<HTML>
<!-- Copyright 2007 Aaron Windsor
     
     Distributed under the Boost Software License, Version 1.0.
     (See accompanying file LICENSE_1_0.txt or copy at
     http://www.boost.org/LICENSE_1_0.txt)
    
  -->
<Head>
<Title>Boost Graph Library: Planar Face Traversal</Title>
<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" 
        ALINK="#ff0000"> 
<IMG SRC="../../../boost.png" 
     ALT="C++ Boost" width="277" height="86"> 

<BR Clear>

<H1>Planar Face Traversal</H1>

<pre>
template&lt;typename Graph, typename PlanarEmbedding, typename PlanarFaceVisitor, typename EdgeIndexMap&gt;
void planar_face_traversal(const Graph& g, PlanarEmbedding embedding, PlanarFaceVisitor& visitor, EdgeIndexMap em);
</pre>

<p>
A graph is <i>planar</i> if it can be drawn in two-dimensional space with no 
two of its edges crossing. Any embedding of a planar graph separates the plane 
into distinct regions that are bounded by sequences of edges in the graph. 
These regions are called <i>faces</i>. 

<br>
<br>
<table align="center" class="image">
<caption align="bottom">
<h5>A plane drawing of a graph (left), and the 8 faces defined by the planar 
embedding (right.) Each connected blue region in the image on the right is a 
face. The large blue region surrounding the graph is the <i>outer face</i>.
</h5>
</caption>
<tr>
<td>
<img src="./figs/face_illustration.png">
</td>
</tr>
<tr></tr>
</table>
<br>


A traversal of the faces of a planar graph involves iterating through all faces
of the graph, and on each face, iterating through all vertices and edges of the
face. The iteration through all vertices and edges of each face follows a 
path around the border of the face.
<p>
In a biconnected graph, like the one shown above, each face is bounded by a 
cycle and each edge belongs to exactly two faces. For this reason, when 
<tt>planar_face_traversal</tt> is called on a biconnected graph, each edge will
be visited exactly twice: once on each of two distinct faces, and no vertex 
will be visited more than once on a particular face. The output of 
<tt>planar_face_traversal</tt> on non-biconnected graphs is less intuitive - 
for example, if the graph 
consists solely of a path of vertices (and therefore a single face), 
<tt>planar_face_traversal</tt> will iterate <i>around</i> the path, visiting 
each edge twice and visiting some vertices more than once. 
<tt>planar_face_traversal</tt> does not visit isolated vertices.
<p>
Like other graph traversal algorithms in the Boost Graph Library, the planar 
face traversal is a generic traversal that can be customized by the 
redefinition of certain visitor event points. By defining an appropriate
visitor, this traversal can be 
used to enumerate the faces of a planar graph, triangulate a planar graph, or 
even construct a dual of a planar graph. 

<br>
<center>
<img src="./figs/face_traversal_example.png">
</center>
<br>

For example, on the above graph, an instance <tt>my_visitor</tt> of the 
following visitor:
<pre>
    struct output_visitor: public planar_face_traversal_visitor
    {
        void begin_face() { std::cout << "New face: "; }
        template &lt;typename Vertex&gt; void next_vertex(Vertex v) { std::cout << v << " "; } 
        void finish_face() { std::cout << std::endl; }
    };
</pre>
can be passed to the <tt>planar_face_traversal</tt> function:
<pre>
    output_visitor my_visitor;
    planar_face_traversal(g, embed, my_visitor); //embed is a planar embedding of g
</pre>
and might produce the output
<pre>
    New face: 1 2 5 4
    New face: 2 3 4 5
    New face: 3 0 1 4
    New face: 2 3 0 1
</pre>

<h3>Visitor Event Points</h3> 
         
<ul>
<li><tt>visitor.begin_traversal()</tt>: called once before any faces are 
visited.
<li><tt>visitor.begin_face()</tt>: called once, for each face, before any 
vertex or edge on that face has been visited.
<li><tt>visitor.end_face()</tt>: called once, for each face, after all vertices
and all edges on that face have been visited.
<li><tt>visitor.next_vertex(Vertex v)</tt>: called once on each vertex in the 
current face (the start and end of which are designated by calls to 
<tt>begin_face()</tt> and <tt>end_face()</tt>, respectively) in order 
according to the order established by the planar embedding.
<li><tt>visitor.next_edge(Edge e)</tt>: called once on each edge in the current
face (the start and end of which are designated by calls to 
<tt>begin_face()</tt> and <tt>end_face()</tt>, respectively) in order 
according to the order established by the planar embedding.
<li><tt>visitor.end_traversal()</tt>: called once after all faces have been 
visited.
</ul>

Although <tt>next_vertex</tt> is guaranteed to be called in sequence for each
vertex as the traversal moves around a face and <tt>next_edge</tt> is
guaranteed to be called in sequence for each edge as the traversal moves
around a face, there's no guarantee about the order in which 
<tt>next_vertex</tt> and <tt>next_edge</tt> are called with respect to each 
other in between calls to <tt>begin_face</tt> and <tt>end_face</tt>. These 
calls may be interleaved, all vertex visits may precede all edge visits, or 
vise-versa.
<p>
<tt>planar_face_traversal</tt> iterates over a copy of the edges of the input 
graph, so it is safe to add edges to the graph during visitor event points.


<h3>Complexity</h3>

If all of the visitor event points run in constant time, the traversal takes 
time <i>O(n + m)</i> for a planar graph with <i>n</i> vertices and <i>m</i>
edges. Note that 
in a simple planar graph with <i>f</i> faces, <i>m</i> edges, and <i>n</i> 
vertices, both <i>f</i> and <i>m</i> are <i>O(n)</i>. 

<H3>Where Defined</H3>

<P>
<a href="../../../boost/graph/planar_face_traversal.hpp">
<TT>boost/graph/planar_face_traversal.hpp</TT>
</a>

<h3>Parameters</h3>

IN: <tt>Graph&amp; g</tt>

<blockquote>
An undirected graph. The graph type must 
be a model of <a href="VertexAndEdgeListGraph.html">VertexAndEdgeListGraph</a>
</blockquote>

IN: <tt>PlanarEmbedding</tt> 

<blockquote>
A model of <a href="PlanarEmbedding.html">PlanarEmbedding</a>.
</blockquote>

IN: <tt>PlanarFaceVisitor</tt>

<blockquote>
A model of <a href="PlanarFaceVisitor.html">PlanarFaceVisitor</a>.
</blockquote>

IN: <tt>EdgeIndexMap vm</tt>

<blockquote>
A <a href="../../property_map/doc/ReadablePropertyMap.html">Readable Property Map
</a> that maps edges from <tt>g</tt> to distinct integers in the range 
<tt>[0, num_edges(g) )</tt><br>
<b>Default</b>: <tt>get(edge_index,g)</tt><br>
</blockquote>


<H3>Example</H3>

<P>
<a href="../example/planar_face_traversal.cpp">
<TT>examples/planar_face_traversal.cpp</TT></a>

<h3>See Also</h3>

<p>
<ul>
<li><a href="./planar_graphs.html">Planar Graphs in the Boost Graph Library</a>
<li><a href="./PlanarFaceVisitor.html">PlanarFaceVisitor</a> concept.
</ul>

<br>
<HR>
Copyright &copy; 2007 Aaron Windsor (<a href="mailto:aaron.windsor@gmail.com">
aaron.windsor@gmail.com</a>)
</BODY>
</HTML>