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Some basic operations: Flipping and collapsing edges

In this section you will learn about some basic operations on a mesh that OpenMesh already provides.

Comprising the flipping of edges in a triangle mesh as well as collapsing edges by joining the two adjacent vertices.

- Flipping edges in triangle meshes
- Collapsing edges

# 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.

Flipping edges in a triangle mesh

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

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].

# 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!

Collapsing will always be performed in the direction the halfedge points to.

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

PolyMesh mesh;
// Request required status flags
mesh.request_vertex_status();
mesh.request_edge_status();
mesh.request_face_status();
// 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( mesh.to_vertex_handle(*it) == vhandle[3] &&
mesh.from_vertex_handle(*it) == vhandle[2])
{
// Collapse edge
mesh.collapse(*it);
break;
}
}
// Our mesh now looks like in the illustration above after the collapsing.