98 lines
3.8 KiB
Plaintext
98 lines
3.8 KiB
Plaintext
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/** \page mesh_navigation How to navigate on a mesh
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\section nav_overview Overview
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\li \ref nav_intro
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\li \ref nav_he
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\li \ref nav_bound
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\li \ref nav_io
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\li \ref nav_opposite
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\section nav_intro Introduction
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In this tutorial you will learn how to navigate on a mesh using
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the %OpenMesh library. In the previous chapter (see \ref mesh_iterators) you have learned
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how to iterate over vertices, edges, halfedges and faces as well as
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circulate over certain structures such as 1-rings and many more.
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So in this tutorial we will focus on efficiently using the
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halfedge data structure and some very useful attributes such as
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the boundary flag. We assume that you already made yourself familiar with the halfedge
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structure which is used in %OpenMesh. Further information on this topic
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can be found in \ref mesh_hds.
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\section nav_he Navigating over halfedges
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So let's start with navigating over halfedges of a mesh.
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Assume we have the following mesh topology:
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\image html mesh.normal.png
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We can now select an arbitrary halfedge of this mesh which then offers
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either one of two possible navigations:
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\li If the chosen halfedge lies at a boundary or in other
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words is not adjacent to a face, we can now navigate
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along the boundary (or hole) of our mesh by using \c next_halfedge_handle()
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or \c prev_halfedge_handle():
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\image html mesh.outer.png
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\li If the chosen halfedge is adjacent to a face, we can then navigate
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along all halfedges that are adjacent to this face. In other words we circulate
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around the inner halfedges of one face:
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\image html mesh.inner.png
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In both cases the code would look something like the following example.
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Depending on whether the initial halfedge is adjacent to a face or not,
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we will either navigate on the boundary halfedges of our mesh or along
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the inner halfedges of a face:
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\include nav_code1.cc
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References:<br />
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OpenMesh::Concepts::KernelT< FinalMeshItems >::next_halfedge_handle()<br />
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OpenMesh::Concepts::KernelT< FinalMeshItems >::prev_halfedge_handle()
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\section nav_bound Mesh boundaries
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As you have seen in the previous section, navigating along boundaries
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is very simple. In general %OpenMesh also offers a boundary attribute
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for edges, vertices and faces. So testing i.e. whether a face is a boundary
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face is quite simple using OpenMesh::PolyConnectivity::is_boundary().<br />
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So for each type we can make use of one of the following functions:
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\include nav_code2.cc
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\section nav_io Using incoming and outgoing halfedges
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%OpenMesh offers quite a lot of iterators and circulators to easily iterate
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over the structures of a mesh. A very helpful iterator is the
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OpenMesh::PolyConnectivity::VertexIHalfedgeIter or the OpenMesh::PolyConnectivity::VertexOHalfedgeIter which are used to iterate
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over all incoming/outgoing halfedges of a vertex.
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So, sticking to the illustration below, a OpenMesh::PolyConnectivity:V:ertexIHalfedgeIter for the
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lower most vertex would iterate over all incoming halfedges (blue),
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whereas the OpenMesh::PolyConnectivity::OpenMesh::PolyConnectivity::VertexOHalfedgeIter would iterate over all outgoing halfedges (red):
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\image html mesh.inout.png
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A schematic code example of how to use the halfedge iterators as described above:
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\include nav_code3.cc
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\section nav_opposite Using opposite halfedges
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The halfedge structure splits every edge into two directional parts by creating two
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directed edges out of one undirected edge. So for every halfedge there exists its
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counterpart pointing in the opposite direction. %OpenMesh allows to easily navigate
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through opposing halfedges via the function OpenMesh::Concepts::KernelT< FinalMeshItems >::opposite_halfedge_handle().
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So in the illustration below \c opposite_halfedge_handle() for the blue halfedge would
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return the red halfedge:
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\image html mesh.opp.png
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There are also a few more functions that offer easy access to opposing structures:
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\include nav_code4.cc
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*/
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