Technical Q&A QD3D65

Mesh Edge Structure Can Not Have More Than 2 Faces

Q: I am using the mesh geometry in my application. Is it possible to make an edge connected to three faces?

A: No. The mesh edge data structure addresses two mesh vertices and two mesh faces (in which either face may be empty). So the mesh edge structure can not have more than two faces. There are provisions in the mesh itself to have more than two faces between two vertices, but this is done by attaching multiple edge structures onto the given vertex structures. So as you see each edge structure still has only two faces adjacent to it. The mesh bookkeeping functions keep track of the adjacencies, but there are no API calls which will cycle through and return all the faces for all those parallel edge structures, so you would have to do this yourself with a loop which looks for identical vertices.

In general, meshes are optimized to work with at most two faces per edge, so it's recommended to use it that way. The structures can handle the case you mention, but the API doesn't allow you to easily extract the information. (e.g., Q3Mesh_GetEdgeFaces() returns only the two faces that are specified in the given edge structure, and therefore you must search for the other parallel edge structures).

[Jul 11 1997]