Creating and Editing Texture Coordinates (UVs) - An Introduction to Computer Graphics for Artists - page 151

Fig. 10.8 A sphere and its default coordinates

10.2.4 Spherical

In the example of a sphere, any continuous row of faces may be unfolded fl at, but
any two rows or more cannot be without distortion or seams. The reason is that the
second row of polygons projects away from the plane of the fi rst row. This causes
the projection to become progressively more distorted as the polygons themselves
taper toward the poles of the sphere (Fig. 10.8 ) .
In the example of the sphere above, you can see that it has the same number of
seams as the cylinder, this is because there is a great deal of distortion allowed, mak-
ing the object seamless in most areas. The problem is that if no distortion is desired,
then the UVs have to be ripped apart.

10.2.5 Scale

Just as the objects in your scene have a scale, so do the texture maps. The measurement
units for your geometry can be inches, feet, yards, miles, millimeters, centimeters,
meters, or kilometers. The measurement units for your texture maps are pixels per
unit. If your scene units are meters, and a 1 m square polygon has a 512 × 512 pixel
map on it, then your texture scale is 512 pixels per meter. If you took this literally,
then a 10 cm square polygon would have a 51 × 51 pixel map. In CG, it doesn't work
exactly like this due to limitations in offl ine and real time renderers. Table 10.1
provides a list of the sizes and their linear equivalents for a scene that has set the
pixel to unit scale at 256 pixels per meter.