When unfolding UVs on a model, several things need to be accomplished:

1. The spaces between the UV coordinates should have roughly the same size proportions as the polygons that the UVs are associated with. If the polygons in the eyelid area of a character are tightly packed together compared to the polygons on the side of the head, then the UV spaces in the eyelids should not be spread out either. Uniform application of texture coordinates depends on a uniform distribution of points relative to the original polygonal model.
2. UV coordinates have a tendency to get tangled up. The mesh of UV coordinates should be organized on the final model in such a way that it is clear that no tangled UVs are on the model.
3. UV coordinates should not overlap with other UV coordinates. When an orthographic texture-mapping scheme is used, the UVs on the front of a model will overlap with the UVs on the back of the model. If the texture that was being applied were a bullet hole that shot directly through the object, this would be fine. Otherwise, organic models should not have UV coordinates that overlap. Overlapping UVs duplicate texture in two or more areas of the model. A common place that this occurs is the ear. The front of the ear will get a map that shows the detail of the ear, but the detail of the ear will often appear behind the ear as well if the coordinates are not taken care of.
4. The UV space of the UV coordinates should fall between the UV space ranging from 0 to 1. Many texture mapping programs allow for the distribution of texture space to fall well outside of these coordinates, and if the texture mapper knows what he or she is doing, this rule can be broken to increase efficiency. However, because paint programs paint maps that fall within the actual map and not outside the map’s own parameterization, then using the parameterization of 0 to 1 will ensure that the map that is being painted will fit correctly.

[Figures 49 & 50] The colored grid map used for unwrapping the UVs on an organic shape (above). The same map applied to the model (right).

UV Editing Using Texture Maps
The best way to check to see if the UVs are working correctly is to preview the UV work using a texture map on the model. Different maps accomplish different things while unmeshing the UVs on a production model. These maps can be used in progression to work out the UV mapping issues one step at a time. The most common test maps, shown in progression, are:

1. A color and number grid. These maps are useful in the initial unwrapping of UVs on a model. Several maps of this type are commonly available. The map shown in Figure 13.49 is a map I made in Photoshop in about 15 minutes. Maps such as these are designed so no numeral (or letter, depending on the map) will fall in the same colored square twice throughout the map. This map has 10 numerals used 10 times each. The placement of the numerals in rows helps establish orientation while viewing the map on the model. The colors are more random but at the same time are somewhat organized diagonally, also helping to establish orientation.

These maps are useful during the initial unwrapping stage because the unique pattern helps establish which areas are overlapping, are being repeated, or are twisted. Because each numeral or color combination only appears once, checking for repeating numeral or color blocks can help eliminate overlapping and tiling.