UVs (pronounced U-VEEZ) 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, called UV texture space. UV texture space uses the letters U and V to indicate the axes in 2D. UV texture space facilitates the placement of image texture maps on a 3D surface.
UVs are essential in that they provide the connection between the surface mesh and how the image texture gets mapped onto the surface mesh. That is, 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 that do not possess UV texture coordinates will not render.
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. Otherwise, changing the model will create a mismatch between the model and the UVs, and affect how any textures appear on the model.
Understanding the concept of UVs and how to map them to a surface, and subsequently lay them out accurately is essential for producing textures on polygonal and subdivision surfaces when working in Maya. Understanding how UVs work is also important when you need to paint textures, fur, or hair onto a 3D model.
Texture mapping polygon and subdivision surfaces in Maya differs from how you texture NURBS surfaces.
For NURBS surface types, each surface mesh is defined as a four-sided square or rectangular patch that has specific U and V directions. For NURBS surface types the texture coordinates (UVs) that control the placement of a texture exist by default and are implicitly connected to the control vertices. When the control vertices get repositioned, so do the positions of the corresponding UV texture coordinates. Any textures mapped to the surface are also affected as a result. That is, moving a CV will affect how the texture map appears on the NURBS surface.
For polygon and subdivision surface types, the shape of surface meshes is usually much more irregular compared to NURBS surface types. Texture coordinates (UVs) associated with these surface types do not always exist by default, and so must be explicitly created, and subsequently modified in most cases, so that the surface mesh can accommodate a texture map.
The process of creating explicit UVs for a surface mesh is called UV mapping. UV mapping is a process whereby you create, edit, and otherwise arrange the UVs (that appear as a flattened, two-dimensional representation of the surface mesh, over top of the two-dimensional 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. UV mapping is a critical skill to master for accurate and realistic textures on polygonal surfaces.
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