Commit 4c69a22a authored by Mike Kremer's avatar Mike Kremer
Browse files

Added a chapter about the navigation on a mesh.

git-svn-id: fdac6126-5c0c-442c-9429-916003d36597
parent 01652469
TriMesh::HalfedgeHandle heh, heh_init;
// Get the halfedge handle assigned to vertex[0]
heh = heh_init = mesh.halfedge_handle(vertex[0].handle());
// heh now holds the handle to the initial halfedge.
// We now get further on the boundary by requesting
// the next halfedge adjacent to the vertex heh
// points to...
heh = mesh.next_halfedge_handle(heh);
// We can do this as often as we want:
while(heh != heh_init) {
heh = mesh.next_halfedge_handle(heh);
// Test if a halfedge lies at a boundary (is not adjacent to a face)
bool is_boundary (HalfedgeHandle _heh) const
// Test if an edge lies at a boundary
bool is_boundary (EdgeHandle _eh) const
// Test if a vertex is adjacent to a boundary edge
bool is_boundary (VertexHandle _vh) const
// Test if a face has at least one adjacent boundary edge.
// If _check_vertex=true, this function also tests if at least one
// of the adjacent vertices is a boundary vertex
bool is_boundary (FaceHandle _fh, bool _check_vertex=false) const
// Get some vertex handle
PolyMesh::VertexHandle v = ...;
for(PolyMesh::VertexIHalfedgeIter vih_it = mesh.vih_iter(v); vih_it; ++vih_it) {
// Iterate over all incoming halfedges...
for(PolyMesh::VertexOHalfedgeIter voh_it = mesh.voh_iter(v); voh_it; ++voh_it) {
// Iterate over all outgoing halfedges...
// Get the face adjacent to the opposite halfedge
// Get the handle to the opposite halfedge
OpenMesh::Concepts::KernelT< FinalMeshItems >::opposite_halfedge_handle();
// Get the opposite vertex to the opposite halfedge
// Get the vertex assigned to the opposite halfedge

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......@@ -26,6 +26,7 @@ most important topics of %OpenMesh in the following sections:
\li \ref mesh_cpp
\li \ref mesh_hds
\li \ref mesh_iterators
\li \ref mesh_navigation
\li \ref mesh_io
\li \ref mesh_type
\li \ref mesh_hierarchy
/** \page mesh_navigation How to navigate on a mesh
\section nav_overview Overview
\li \ref nav_intro
\li \ref nav_he
\li \ref nav_bound
\li \ref nav_io
\li \ref nav_opposite
\section nav_intro Introduction
In this tutorial you will learn how to navigate on a mesh using
the %OpenMesh library. In the previous chapter (see \ref mesh_iterators) you have learned
how to iterate over vertices, edges, halfedges and faces as well as
circulate over certain structures such as 1-rings and many more.
So in this tutorial we will focus on efficiently using the
halfedge data structure and some very useful attributes such as
the boundary flag. We assume that you already made yourself familiar with the halfedge
structure which is used in %OpenMesh. Further information on this topic
can be found in \ref mesh_hds.
\section nav_he Navigating over halfedges
So let's start with navigating over halfedges of a mesh.
Assume we have the following mesh topology:
\image html mesh.normal.png
We can now select an arbitrary halfedge of this mesh which then offers
either one of two possible navigations:
\li If the chosen halfedge lies at a boundary or in other
words is not adjacent to a face, we can now navigate
along the boundary (or hole) of our mesh by using \c next_halfedge_handle()
or \c prev_halfedge_handle():
\image html mesh.outer.png
\li If the chosen halfedge is adjacent to a face, we can then navigate
along all halfedges that are adjacent to this face. In other words we circulate
around the inner halfedges of one face:
\image html mesh.inner.png
In both cases the code would look something like the following example.
Depending on whether the initial halfedge is adjacent to a face or not,
we will either navigate on the boundary halfedges of our mesh or along
the inner halfedges of a face:
References:<br />
OpenMesh::Concepts::KernelT< FinalMeshItems >::next_halfedge_handle()<br />
OpenMesh::Concepts::KernelT< FinalMeshItems >::prev_halfedge_handle()
\section nav_bound Mesh boundaries
As you have seen in the previous section, navigating along boundaries
is very simple. In general %OpenMesh also offers a boundary attribute
for edges, vertices and faces. So testing i.e. whether a face is a boundary
face is quite simple using OpenMesh::PolyConnectivity::is_boundary().<br />
So for each type we can make use of one of the following functions:
\section nav_io Using incoming and outgoing halfedges
%OpenMesh offers quite a lot of iterators and circulators to easily iterate
over the structures of a mesh. A very helpful iterator is the
OpenMesh::PolyConnectivity::VertexIHalfedgeIter or the OpenMesh::PolyConnectivity::VertexOHalfedgeIter which are used to iterate
over all incoming/outgoing halfedges of a vertex.
So, sticking to the illustration below, a OpenMesh::PolyConnectivity:V:ertexIHalfedgeIter for the
lower most vertex would iterate over all incoming halfedges (blue),
whereas the OpenMesh::PolyConnectivity::OpenMesh::PolyConnectivity::VertexOHalfedgeIter would iterate over all outgoing halfedges (red):
\image html mesh.inout.png
A schematic code example of how to use the halfedge iterators as described above:
\section nav_opposite Using opposite halfedges
The halfedge structure splits every edge into two directional parts by creating two
directed edges out of one undirected edge. So for every halfedge there exists its
counterpart pointing in the opposite direction. %OpenMesh allows to easily navigate
through opposing halfedges via the function OpenMesh::Concepts::KernelT< FinalMeshItems >::opposite_halfedge_handle().
So in the illustration below \c opposite_halfedge_handle() for the blue halfedge would
return the red halfedge:
\image html mesh.opp.png
There are also a few more functions that offer easy access to opposing structures:
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