Building, rigging, and animating characters is a lot of work. With all that work, many people overlook one of the most important tasks in creating a character - the textures. Textures play a very important part in the way a final image looks. Even the best model will only look average with poor texturing.
Texturing a character is very much a visual exercise. If the texture looks right, it usually is. There are other issues, such as getting the resolution of the texture maps correct, and making sure the textures fit properly on the character.
There are two major ways to create textures, bitmaps and procedurals.
Most people use bitmaps to texture characters. The reasons are pretty straightforward - textures allow for artists to scan, photograph, or paint images. This speaks directly to the creative side of the artist, who simply wants to paint the character.
The downside to bitmaps is that they are tied to the resolution of the output medium as well as where the character is on the screen. The textures on a character created for television (640x480 pixels) will not scale up for a motion picture (2048x1170) for example. The larger the final rendered image, the larger the bitmaps need to be.
If the character is standing full screen, then the bitmaps will need to be at least the size of the output medium. If the camera gets closer, the bitmaps will need to be even larger. On some films, the bitmaps can be as large as 5K pixels per image.
Procedural textures allow you to define a texture using a predefined formula. This can be something as simple as noise to something as complex as a Renderman shader, which resembles a computer program. These are certainly not as intuitive to use as bitmaps, but they have advantages.
One big advantage to using procedural shaders is that they work at all resolutions. This is important for those who create feature films. In addition, procedural shaders can be manipulated by changing a few software parameters. This allows you to take one shader and tweak it to fit many situations.
To keep things simple, we will map this guy using a planar map created in Photoshop. I find it is easiest to paint the maps if you have a template to use as reference. To do this, simply render an image of the head from the front view. This image should be approximately as big as the character
GETTING READY TO PAINT
PAINTING COLOR MAPS
Color is the most apparent of the maps you will be creating. The color of the skin needs to fit the character and also look natural. A good starting point for skin textures are photographic maps of real skin. The type of map you choose depends on the character. For this character, we're using a reptile-like texture. Using the clone stamp tool, paint the textures over the reference image to match the surface of the face. Instead of trying to match the outside edges of the model exactly, it's best just to let the color bleed out beyond the edges. This will make it easy to reposition the map later.
PAINTING BUMP MAPS
Bump maps are very important to creating a realistic surface. Skin is not a smooth surface, it has all sorts of imperfections. Characters have wrinkles, pores, blemishes, and so on. These sorts of details can be best created using a bump or a displacement map. Bump maps create the illusion that a surface is rough. A displacement map actually distorts the geometry of the underlying surface to get the same effect.
Both of these maps work by using a greyscale image that defines the intensity of the bump or displacement. Black pixels do not alter the surface at all, while white pixels give maximum effect.
The easiest way to create a slightly rough surface is to use a simple procedural map, such as noise. This will give the surface a rougher and more natural feel. Just adding noise, however, will give all parts of the surface equal roughness. Different areas of the face will need different bump maps - the lips, for example, have a distinct texture that's different from the pores areoing the edges of the nose, which are different from the forehead. To get more accurate bumps, its best to paint a map specifically for your model.
The procedure for painting a bump map is the same as for the color map, but simply done in greyscale. The rendered reference image of the character is opened in Photoshop, and the bump map painted on a layer above the reference. When creating a bump map, try to make sure you gets a much contrast as possible.
Specularity defines how shiny a surface is at any given point. The face is shinier at different areas - the side of the nose usually has more oil, so it appears slicker. This is why many women powder their nose - to remove the shine. In any event, painting a specularity map allows you to define where the face is matte and where it is shiny.
Painting the specularity map is much like painting any other map type. The map is usually greyscale, much like the bump map.
CREATING THE SHADER
Exactly how you create the shader will depend on your software, but if you created the maps using the same reference image, all the maps should be aligned properly. For skin, Blinn shaders work well. If your software supports Oren-Nayer Blinn, then choose that as it has a much softer look, which usually helps when texturing characters. In a pinch, a Phong shader will also work, as will many other types.
Place the color, bump, and specularity maps in the appropriate slots. Set the color map to 100%, and adjust the strength of the other maps until the surface looks right.
MAPPING THE CHARACTER
Finally you need to apply the texture and the associated maps to the character. Apply a planar map to the character, then apply the shader. Usually, the map will not be aligned perfectly, so you'll need to adjust the size and position of the mapping until they fit the character perfectly.
With planar mapping, we only textured the front part of the character. You will need to repeat the process for the back of the head. Once the maps are applied, you will need to render some tests to make sure the character looks good from all angles and in a number of different lighting situations.
Render a final test and tweak if necessary.
These are the basics of texturing for characters. There are plenty of other methods, including the use of 3D paint systems, to create and place textures on your characters. Experiment and have fun.