/** \page mesh_operations Some basic operations: Flipping and collapsing edges

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)) {
		// Flip edge
		mesh.flip(*it);
	}
}

// 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(). 

\note You have to request status attributes in order to use the collapse and delete functions!

\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) == vhandle[3] &&
	   from_vertex_handle(*it) == vhandle[2]) {

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

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

\endcode

*/