UV mapping for subdivision surfaces
UVs are two-dimensional texture coordinates that reside with the vertex component information for polygonal and subdivision surface meshes. UVs exist to define a two-dimensional texture coordinate system for a surface mesh and to facilitate the placement of image texture maps on the surface. UVs act as marker points that control which points (pixels) on the texture map correspond to which points (vertices) on the mesh. Textures applied to polygon or subdivision surfaces without UV texture coordinates will not render.
UV mapping is a process whereby you create, edit, and otherwise arrange a flattened (two-dimensional) representation of the surface mesh over the bitmap image to be used as a texture as it appears in the UV Texture Editor. The UV mapping process results in a correlation between the image and how it appears as a texture when mapped onto the three-dimensional surface mesh.
Although Maya creates UVs by default for many primitive types, you’ll need to rearrange the UVs in most cases, because the default arrangement will usually not match any subsequent edits to the model you may make. In addition, the location of the UV texture coordinates do not automatically update when you edit a surface mesh.
In most cases, you map and arrange UVs after you have completed your modeling and before you assign textures to the model.
Subdivision surfaces are similar to polygon surfaces in that they have UV texture coordinates. However, subdivision surfaces have their own mapping and editing operations, separate from the polygon operations. (It’s also possible to map UVs with the polygon operations; see Edit UVs in Polygon Proxy Mode.)
For subdivision surfaces, there are two UV mapping operations available: Planar Mapping and Automatic Mapping. These items are located in the Subdiv Surfaces > Texture menu. They are equivalent to the Planar Mapping and Automatic Mapping features in the Create UVs menu. However, one key difference is that the Subdiv Surfaces operations use only one UV set, whereas polygons can have multiple UV sets.
To create a UV mapping arrangement that works best for your model, you may need to map several times, until you find a mapping arrangement that is suitable.