Validation of CG Models - An Introduction to Computer Graphics for Artists - page 129

Fig. 9.18 The polygon on the far left is a non-planar quad. To the right are the two possible out-
comes of triangulating the polygon

Fig. 9.19 The cube on the left has the correct vertex order. On the right , the vertex order has been
scrambled

to cut the polygon. Depending on how far from being in the same plane the vertices
are, the error can be anything from invisible to a serious fault (Fig. 9.18 ).
This is an extremely common but usually minor mistake, even among profes-
sionals. The most common solution is to disguise the problem by subdividing the
object globally to increase the poly count. This does not eliminate the error, but
reduces its prominence to a level that is diffi cult to detect. When working on proj-
ects where polycount is an important factor (that would be almost anything) it is
better to modify this by hand, without recourse to global subdivision.

9.3.13 Offset Vertex Order

Polygonal models are stored, in part, as vertex lists. These lists are either numbered or
stored in the order they are connected. For example, vertex 1, 2, and 3 in a list would
make the fi rst triangle of an object. If a quad is listed, but only the fi rst three vertices are
listed as belonging to that face, then the remaining vertex gets added to the next poly-
gon. This will offset vertices from subsequent polygons, causing edges to be drawn
between the wrong vertices. The result is severely corrupted geometry (Fig. 9.19 ).
This usually happens during corrupted save operations and is not evident until
reopening the fi le. This error is impossible to miss. It causes your vertices to be con-
nected improperly. The resulting shape usually looks like a number of jagged