openmesh/Doc/operations.docu

/** \page mesh_operations Some basic operations on a mesh

In this section you will learn about some basic operations on a mes that %OpenMesh
already provides. Comprising the flipping of edges in a triangle mesh as well as
collapsing edges by joining the two adjacent vertices.

\li \ref op_flip
\li \ref op_collapse

\section op_flip Flipping edges in triangle meshes

Considering two adjacent faces of a triangle mesh, there exist exactly two
different configurations of the inner edge. Calling the function
OpenMesh::TriConnectivity::flip(EdgeHandle _eh) will flip the specified edge to
its opposite orientation as shown in the illustration below.

\image html mesh.flip.png "Flipping edges in a triangle mesh"

So, the following snippet of code shows how to use this in your applications:

\code
TriMesh mesh;

// Add some vertices
TriMesh::VertexHandle vhandle[4];

vhandle[0] = mesh.add_vertex(MyMesh::Point(0, 0, 0));
vhandle[1] = mesh.add_vertex(MyMesh::Point(0, 1, 0));
vhandle[2] = mesh.add_vertex(MyMesh::Point(1, 1, 0));
vhandle[3] = mesh.add_vertex(MyMesh::Point(1, 0, 0));

// Add two faces
std::vector<TriMesh::VertexHandle> face_vhandles;

face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[0]);
mesh.add_face(face_vhandles);

face_vhandles.clear();

face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[3]);
mesh.add_face(face_vhandles);

// Now the edge adjacent to the two faces connects
// vertex vhandle[0] and vhandle[2].

// Find this edge and then flip it
for(TriMesh::EdgeIter it = mesh.edges_begin(); it != mesh.edges_end(); ++it) {

	if(!mesh.is_boundary(it.handle())) {
		// Flip edge
		mesh.flip(it.handle());
	}
}

// The edge now connects vertex vhandle[1] and vhandle[3].
\endcode


\section op_collapse Collapsing edges

In this section you will learn how to collapse edges such that the
two adjacent vertices join. %OpenMesh provides the function OpenMesh::PolyConnectivity::collapse(HalfedgeHandle _heh) to
perform this operation. This will collapse the from-vertex (remeber that halfedges are directed)
to the to-vertex of the halfedge as illustrated below. Note that collapsing edges
might cause topological inconsistencies to your mesh. You should verify consistency after
collapsing edges by calling OpenMesh::PolyConnectivity::is_collapse_ok().

\image html mesh.collapse.png "Collapsing will always be performed in the direction the halfedge points to."

A simple code example related to the illustration might look like this:

\code
PolyMesh mesh;

// Add some vertices as in the illustration above
PolyMesh::VertexHandle vhandle[7];

vhandle[0] = mesh.add_vertex(MyMesh::Point(-1, 1, 0));
vhandle[1] = mesh.add_vertex(MyMesh::Point(-1, 3, 0));
vhandle[2] = mesh.add_vertex(MyMesh::Point(0, 0, 0));
vhandle[3] = mesh.add_vertex(MyMesh::Point(0, 2, 0));
vhandle[4] = mesh.add_vertex(MyMesh::Point(0, 4, 0));
vhandle[5] = mesh.add_vertex(MyMesh::Point(1, 1, 0));
vhandle[6] = mesh.add_vertex(MyMesh::Point(1, 3, 0));

// Add three quad faces
std::vector<PolyMesh::VertexHandle> face_vhandles;

face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[0]);
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[3]);
mesh.add_face(face_vhandles);

face_vhandles.clear();

face_vhandles.push_back(vhandle[1]);
face_vhandles.push_back(vhandle[3]);
face_vhandles.push_back(vhandle[5]);
face_vhandles.push_back(vhandle[4]);
mesh.add_face(face_vhandles);

face_vhandles.clear();

face_vhandles.push_back(vhandle[3]);
face_vhandles.push_back(vhandle[2]);
face_vhandles.push_back(vhandle[6]);
face_vhandles.push_back(vhandle[5]);
mesh.add_face(face_vhandles);

// Now find the edge between vertex vhandle[2]
// and vhandle[3]
for(PolyMesh::HalfedgeIter it = mesh.halfedges_begin(); it != mesh.halfedges_end(); ++it) {

	if(to_vertex_handle(it.handle()) == vhandle[3] &&
	   from_vertex_handle(it.handle()) == vhandle[2]) {

		// Collapse edge
		mesh.collapse(it.handle());
		break;
	}
}

// Our mesh now looks like in the illustration above after the collapsing.

\endcode

*/