Creating and Editing Texture Coordinates (UVs) - An Introduction to Computer Graphics for Artists - page 149

Fig. 10.5 Shadows cast by two octahedrons illustrate why seams are necessary. Imagine the
grid the shadows are cast upon as UV space, and see what happens when a polygon is rotated
to fl atten it out

to edit them carefully, and if you don't have to, you should be pleasantly surprised.
For the rest of the time, you will get your work done faster if you assume to begin
with that editing is required.

10.2.1 Non-manifold vs. Manifold Geometry

Only a tiny class of objects may be mapped without distortions and seams. These
are objects that have only slight topology changes and no insets. The reason is that
true 3D space cannot be represented in 2D texture space. Every part of an object that
is angled away from a single primary projection plane must be folded into that plane
in order to avoid distortion (Fig. 10.5 ) . Folding all the planes of a three-dimensional
object into the same two-dimensional plane requires the creation of seams. Non-
manifold geometry cannot be unfolded this way, so overlapping sections must be
cut away and treated separately.

10.2.2 Cubic Projection

A cube can be unfolded into a continuous fl at series of co-planar polygons
(Fig. 10.6 ) . After unfolding a cube into a fl at sheet, a seam is created around its
perimeter. Therefore, the cube cannot have a completely seamless texture. It will be
seamless in some areas, but not all.