Validation of CG Models - An Introduction to Computer Graphics for Artists - page 128

Fig. 9.17 If unfolded, the polygon on the right would have to either overlap the polygon on the
left or be cut free to have non-overlapping texture coordinates

object using a global function. The drawback to this method is that it may cut
polygons in unexpected ways. Artists should check a model for all the more promi-
nent locations an error might reside, fi x them by hand, and then triangulate the
model to catch any small examples that were missed.

9.3.11 Non-manifold Geometry

It is not literally an error to make non-manifold geometry, but it can interfere with some
editing functions, so it is good to know what it looks like (Fig. 9.17 ). Non- manifold
geometry describes polygons that cannot be unfolded fl at without overlap due to a
convergence of three or more edges at a single vertex. This causes problems with some
tools because they cannot determine the orientation of the object's normals.

9.3.12 Non-planar Faces

Faces with four or more sides where one or more vertices do not lie within the same
plane as any group of three vertices from the same face are non-planar. Non-planar
faces are renderable in most renderers, but can cause unpredictable results. The
greater the distance a non-planar vertex is from the primary plane of the face, the
greater the distortion is likely to be. The reason is that the renderer triangulates all
non-planar faces. If the renderer triangulates a face, it must fi rst decide which way