Initializing Half-edge data structure from vertices - algorithm

Question

I'm working on implementing various subdivision algorithms (such as catmull-clark); to do this efficiently requires a good way to store information about a grid of tesselated polygons. I implemented the half-edge data structure as outlined by flipcode, but now I'm not sure how to populate the data structure from vertices!
My initial attempt was to
create vertices
group vertices into faces
sort vertices within faces (using their angle relative to the centroid)
for each face, grab the first vertex and then walk through the sorted vertex list to create a half-edge list.
However, this creates a list of faces (with half-edges) that don't have any information about adjacent faces! This also feels a bit wrong, because it seems as if the faces are really the first-class object and the edges provide auxiliary information; I really feel like I should be creating edges from the vertices and then sorting out the faces from there. But again, I'm not really sure how to go about it that way -- I can't think of a way to create a list of half-edges without creating the faces first.
Any suggestions for what the best way to go turning data about vertices (and faces) into half-edges?

Answer 1

First, I'd like to point you to an excellent C++ implementation of the half-edge data structure: OpenMesh. If you want to use it, make sure you work you way through the tutorial. If (and only if) you do that, working with OpenMesh is quite straightforward. It also contains some nice methods on top of which you can implement subdivision or reduction algorithms.
Now to your question:
However, this creates a list of faces (with half-edges) that don't have any information about adjacent faces! This also feels a bit wrong, because it seems as if the faces are really the first-class object and the edges provide auxiliary information
I think this somewhat misses the point of the half-edge data structure. In a half-edge structure, it is the half-edges that carry the most information!
Quoting shamelessly from the OpenMesh documentation (see also the figure there):
Each vertex references one outgoing halfedge, i.e. a halfedge that starts at this vertex.
Each face references one of the halfedges bounding it.
Each halfedge provides a handle to
the vertex it points to ,
the face it belongs to
the next halfedge inside the face (ordered counter-clockwise) ,
the opposite halfedge ,
(optionally: the previous halfedge in the face ).
As you see, most information is stored in the half-edges - these are the primary objects. Iterating over meshes in this data-structure is all about cleverly following pointers.
However, this creates a list of faces (with half-edges) that don't have any information about adjacent faces!
This is perfectly ok! As you see above, a face references only one bounding half edge. Assuming a triangle mesh, the chain of pointers you follow to get the 3 adjacent triangles to a given face F is the following:
F -> halfEdge -> oppositeHalfEdge -> face
F -> halfEdge -> nextHalfEdge -> oppositeHalfEdge -> face
F -> halfEdge -> previousHalfEdge -> oppositeHalfEdge -> face
Optionally, you can use nextHalfEdge -> nextHalfEdge if you don't use the 'previous' pointers. This, of course, generalizes easily to quads or higher order polygons.
If you set the pointers listed above correctly when building your mesh, then you can iterate over all kinds of adjacencies in your mesh like this. If you use OpenMesh, you can use a bunch of special iterators that to the pointer chasing for you.
Setting the "opposite half edge" pointers is of course the tricky part when building a half-edge structure from a "triangle soup". I suggest to use a map data-structure of some kind to keep track of half-edges already created.
To be more specific, here is some very conceptual pseudo-code for creating a half-edge mesh from faces. I omitted the vertex part, which is simpler, and can be implemented in the same spirit. I assume that iteration over a face edges is ordered (e.g. clock-wise).
I assume half edges are implemented as structs of type HalfEdge, which contain the pointers listed above as members.
struct HalfEdge
{
HalfEdge * oppositeHalfEdge;
HalfEdge * nextHalfEdge;
Vertex * vertex;
Face * face;
}
Let Edges be a map from pairs of vertex identifiers to pointers to the actual half-edge instances, e.g.
map< pair<unsigned int, unsigned int>, HalfEdge* > Edges;
in C++. Here is the construction pseudo-code (without the vertex and face part):
map< pair<unsigned int, unsigned int>, HalfEdge* > Edges;
for each face F
{
for each edge (u,v) of F
{
Edges[ pair(u,v) ] = new HalfEdge();
Edges[ pair(u,v) ]->face = F;
}
for each edge (u,v) of F
{
set Edges[ pair(u,v) ]->nextHalfEdge to next half-edge in F
if ( Edges.find( pair(v,u) ) != Edges.end() )
{
Edges[ pair(u,v) ]->oppositeHalfEdge = Edges[ pair(v,u) ];
Edges[ pair(v,u) ]->oppositeHalfEdge = Edges[ pair(u,v) ];
}
}
}
EDIT: Made the code a bit less pseudo, to be more clear about the Edges map and the pointers.

Answer 2

Consider we have a list of triangles, each of which is specified by 3 vertex IDs in counterclockwise order, and the task is to construct the half-edge data structure representing this triangulation. If the vertices are given as 3d-vectors (as in STL file format) then the works starts from giving unique identifiers to each distinct vertex, which is accomplish via a hash map from 3d-vector to vertex ID.
After that we progressively consider every triangle from the list. And the first thing is to find or create its three edges (each of which consists of a pair of half-edges). If some edge is shared by two triangles and the other triangle was already added in the data structure then we find that edge, otherwise new edge must be created.
To find whether an edge between vertices v1 and v2 already exists, we use a flat map from a vertex to one of half-edges with the origin in it. All other half-edges with the same origin can be enumerated using already constructed half-edge data structure. So the algorithm for this step is to look at every half-edge originating in v1 and test whether its destination is v2. If no such edge is found, then two half-edges must be created. In the terms of flipcode: pair of one created half-edge points on the other, vert on v1 and v2 respectively, and next on existing half-edges in v1 and v2, or if it was the first half-edge in a vertex then on itself.
When all 3 edges of the current triangle are found or created, face field of half-edges having that triangle at the left points to new HE_face record representing the triangle.
After all triangles are passed, the data structure is ready. There are certain optimizations and improvements that can be implemented, but the basic idea is as presented above.
All mesh libraries based on half-edge data structure construct it during mesh opening from a standard file format, such as STL/PLY/OBJ/OFF/… So it is a good idea at least to look at their code, and probably even integrate them in your software instead of creating your own implementation:
OpenMesh is a good C++ library already presented in the other answer.
MeshLib is a younger library also written in C++, which opens standard file formats considerably faster (based on my observations especially STLs); this suggests better optimizations in the conversion (see MRMeshBuilder.cpp) in half-edge data structure (MRMeshTopology.h).