Understanding UVs - Love Them or Hate Them, They're Essential to Know
UVs are two-dimensional texture coordinates that correspond with the vertex information for your geometry. UVs are vital because they provide the link between a surface mesh and how an image texture gets applied onto that surface. They are basically marker points that control which pixels on the texture corresponds to which vertex on the 3D mesh.
By default, most 3D applications will create an automatic UV layout when the mesh is originally created. However if you were to drop the texture for your character’s head directly onto the 3D model, the chances are good that you would see very undesirable results. This occurs because during the modeling process, the UVs aren't usually taken into account and, as a result, the 2D image can't wrap around a 3D object the way you would expect to see.
Once your model is complete, in order to properly texture your model, you need to begin the process of laying out the UVs (often referred to as UV mapping). This basically is the process of creating a 2D representation of your 3D object. Imagine your model unfolded and flattened out into a flat 2D image. Where would the natural seams occur? Where on the 3D model would the most detail be needed? Those are the types of things you'll need to take into account when creating your UV layout.
Each 3D application has a UV editor that you'll use to unfold and edit the UVs for your model. Depending on your 3D application, each face or polygon on your 3D model is tied to a face on the UV map. UV mapping is a critical skill to master for accurate textures on a surface.
The actual creation of UVs is done through a projection technique. Think of this as a projector showing a movie on a screen. The concept is the same, except in a 3D application there are typically several different UV projection types available to you. These are based off of simple geometric shapes and often times are a great starting point when beginning to layout UVs for a single object.
If you have a 3D object that is spherical you can apply a spherical projection to it. This creates UVs that are based on a spherical shape wrapped around the mesh.
Cylindrical mapping creates UVs for a 3D object based on a cylindrical projection shape. This is great for objects that can be completely enclosed and visible within the cylinder. So something like an arm or leg.
Planar mapping projects UVs onto a mesh through a plane. This projection type would be best for objects that are relatively flat. If your models form is very complex, a planar projection can produce UVs that overlap and look distorted. So a planar map should be used on very simple shapes.
While these projection types are great, they generally aren't an all-in-one solution to every UV layout. As soon as you start creating complex meshes you'll quickly find that a single planar or cylindrical map simply won't create the desired result. Fortunately, every face on a mesh can have its own projection applied and the UVs themselves can be manipulated and edited extensively after projection. This really gives you a fine level of control over exactly how the UVs look and, by extension, how the final 2D textures will be applied to your 3D model.
Tips for UV Editing
When laying out the UVs for your 3D object there will always be a seam on your texture, so plan out where you want to your seams to be. Look for places where they can be hidden or less likely to be visible on your 3D model. You can also use the paint tool inside your 3D application to paint directly over the seam in the texture.
When you have the UVs properly laid out in the UV editor you can create a snapshot of the UVs using a UV Snapshot tool or the Render UV tool (it'll have slightly different names depending on your 3D application). Basically this tool will take a picture of the UV layout and save it as your desired image format so you can import it directly into your favorite 2D paint tool. This gives you a guide to see where you are painting on the 3D model.
In most 3D applications there is an automatic mapping projection tool. Automatic mapping creates UVs for a mesh by attempting to find the best possible UV placement by projecting from multiple planes. This is useful when you have more complex shapes where the basic projections don't produce UVs that are useful. Manual editing of the UVs is usually still required, but automatic mapping is a good place to start from.
Like the geometry they represent, UV points can be connected together to form a larger shape that is referred to as a UV shell in Maya, a UV island in Softimage and a UV cluster in 3ds Max. Despite their different names in different applications, they serve a common purpose. Often when working with these shells, some of them may overlap in the UV editor. If this happens the texture will appear to repeat. Unless there is a specific need for it, as there sometimes is with game textures, overlapping UV shells should generally be avoided.
Beneficial Software for UV Mapping and Texturing
To help with your texturing and UV layout process there have been a few programs that can help speed up your workflow. UVLayout is a great application designed specifically for laying out the UVs for your 3D model. It has some great features to speed up the process so you can spend more time texturing and less time setting up the UVs.
The Foundry's MARI is an application designed for the sole purpose of painting textures for your 3D model, and has some great features that significantly speed up the process when designing the textures. For example, you can paint directly onto your 3D model which allows you to see exactly where these textures are going to be placed. Once completed, you can simply export the texture map.
Now that you are familiar with UVs and their purpose, make sure you utilize them when creating complex textures for your 3D models. For reference here are some great introduction tutorials for UV mapping in your 3D application in UV mapping in 3ds Max, UV mapping in Maya and UV mapping in Softimage.