Automatic Hair
The method outlined previously will work with just about any 3D package. Many software applications have hair plug-ins available to automate the process. Many of these produce excellent results. Still, you will need to know how they work to get the most out of them. Hair software falls into several broad categories.
Hair from the Environment
One of the more economical ways to create hair is as a 2D environment effect. The hair is calculated after the image is rendered and automatically is composited into the shot. Some examples of this are Digimation's Shag:Fur plug-in for 3DS max, as well as Softimage's Mental Ray fur shader.
Because no actual geometry is created, environment hair renders reasonably quickly. The downfall is that this technology works best with shorter hairstyles, such as crew cuts and fur. Most of this type of software tends to choke when the hair needs to interact with the character, such as long hair flowing over the shoulders.
Geometry Hair
The more sophisticated plug-ins, such as Maya's Hair, actually create hair as individual strands. To get a realistic look, however, you need to create lots of geometry, which drives up render times considerably. The big advantage to creating hair as geometry is that the hair can collide with obstacles, so it allows for longer hairstyles that can interact with the environment. Another advantage is that rendering looks more realistic because the individual hairs can cast shadows into the scene file.
Manipulating hair using third-party software can be difficult, particularly when there are lots of hairs. To help speed things along, most packages provide an interface that allows for a few "master" hairs to control the rest. These will allow you to control and sculpt the direction of the hair (see Figure 7).
Figure 7 In Maya, these master hairs (green) show the general shape and direction of the hair.
Many other parameters can be added. Some packages use a bitmap to define the color of the hair, so creating a leopard with spots is an easy task. Density of the hair is also important (see Figure 8.) Some characters will need to be totally fluffy, with a very high hair density. Others will need to be more scraggly, with patches of skin showing through.
Figure 8 When the sphere is rendered, differing hair densities create different looks.
As you can see, there are quite a few methods for creating hair. Which technique you choose depends on a variety of factors. Many times, your choice of software dictates the way the hair is produced. Remember that the technique you choose should be tested thoroughly before you go into actual production so the technique that can be fitted into the production pipeline from the start.
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