Creating and Editing Texture Coordinates (UVs) - An Introduction to Computer Graphics for Artists - page 154

Fig. 10.11 A reference cube
is used as a size reference for
projecting UVs
elements combined, but not smaller (Fig. 10.11 ) . The reference cube is not a bounding
box, so it is not meant to fi t the greatest dimension in each of the three global
axes exactly. Instead, all of its sides must be at least as long as or longer than the
largest dimension in your scene, whatever that is. It is very important to scale
the cube proportionately. If not, the aspect ratio of some faces will be changed and
corresponding texture coordinates will be distorted.
Different applications vary in the method required to make effective use of the
reference cube. In some, you will have to project each axis separately, in others; you
can project them all simultaneously. The difference is whether your application
always projects into legal 0-1 texture space and whether it allows overlapping
projections. Your goal is to project in such a way that each projection does not
overlap the other. All applications have a way to do this, but whether it is automatic
or manual will depend on the application.
An object to be mapped is completely enclosed by a reference cube object. If it
were not completely enclosed, the reference object would be of no use because
it would no longer constrain the outer bounds of a projection to the size of any of
its identical faces.
The reference cube may be rotated and moved as necessary for multiple pro-
jections, but never scaled. During the course of making projections for a compli-
cated object, a reference cube will have its faces mapped many times over. Don't
worry about overwriting its UVs or whether the cube's UVs look acceptable,
the only thing that counts is that the object you are mapping gets the UVs it needs.
That will occasionally result in poor UVs for the reference cube. When you are
fi nished, you may want to hide the cube but keep it around in case you need to make
changes later on.