as the following function:

Similarly, the upper envelope of S is the pointwise maximum of the xymonotone surfaces in the set:

Given a set of xymonotone surfaces S, the minimization diagram of S is a subdivision of the xyplane into cells, such that the identity of the surfaces that induce the lower diagram over a specific cell of the subdivision (be it a face, an edge, or a vertex) is the same. In nondegenerate situation, a face is induced by a single surface (or by no surfaces at all, if there are no xymonotone surfaces defined over it), an edge is induced by a single surface and corresponds to its projected boundary, or by two surfaces and corresponds to their projected intersection curve, and a vertex is induced by a single surface and corresponds to its projected boundary point, or by three surfaces and corresponds to their projected intersection point. The maximization diagram is symmetrically defined for upper envelopes. In the rest of this chapter, we refer to both these diagrams as envelope diagrams.
It is easy to see that an envelope diagram is no more than a planar arrangement (see Chapter 28), represented using an extended Dcel structure, such that every Dcel record (namely each face, halfedge and vertex) stores an additional container of it originators: the xymonotone surfaces that induce this feature.
Lower and upper envelopes can be efficiently computed using a divideandconquer approach. First note that the envelope diagram for a single xymonotone curve S_{k} is trivial to compute: we project the boundary of its range of definition R_{Sk} onto the xyplane, and label the faces it induces accordingly. Given a set D of (non necessarily xymonotone) surfaces in ℝ^{3}, we subdivide each surface into a finite number of weakly xymonotone surfaces,^{1} and obtain the set S. Then, we split the set into two disjoint subsets S_{1} and S_{2}, and we compute their envelope diagrams recursively. Finally, we merge the diagrams, and we do this by overlaying them and then applying some postprocessing on the resulting diagram. The postprocessing stage is nontrivial and involves the projection of intersection curves onto the xyplane  see [Mey06] for more details.
The implementation of the envelopecomputation algorithm is generic and can handle arbitrary surfaces. It is parameterized with a traits class, which defines the geometry of surfaces it handles, and supports all the necessary functionality on these surfaces, and on their projections onto the xyplane. The traits class must model the EnvelopeTraits_3 concept, the details of which are given below.
As the representation of envelope diagrams is based on 2D arrangements, and the envelopcomputation algorithm employs overlay of planar arrangements, the EnvelopeTraits_3 refines the ArrangementXMonotoneTraits_2 concept. Namely, a model of this concept must define the planar types Point_2 and X_monotone_curve_2 and support basic operations on them, as listed in Section 28.6. Moreover, it must define the spacial types Surface_3 and Xy_monotone_surface_3 (in practice, these two types may be the same). Any model of the envelopetraits concept must also support the following operations on these spacial types:
(a)  (b) 
This operation is used at the bottom of the recursion to build the minimization diagram of a single xymonotone surface.
This operation is used by the algorithm to determine the surface that induce the envelope over a face incident to c.
This operation is used by the algorithm to determine which surface induce the envelope over an edge associated with the xmonotone curve c, or of a face incident to c, in situations where the previous predicate cannot be used, as c is not an intersection curve of S_{1} and S_{2} (see Figure 32.1(b) for an illustration of a situation where this operation is used).
The package currently contains a traits class for named Env_triangle_traits_3<Kenrel> handling 3D triangles, and another named Env_sphere_traits_3<ConicTraits> for 3D spheres, based on geometric operations on conic curves (ellipses). In addition, the package includes a traitsclass decorator that enables users to attach external (nongeometric) data to surfaces. The usage of the various traits classes is demonstrated in the next section.
(a)  (b)  (c) 
The following example shows how to use the envelopetraits class for 3D triangles and how to traverse the envelope diagram. It constructs the lower and upper envelopes of the two triangles, as depicted in Figure 32.2(a) and prints the triangles that induce each face and each edge in the output diagrams. For convenience, we use the traitsclass decorator Env_surface_data_traits_3 to label the triangles. When printing the diagrams, we just output the labels of the triangles:
File: examples/Envelope_3/ex_envelope_triangles.cpp
#include <CGAL/Gmpq.h> #include <CGAL/Cartesian.h> #include <CGAL/Env_triangle_traits_3.h> #include <CGAL/Env_surface_data_traits_3.h> #include <CGAL/envelope_3.h> #include <iostream> #include <list> typedef CGAL::Gmpq Number_type; typedef CGAL::Cartesian<Number_type> Kernel; typedef CGAL::Env_triangle_traits_3<Kernel> Traits_3; typedef Kernel::Point_3 Point_3; typedef Traits_3::Surface_3 Triangle_3; typedef CGAL::Env_surface_data_traits_3<Traits_3, char> Data_traits_3; typedef Data_traits_3::Surface_3 Data_triangle_3; typedef CGAL::Envelope_diagram_2<Data_traits_3> Envelope_diagram_2; /* Auxiliary function  print the features of the given envelope diagram. */ void print_diagram (const Envelope_diagram_2& diag) { // Go over all arrangement faces. Envelope_diagram_2::Face_const_iterator fit; Envelope_diagram_2::Ccb_halfedge_const_circulator ccb; Envelope_diagram_2::Surface_const_iterator sit; for (fit = diag.faces_begin(); fit != diag.faces_end(); ++fit) { // Print the face boundary. if (fit>is_unbounded()) { std::cout << "[Unbounded face]"; } else { // Print the vertices along the outer boundary of the face. ccb = fit>outer_ccb(); std::cout << "[Face] "; do { std::cout << '(' << ccb>target()>point() << ") "; ++ccb; } while (ccb != fit>outer_ccb()); } // Print the labels of the triangles that induce the envelope on this face. std::cout << "> " << fit>number_of_surfaces() << " triangles:"; for (sit = fit>surfaces_begin(); sit != fit>surfaces_end(); ++sit) std::cout << ' ' << sit>data(); std::cout << std::endl; } // Go over all arrangement edges. Envelope_diagram_2::Edge_const_iterator eit; for (eit = diag.edges_begin(); eit != diag.edges_end(); ++eit) { // Print the labels of the triangles that induce the envelope on this edge. std::cout << "[Edge] (" << eit>source()>point() << ") (" << eit>target()>point() << ") > " << eit>number_of_surfaces() << " triangles:"; for (sit = eit>surfaces_begin(); sit != eit>surfaces_end(); ++sit) std::cout << ' ' << sit>data(); std::cout << std::endl; } return; } /* The main program: */ int main () { // Construct the input triangles, makred A and B. std::list<Data_triangle_3> triangles; triangles.push_back (Data_triangle_3 (Triangle_3 (Point_3 (0, 0, 0), Point_3 (0, 6, 0), Point_3 (5, 3, 4)), 'A')); triangles.push_back (Data_triangle_3 (Triangle_3 (Point_3 (6, 0, 0), Point_3 (6, 6, 0), Point_3 (1, 3, 4)), 'B')); // Compute and print the minimization diagram. Envelope_diagram_2 min_diag; CGAL::lower_envelope_3 (triangles.begin(), triangles.end(), min_diag); std::cout << std::endl << "The minimization diagram:" << std::endl; print_diagram (min_diag); // Compute and print the maximization diagram. Envelope_diagram_2 max_diag; CGAL::upper_envelope_3 (triangles.begin(), triangles.end(), max_diag); std::cout << std::endl << "The maximization diagram:" << std::endl; print_diagram (max_diag); return (0); }
The next example demonstrates how to instantiate and use the envelopetraits class for spheres, based on the Arr_conic_traits_2 class that handles the projected intersecion curves. The program reads a set of spheres from an input file and constructs their lower envelope:
File: examples/Envelope_3/ex_envelope_spheres.cpp
#include <CGAL/basic.h> #ifndef CGAL_USE_CORE #include <iostream> int main() { std::cout << "Sorry, this example needs CORE ..." << std::endl; return 0; } #else #include <CGAL/Cartesian.h> #include <CGAL/CORE_algebraic_number_traits.h> #include <CGAL/Arr_conic_traits_2.h> #include <CGAL/Env_sphere_traits_3.h> #include <CGAL/envelope_3.h> #include <CGAL/Timer.h> #include <iostream> #include <list> typedef CGAL::CORE_algebraic_number_traits Nt_traits; typedef Nt_traits::Rational Rational; typedef Nt_traits::Algebraic Algebraic; typedef CGAL::Cartesian<Rational> Rat_kernel; typedef Rat_kernel::Point_3 Rat_point_3; typedef CGAL::Cartesian<Algebraic> Alg_kernel; typedef CGAL::Arr_conic_traits_2<Rat_kernel, Alg_kernel, Nt_traits> Conic_traits_2; typedef CGAL::Env_sphere_traits_3<Conic_traits_2> Traits_3; typedef Traits_3::Surface_3 Sphere_3; typedef CGAL::Envelope_diagram_2<Traits_3> Envelope_diagram_2; int main(int argc, char **argv) { // Get the name of the input file from the command line, or use the default // fan_grids.dat file if no commandline parameters are given. const char * filename = (argc > 1) ? argv[1] : "spheres.dat"; // Open the input file. std::ifstream in_file(filename); if (! in_file.is_open()) { std::cerr << "Failed to open " << filename << " ..." << std::endl; return 1; } // Read the spheres from the file. // The input file format should be (all coordinate values are integers): // <n> // number of spheres. // <x_1> <y_1> <x_1> <R_1> // center and squared radious of sphere #1. // <x_2> <y_2> <x_2> <R_2> // center and squared radious of sphere #2. // : : : : // <x_n> <y_n> <x_n> <R_n> // center and squared radious of sphere #n. int n = 0; std::list<Sphere_3> spheres; int x = 0, y = 0, z = 0, sqr_r = 0; int i; in_file >> n; for (i = 0; i < n; ++i) { in_file >> x >> y >> z >> sqr_r; spheres.push_back(Sphere_3(Rat_point_3(x, y, z), Rational(sqr_r))); } in_file.close(); // Compute the lower envelope. Envelope_diagram_2 min_diag; CGAL::Timer timer; std::cout << "Constructing the lower envelope of " << n << " spheres." << std::endl; timer.start(); CGAL::lower_envelope_3(spheres.begin(), spheres.end(), min_diag); timer.stop(); // Print the dimensions of the minimization diagram. std::cout << "V = " << min_diag.number_of_vertices() << ", E = " << min_diag.number_of_edges() << ", F = " << min_diag.number_of_faces() << std::endl; std::cout << "Construction took " << timer.time() << " seconds." << std::endl; return 0; } #endif
The next example demonstrates how to instantiate and use the envelopetraits class for planes, based on the Arr_linear_traits_2 class that handles infinite linear objects such as lines and rays.
File: examples/Envelope_3/ex_envelope_planes.cpp
#include <CGAL/Gmpq.h> #include <CGAL/Cartesian.h> #include <CGAL/Env_plane_traits_3.h> #include <CGAL/envelope_3.h> #include <iostream> #include <list> typedef CGAL::Gmpq Number_type; typedef CGAL::Cartesian<Number_type> Kernel; typedef Kernel::Plane_3 Plane_3; typedef CGAL::Env_plane_traits_3<Kernel> Traits_3; typedef Traits_3::Surface_3 Surface_3; typedef CGAL::Envelope_diagram_2<Traits_3> Envelope_diagram_2; /* Auxiliary function  print the features of the given envelope diagram. */ void print_diagram (const Envelope_diagram_2& diag) { // Go over all arrangement faces. Envelope_diagram_2::Face_const_iterator fit; Envelope_diagram_2::Ccb_halfedge_const_circulator ccb; Envelope_diagram_2::Surface_const_iterator sit; for (fit = diag.faces_begin(); fit != diag.faces_end(); ++fit) { // Print the face boundary. // Print the vertices along the outer boundary of the face. ccb = fit>outer_ccb(); std::cout << "[Face] "; do { if(!ccb>is_fictitious()) std::cout << '(' << ccb>curve() << ") "; ++ccb; } while (ccb != fit>outer_ccb()); // Print the planes that induce the envelope on this face. std::cout << "> " << fit>number_of_surfaces() << " planes:"; for (sit = fit>surfaces_begin(); sit != fit>surfaces_end(); ++sit) std::cout << ' ' << sit>plane(); std::cout << std::endl; } return; } /* The main program: */ int main () { // Construct the input planes. std::list<Surface_3> planes; planes.push_back (Surface_3(Plane_3(0, 1, 1, 0))); planes.push_back (Surface_3(Plane_3(1, 0, 1, 0))); planes.push_back (Surface_3(Plane_3(0, 1 , 1, 0))); planes.push_back (Surface_3(Plane_3(1, 0, 1, 0))); // Compute and print the minimization diagram. Envelope_diagram_2 min_diag; CGAL::lower_envelope_3 (planes.begin(), planes.end(), min_diag); std::cout << std::endl << "The minimization diagram:" << std::endl; print_diagram (min_diag); // Compute and print the maximization diagram. Envelope_diagram_2 max_diag; CGAL::upper_envelope_3 (planes.begin(), planes.end(), max_diag); std::cout << std::endl << "The maximization diagram:" << std::endl; print_diagram (max_diag); return (0); }
^{1}  We consider vertical surfaces, namely patches of planes that are perpendicular to the xyplane, as weakly xymonotone, to handle degenerate inputs properly. 