Creating and Editing Texture Coordinates (UVs) - An Introduction to Computer Graphics for Artists - page 162

Fig. 10.23 The Mercator projection method causes serious distortion

10.4.2 Default Sphere, Cut Poles

Some of the distortion inherent in a cylindrical projection can be mitigated by splitting
polar triangles from their common vertex and moving the new vertices directly over
the other two vertices of the polygon they defi ne (Fig. 10.24 ) . This solution eliminates
the most egregious stretching, but it does not address scale errors that result from the
cylindrical mapping method. Another disadvantage to this solution is the large num-
ber of seams created at the poles, one for every triangle. Cutting the polar triangles
apart and adjusting the position of the UVs allows the object to be painted in these
areas, but 3D paint is still recommended because of the large number of seams.

10.4.3 Interrupted Mapping

An interrupted map is one where the projection has been split along seams to pre-
serve the area of polygons (Fig. 10.25 ). This type of projection eliminates the distor-
tion caused by trying to reconcile the distance between progressively smaller
polygons that are forced to be the same size as their neighbors. It is the artist's
choice where the map will be interrupted and how often, but if done for every verti-
cal section of polygons, the resulting layout will be distortion-free. It will also have
a very high number of seams. This solution is desirable when the texture to be
painted on the map is likely to suffer from distortion. If, for instance, the texture
map is a map of the world, complete with place names, this would be one of the best
solutions because it would not distort the fi ne detail of such a texture.
Making an interrupted map takes some effort on the modeler's part if it is done
by hand, but for applications that have scripting tools, it is possible to make a script
to generate this result automatically.