Validation of CG Models - An Introduction to Computer Graphics for Artists - page 124

Fig. 9.12 Face number two cannot be resolved into triangles because the inner and outer ring of
edges are not connected as in face number one, on left

another edge might not be properly connected to the rest of the face, and so your cut
attempt is impossible because you are unintentionally selecting edges that do not
belong to the same face.
A partially coincident edge is easier to detect than a coincident edge because you
may select an edge and see that its length is not correct. It either terminates before
reaching a visible vertex, or it extends straight through and beyond a visible vertex. In
either case, this observation indicates the presence of two or more edges where you
expect to see one. With partially coincident edges, there are usually three or more.
The most common cause of partially coincident edges is manual editing of poly-
gons that have fi ve or more sides combined with later use of vertex merging, face
deletion, or other optimization efforts.

9.3.6 Hole Geometry

A hole is a polygon with two or more complete borders, one inner and one outer,
made of adjacent edges that form two or more unbroken loops (Fig. 9.12 ). One way
to make this error is to make a pair of concentric edges, and then delete the edge
connections.
Unlike a non-hole polygon, a polygon with a hole does not have an easily defi ned
start and end point for triangulation purposes. This can cause some geometry edit-
ing functions fail. A face with a hole must be an n-gon or the hole would be impos-
sible. This increases the risk of the n-gon also being non-planar. Combined with loss
of editing functionality, it is usually best to triangulate a hole and then edit any
remaining problems afterwards.
Some applications will render a polygon with a hole, but they may not render it
correctly. Some renderers will crash trying to render hole geometry.