Validation of CG Models - An Introduction to Computer Graphics for Artists - page 119

Fig. 9.6 Smoothing on left is smooth where it should be hard. On right , it is correct after splitting
the normal along the edge between the two smoothed faces

Smoothing errors signifi cantly degrade the quality of rendered output. Therefore,
it is best to check your model for this type of error and fi x any examples of it that
you fi nd before moving on.

9.2.6 Visible Gaps in Polyset

An opening in a polyset where the polyset is supposed to be closed creates an
unwanted gap (Fig. 9.7 ). This error is most often caused by translation or rotation
errors. Another common cause is when two or more pieces are brought together into
the same polyset, but edges that should be adjacent are not sealed.
A false gap is when a polygon has a reversed normal and appears to be missing,
but is not. When you think you have a gap, you should always check to see if it is a
false gap before you fi ll it, or risk creating coincident faces (Sect. 9.4.3 ) or even
lamina faces (Sect. 9.3.8 ).
Frequently, unless great care is taken during the design process, shared edges
between different components of an object have a different number of vertices. For a
polyset to be free of gaps, either side of a common edge must have the same number of
vertices. The most practical solution is usually to either add or subtract vertices from
one side or the other, then to snap the vertices together and merge them when done.