Validation of CG Models - An Introduction to Computer Graphics for Artists - page 131

fi nished adjusting a model, all transforms should be deleted before the model is
delivered to anyone for animation or as an in-game asset.

9.3.16 UV Distortion

There are a number of ways to distort UVs. Some are more acceptable than others.
The most severe distortion errors happen when UVs for the same polygon are coin-
cident. In the UV editor, a quad that has been distorted like this will look like a
single edge (two pairs of coincident UVs) or a single point (all UVs coincident).
When this is true of a polygon, a texture map will display as either stretched parallel
rows of single pixels (collapsed to edge) or a single color (collapsed to a point.) This
error can be fi xed by editing the UVs manually or by reprojecting them.
All other types of distortion are caused when the shape of the mapped polygon
does not match exactly the shape of the UVs that are attached to it. It is very diffi cult
to completely eliminate distortion from objects that have any amount of curvature,
but it can be minimized. See Chap. 11 for more details on this subject.

9.3.17 Zero Edge-Length Face

A face whose edges are each zero units in length (Fig. 9.21 ).
These faces occur during merge vertex, decimation routines, and other derezing
operations. They can be very diffi cult to delete because their zero edge length condi-
tion makes them illegal in a way that makes it diffi cult to perform many editing
operations on them, including deletion. For instance, they are literally faces , but
because they have no physical dimension, they are not treated as faces. Deleting
them directly is not always possible, nor can their component edges and vertices be
deleted. Sometimes the only way to delete these is to use a global operation on your
object that selectively truncates an entire side of it, and make sure it is the side con-
taining the zero edge length faces.
If you have a cleanup tool that can fi nd these, use it. Faces of this type are highly
illegal and can cause crashes in game engines and renderers.

9.4 Construction

9.4.1 Introduction

Construction errors are not errors until the model is considered fi nished and deliv-
ered. During the process of creating a model, artists will modify the model in many
different ways, some of them meant to be temporary, as other parts are worked on
or reference objects are created. It is a legitimate modeling practice to make