Creating and Editing Texture Coordinates (UVs) - An Introduction to Computer Graphics for Artists - page 157

Fig. 10.16 The orange dotted line represents a projection plane aligned with the global axes. The
white dotted line is the boundary of the same projection plane, modifi ed to be aligned with the
object (Model courtesy of Andrius Drevinskas)

10.2.6 Projection Orientation

To project texture coordinates without distortion, the projection matrix should be
oriented to each plane's normal, or as close as possible. You may be able to use an
automatic tool that will do this for you, or you will have to adjust the projection
angle manually (Fig. 10.16 ) . If your application doesn't allow you to adjust the
orientation of the projection, you can rotate your object until it is aligned with
the global axes, then project from that position. After you have done this, you can
rotate the object back to its correct orientation in the scene.

10.2.6.1 Neutral Poses

An object can have a neutral pose for animation (see Sect. 13.3.6 ), but this is also
true for projecting UV coordinates. Complex objects like characters can be very
diffi cult to apply texture coordinates to. This job is made even more diffi cult if the
model is not in a neutral pose. This is because the texture coordinates for the parts
of a character must be projected separately if they are oriented differently. This is
always the case when the character is not in a neutral pose. To make the pose neutral
for projecting UVs, the parts of the model should be oriented to match the global
axes as much as possible. In some areas like the fi ngers, this won't be completely
possible because their orientation will not exactly match the arm (Fig. 10.17 ) , but
if the fi ngers are straight, they can each be projected as a unit instead of three
separate joints per fi nger, as would be required if they were posed differently.

10.3 Distortion

The fi rst rule of UV editing is that if the shape of your polygon is not exactly the
same as the shape of the UVs attached to it, your texture will be distorted (Fig. 10.18 ).
There is no way around this. It is always true.