Validation of CG Models - An Introduction to Computer Graphics for Artists - page 127

Fig. 9.16 Many of the fl aps in this carton have not been subdivided into triangles or quads, caus-
ing many n-gons to be present in the model

The reason is that it will only accept light from the same position as the lights from
the scene it was in when the normals were locked.
Most applications have a button that will allow you to unlock normals. If this
does not work, you should be able to recalculate them by re-importing your object.
This error is increasingly rare. You may only encounter it occasionally, but it is
disturbing if you don't know what it is.

9.3.10 N-Gons

Many game engines and commercial renderers will not render a polygon with more
than four sides (Fig. 9.16 ). These are called n-gons , short for n-sided polygon. Some
renderers will accept n-gons, but there is a risk that it will not render properly.
N-gons are frequently, but not always, non-planar (Sect. 9.3.12 ). A non-planar
polygon usually does not render as expected and should be reduced to either quads
(four-sided faces) or triangles. Planar quadrilaterals and triangles only are legal.
(Figure 9.16 ) Note the number of vertices along a single edge. Whenever you
have only two edges on either side of a vertex, you have a polygon that needs to be
further subdivided or risk creation of an illegal n-gon.
N-gons cannot be tolerated in a model, but are a very common by-product of the
modeling process. Most professionals will create these unintentionally while build-
ing an object, and then carefully check the model when they are done and eliminate
any n-gons they fi nd. A fast solution to this is to triangulate or quadrangulate the