2D Apollonius Graphs (Delaunay Graphs of Disks)

This chapter describes the two-dimensional Apollonius graph of CGAL. We start with a few definitions in Section 27.1. The software design of the 2D Apollonius graph package is described in Section 27.2. In Section 27.3 we discuss the geometric traits of the 2D Apollonius graph package and in Section 27.4 the Apollonius graph hierarchy, a data structure suitable for fast nearest neighbor queries, is briefly described.

The 2D Apollonius graph class of CGAL is designed to compute the
dual of the *Apollonius diagram* or, as it is also known, the
*Additively weighted Voronoi diagram*. The algorithm that has been
implemented is dynamic, which means that we can perform insertions and
deletions on line. The corresponding CGAL class is called
*Apollonius_graph_2<ApolloniusGraphTraits_2,ApolloniusGraphDataStructure_2>*
and will be discussed in more detail in the sequel. The interested
reader may want to refer to the paper by Karavelas and Yvinec
[KY02] for the general idea as well as the details of the
algorithm implemented.

Before describing the details of the implementation we make a brief
introduction to the theory of Apollonius diagrams.
The Apollonius diagram is defined over a set of sites
$$*P _{i}=(c_{i},w_{i})*, $$

$$* (x,P _{i})= x-c_{i} -w_{i},*

where $$* · * denotes the Euclidean norm.
It can easily be seen that it is a generalization of the Voronoi
diagram for points, which can actually be obtained if all the weights
$$*w _{i}* are equal. Unlike the case of points, however, it is
possible that a site $$

If all weights $$*w _{i}* are non-negative, the Apollonius
diagram can be viewed as the Voronoi diagram of the set of circles
$$

The Apollonius diagram is a planar graph, and so is its dual, the
Apollonius graph. There are many ways to embed it on the plane and one
such way is shown in Fig. 27.1(right).
The Apollonius graph is uniquely defined once we have
the Apollonius diagram. If the circles are in *general position*
(see precise definition below), then the Apollonius graph is a graph
with triangular faces away from
the convex hull of the set of circles (by triangular we mean that
every face has exactly three edges). Near the convex hull we may
have some spikes (i.e., vertices of degree 1). To unify our approach
and handling of the Apollonius graph we add to the set of (finite)
circles a fictitious circle at infinity, which we call the
*site at infinity*. We can then connect all vertices of the outer
face of the Apollonius graph to the site at infinity which gives us
a graph with the property that all of its faces are now
triangular. However, the Apollonius graph is not a triangulation for
two main reasons: we cannot always embed it on the plane with straight
line segments that yield a triangulation and, moreover, we may have two
faces of the graph that have two edges in common, which is not allowed
in a triangulation. Both of these particularities appear when we
consider the Apollonius graph of the set of circles in
Fig. 27.1(right).

We would like to finish our brief introduction to the theory of
Apollonius graphs by discussing the concept of general position. We say
that a set of circles is in general position if no two triplets of
circles have the same tritangent circle. This statement is rather
technical and it is best understood in the context of points. The
equivalent statement for points is that we have no two triplets of
points that define the same circumcircle, or equivalently that no
four points are co-circular. The statement about general position made
above is a direct generalization of the (much simpler to understand)
statement about points. On the contrary, when we have circles in
degenerate position, the Apollonius graph has faces with more than
three edges on their boundary. We can get a triangulated version of
the graph by simply *triangulating* the corresponding faces in an
arbitrary way. In fact the algorithm that has been implemented in
CGAL has the property that it always returns a valid
*triangulated* version of the Apollonius graph. By valid we mean
that it contains the actual Apollonius graph (i.e., the actual dual of
the Apollonius diagram) and whenever there are faces with more than
three faces then they are triangulated. The way that they are
triangulated depends on the order of insertion and deletion of the
circles in the diagram.

One final point has to be made about hidden circles. First of all we would like to be more precise about our definition of hidden circles: we say that a circle is hidden if its cell has empty interior. This definition allows us to guarantee that all visible circles have cells that are two-dimensional regions. Geometrically the fact that a circle is hidden means that it is contained in the closure of the disk of another circle (see again Fig. 27.1). Note that a circle contained in the union of several disks, but not in the closure of any one of them, is not hidden.

Hidden circles pose an additional difficulty to our algorithm and software design. Since we allow circles to be inserted and deleted at wish, it is possible that a circle that was hidden at some point in time, may become visible at a later point in time; for example this can happen if we delete the circle that hides it. For this purpose we store hidden circles and have them reappear when they become visible. We will discuss this issue in detail below. For the time being it suffices to say that the user has the ability to control this behavior. More specifically it is possible to discard the circles that become hidden. This choice is totally natural when for example we expect to do only insertions, since in this case a circle that becomes hidden will never reappear. On the other hand if deletions are expected as well, then we lose the ability to have the hidden circles reappear.

**Degenerate Dimensions.**
The dimension of the Apollonius graph is in general 2. The exceptions
to this rule are as follows:

- The dimension is $$
*-1*if the Apollonius graph contains no circles. - The dimension is $$
*0*if the Apollonius graph contains exactly one visible circle. - The dimension is $$
*1*is the Apollonius graph contains exactly two visible circles.

The 2D Apollonius graph class
*Apollonius_graph_2<ApolloniusGraphTraits_2,ApolloniusGraphDataStructure_2>*
follows the design of the triangulation package of CGAL. It is
parametrized by two arguments:

- the
**geometric traits**class. It provides the basic geometric objects involved in the algorithm, such as sites, points etc. It also provides the geometric predicates for the computation of the Apollonius graph, as well as some basic constructions that can be used, for example, to visualize the Apollonius graph or the Apollonius diagram. The geometric traits for the Apollonius graph will be discussed in more detail in the next section. - the
**Apollonius graph data structure**. This is essentially the same as the triangulation data structure (discussed in Chapter 21), augmented with some additional operations that are specific to Apollonius graphs. The corresponding concept is that of*ApolloniusGraphDataStructure_2*, which in fact is a refinement of the*TriangulationDataStructure_2*concept. The class*Triangulation_data_structure_2<Vb,Fb>*is a model of the concept*ApolloniusGraphDataStructure_2*. A default value for the corresponding template parameter is provided, so the user does not need to specify it.

**Storing Hidden Sites.**
As we have already mentioned a circle is hidden if it is contained
inside some visible circle. This creates a parent-child relationship
between visible and hidden circles: the parent of a hidden circle is the
visible circle that contains it. If more than one visible circles
contain a hidden circle then the hidden circle can be assigned to any of
the visible circles arbitrarily.

To store hidden circles we assign to every visible circle a list. This
list comprises the hidden circles that are contained in the
visible circle. The user can access the hidden circles associated with
a visible circle through an iterator called
*Hidden_sites_iterator*. This iterator is defined in the
*ApolloniusGraphVertexBase_2* concept and is implemented by its
model, the *Apollonius_graph_vertex_base_2<Gt,StoreHidden>*
class. It is also possible to iterate through the entire set of hidden
sites using an homonymous iterator defined by the
*Apollonius_graph_2<Gt,Agds>* class.

Since storing hidden sites may not be of interest in some cases (e.g.,
for example this is the case if we only perform insertions in the
Apollonius graph), the user has the possibility of controllong this
behavior. More precisely, the class
*Apollonius_graph_vertex_base_2<Gt,StoreHidden>* has two template
parameters, the second of which is a boolean value. This value is by
default *true* and it indicates that hidden sites should be
stored. The user can indicate that hidden sites may be discarded
by setting this value to *false*.

The predicates required for the computation of the Apollonius graph are rather complicated. It is not the purpose of this document to discuss them in detail. The interested reader may refer to the papers by Karavelas and Emiris for the details [KE02, KE03]. However, we would like to give a brief overview of what they compute. There are several predicates needed by this algorithm. We will discuss the most important/complicated ones. It turns out that it is much easier to describe them in terms of the Apollonius diagram, rather than the Apollonius graph. Whenever it is applicable we will also describe their meaning in terms of the Apollonius graph.

The first two geometric predicates are called
*Is_hidden_2* and *Oriented_side_of_bisector_2*. The first one
involves two circles, say $$*P _{1}* and $$

The next geometric predicate is called *Vertex_conflict_2* and it
involves four circles $$*P _{1}*, $$

$$* (K _{1}, K_{2}) = c_{1}-c_{2} -r_{1}-r_{2}. *

This predicate determines if a vertex in the Apollonius diagram (the center of the yellow circle) is destroyed when a new circle is inserted in the diagram (the green circle). In the Apollonius graph it tells us if a triangular face of the diagram is to be destroyed or not.

What we essentially want to compute when we construct incrementally a
Voronoi diagram, is whether the object to be inserted destroys an edge
of the Voronoi diagram or not. In the case of points this is really
easy and it amounts to the well known *incircle* test.
In the case
of circles the situation is more complicated. We can have six possible
outcomes as to what portion of an edge of the Apollonius diagram the
new circle destroys (see Fig. 27.3). The first
two can be answered directly by the *Vertex_conflict_2* predicate
evaluated for the two endpoints of the Apollonius diagram edge. This
is due to the fact that the value of the *Vertex_conflict_2*
predicate is different for the two endpoints. If the two values are
the same then we need an additional test which determines if the interior
of the Apollonius diagram edge is destroyed by the new circle. This is
what the *Finite_edge_interior_conflict_2* and
*Infinite_edge_interior_conflict_2* predicates do. In essense, it
is the same predicate (same idea) applied to two different types of
edges in the Apollonius diagram: a finite or an infinite edge. An edge
is infinite if its dual edge in the Apollonius graph connects the
site at infinity with the vertex corresponding to a (finite) circle;
otherwise it is a finite edge.

The last predicate that we want to discuss is called
*Is_degenerate_edge_2*. It tells us whether an edge in the
Apollonius diagram is degenerate, that is if its two endpoints
coincide. In the Apollonius graph such an edge corresponds to one of
the additional edges that we use to triangulate the non-triangular
faces.

The afore mentioned predicates are part of the
*ApolloniusGraphTraits_2* concept of CGAL. CGAL also provides
a model for this concept, the
*Apollonius_graph_traits_2<K,Method_tag>* class. The first
template parameter of this class must be a model of the *Kernel*
concept. The second template parameter is a tag that indicates what
operations are allowed in the computations that take place within the
traits class.
The two possible values of the *Method_tag* parameter are
*CGAL::Ring_tag* and *CGAL::Sqrt_field_tag*. When
*CGAL::Ring_tag* is used, only ring operations are used during the
evaluation of the predicates, whereas if *CGAL::Sqrt_field_tag* is
chosen, all four field operations, as well as square roots, are used
during the predicate evaluation.

The *Apollonius_graph_traits_2<K,Method_tag>* class provides exact
predicates if the number type in the kernel *K* is an exact number
type. This is to be associated with the type of operations allowed for
the predicate evaluation. For example *CGAL::MP_Float* as number
type, with *CGAL::Ring_tag* as tag will give exact predicates,
whereas *CGAL::MP_Float* with *CGAL::Sqrt_field_tag* will give
inexact predicates.

Since using an exact number type may be too slow, the
*Apollonius_graph_traits_2<K,Method_tag>* class is designed to
support the dynamic filtering of CGAL through the
*CGAL::Filtered_exact<CT,ET>* mechanism. In particular if *CT*
is an inexact number type that supports the operations denoted by the
tag *Method_tag* and *ET* is an exact number type for these
operations, then kernel with number type
*CGAL::Filtered_exact<CT,ET>* will yield exact predicates for the
Apollonius graph traits. To give a concrete example,
*CGAL::Filtered_exact<double,CGAL::MP_Float>* with
*CGAL::Ring_tag* will produce exact predicates.

Another possibility for fast and exact predicate evalutation is to use
the
*Apollonius_graph_filtered_traits_2<CK,CM,EK,EM,FK,FM>*
class. This class is the analog of a filtered kernel. It takes a
constructions kernel *CK*, a filtering kernel *FK* and an
exact kernel *EK*, as well as the corresponding tags
(*CM*, *FM* and *EM*, respectively).
It evaluates the predicates by first using the filtering kernel, and
if this fails the evaluation is performed using the exact kernel. The
constructions are done using the kernel *CK*, which means that
they are not necessarily exact. All template parameters except
*CK* have default values, which are explained in the reference
manual.

The *Apollonius_graph_hierarchy_2<ApolloniusGraphTraits_2,ApolloniusGraphDataStructure_2>* class is nothing but the equivalent of the *Triangulation_hierarchy_2*
class, applied to the Apollonius graph. It consists of a series of
Apollonius graphs constructed in a manner analogous to the Delaunay
hierarchy by Devillers [Dev98]. The class
*Apollonius_graph_hierarchy_2<ApolloniusGraphTraits_2,ApolloniusGraphDataStructure_2>*
has exactly the same interface and functionality as the
*Apollonius_graph_2<ApolloniusGraphTraits_2,ApolloniusGraphDataStructure_2>*
class. Using the Apollonius graph hierarchy involves an additional
cost in space and time for maintaining the hierarchy. Our experiments
have shown that it usually pays off to use the hierarchy for inputs
consisting of more than 1,000 circles. This threshold holds for both
the construction of the Apollonius diagram itself, as well as for
nearest neighbor queries.

// file: examples/Apollonius_graph_2/ag2_exact_traits.C #include <CGAL/basic.h> // standard includes #include <iostream> #include <fstream> #include <cassert> // the number type #include <CGAL/MP_Float.h> // example that uses an exact number type typedef CGAL::MP_Float NT; // choose the kernel #include <CGAL/Simple_cartesian.h> typedef CGAL::Simple_cartesian<NT> Kernel; // typedefs for the traits and the algorithm #include <CGAL/Apollonius_graph_2.h> #include <CGAL/Apollonius_graph_traits_2.h> typedef CGAL::Apollonius_graph_traits_2<Kernel> Traits; typedef CGAL::Apollonius_graph_2<Traits> Apollonius_graph; int main() { std::ifstream ifs("data/sites.cin"); assert( ifs ); Apollonius_graph ag; Apollonius_graph::Site_2 site; // read the sites and insert them in the Apollonius graph while ( ifs >> site ) { ag.insert(site); } // validate the Apollonius graph assert( ag.is_valid(true, 1) ); std::cout << std::endl; return 0; }

// file: examples/Apollonius_graph_2/ag2_exact_traits_sqrt.C #include <CGAL/basic.h> // standard includes #include <iostream> #include <fstream> #include <cassert> #if defined CGAL_USE_LEDA # include <CGAL/leda_real.h> #elif defined CGAL_USE_CORE # include <CGAL/CORE_Expr.h> #endif #if defined CGAL_USE_LEDA // If LEDA is present use leda_real as the exact number type typedef leda_real NT; #elif defined CGAL_USE_CORE // Otherwise if CORE is present use CORE's Expr as the exact number type typedef CORE::Expr NT; #else // Otherwise just use double. This may cause numerical errors but it // is still worth doing it to show how to define correctly the traits // class typedef double NT; #endif #include <CGAL/Simple_cartesian.h> typedef CGAL::Simple_cartesian<NT> Kernel; // typedefs for the traits and the algorithm #include <CGAL/Apollonius_graph_2.h> #include <CGAL/Apollonius_graph_traits_2.h> // the traits class is now going to assume that the operations // +,-,*,/ and sqrt are supported exactly typedef CGAL::Apollonius_graph_traits_2<Kernel,CGAL::Sqrt_field_tag> Traits; typedef CGAL::Apollonius_graph_2<Traits> Apollonius_graph; int main() { std::ifstream ifs("data/sites.cin"); assert( ifs ); Apollonius_graph ag; Apollonius_graph::Site_2 site; // read the sites and insert them in the Apollonius graph while ( ifs >> site ) { ag.insert(site); } // validate the Apollonius graph assert( ag.is_valid(true, 1) ); std::cout << std::endl; return 0; }

// file: examples/Apollonius_graph_2/ag2_filtered_traits_no_hidden.C #include <CGAL/basic.h> // standard includes #include <iostream> #include <fstream> #include <cassert> #include <CGAL/basic.h> // example that uses the filtered traits // choose the representation #include <CGAL/Simple_cartesian.h> typedef CGAL::Simple_cartesian<double> Rep; #include <CGAL/Apollonius_graph_2.h> #include <CGAL/Triangulation_data_structure_2.h> #include <CGAL/Apollonius_graph_vertex_base_2.h> #include <CGAL/Triangulation_face_base_2.h> #include <CGAL/Apollonius_graph_filtered_traits_2.h> // typedef for the traits; the filtered traits class is used typedef CGAL::Apollonius_graph_filtered_traits_2<Rep> Traits; // typedefs for the algorithm // With the second template argument in the vertex base class being // false, we indicate that there is no need to store the hidden sites. // One case where this is indeed not needed is when we only do // insertions, like in the main program below. typedef CGAL::Apollonius_graph_vertex_base_2<Traits,false> Vb; typedef CGAL::Triangulation_face_base_2<Traits> Fb; typedef CGAL::Triangulation_data_structure_2<Vb,Fb> Agds; typedef CGAL::Apollonius_graph_2<Traits,Agds> Apollonius_graph; int main() { std::ifstream ifs("data/sites.cin"); assert( ifs ); Apollonius_graph ag; Apollonius_graph::Site_2 site; // read the sites and insert them in the Apollonius graph while ( ifs >> site ) { ag.insert(site); } // validate the Apollonius graph assert( ag.is_valid(true, 1) ); std::cout << std::endl; // now remove all sites std::cout << "Removing all sites... " << std::flush; while ( ag.number_of_vertices() > 0 ) { ag.remove( ag.finite_vertex() ); } std::cout << "done!" << std::endl << std::endl; return 0; }

// file: examples/Apollonius_graph_2/ag2_hierarchy.C #include <CGAL/basic.h> // standard includes #include <iostream> #include <fstream> #include <cassert> // example that uses the filtered traits #include <CGAL/MP_Float.h> #include <CGAL/Simple_cartesian.h> // constructions kernel (inexact) typedef CGAL::Simple_cartesian<double> CK; // exact kernel typedef CGAL::Simple_cartesian<CGAL::MP_Float> EK; // typedefs for the traits and the algorithm #include <CGAL/Apollonius_graph_hierarchy_2.h> #include <CGAL/Apollonius_graph_filtered_traits_2.h> // Type definition for the traits class. // In this example we explicitly define the exact kernel. We also // explicitly define what operations to use for the evaluation of the // predicates and constructions, when the filtering and the exact // kernels are used respectively. // Note that the operations allowed for the filtering and the // constructions (field operations plus square roots) are different // from the operations allowed when the exact kernel is used (ring // operations). typedef CGAL::Sqrt_field_tag CM; typedef CGAL::Ring_tag EM; typedef CGAL::Apollonius_graph_filtered_traits_2<CK,CM,EK,EM> Traits; // Now we use the Apollonius graph hierarchy. // The hierarchy is faster for inputs consisting of about more than // 1,000 sites typedef CGAL::Apollonius_graph_hierarchy_2<Traits> Apollonius_graph; int main() { std::ifstream ifs("data/hierarchy.cin"); assert( ifs ); Apollonius_graph ag; Apollonius_graph::Site_2 site; // read the sites and insert them in the Apollonius graph while ( ifs >> site ) { ag.insert(site); } // validate the Apollonius graph assert( ag.is_valid(true, 1) ); return 0; }

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