Chapter 10. Skeletons And Rigging

Once you model a character, you need to place a skeleton inside it in order to animate it (Figure 10.1). The skeleton is built as a hierarchy of individual joints connected by bones.

Figure 10.1. Inside this eyeball character, you can see the skeleton, which is used to pose and animate the character.

These bones typically aren't used directly to animate. Instead, you place a set of constraints on the bones. These constraints limit or alter the movement of the bones and allow you both greater control of the whole skeleton and the ability to pose the character with just a few bones or curves.

In general, nature serves as the best template for placing joints: Use pictures of skeletons, human or animal. A shoulder joint should go at the shoulder; an elbow joint should go at the elbow; and so on. However, you don't need to be too literal. For example, although the human foot has 26 bones, you can animate a shoe with three bones. Wherever you want something to bend, that's where you need a joint.

Joints are hierarchicalthat is, the joints at the top of the hierarchy move those beneath them. The first joint you place is at the top of the hierarchy and is often referred to as the root joint. This joint moves the whole skeleton. Because the knee, ankle, and foot joints are below the hip joint, the rest of the leg moves when the hip joint is rotated. This type of character posing uses forward kinematics (FK) (Figure 10.2).

Figure 10.2. Illustrating the principle of forward kinematics. When the hip joint is rotated, all the joints below rotate with it.

Inverse kinematics (IK) refers to posing from the bottom of the hierarchy up. In this type of rig, if you move the foot around, the knee and hip rotate accordingly (Figure 10.3). Sections of your skeleton must specifically be set up for IK use, whereas FK control is inherent in the bone hierarchy.

Figure 10.3. The foot is moved using an IK handle. Some animators avoid IK because it tends to move limbs in straight lines rather than natural arcs.

FK or IK?

New animators always wonder which is betterFK or IK? The answer is that they both have advantages and weaknesses.

Forward kinematics allows for very precise motion, smooth arcs, and easy layering of rotations. FK can be more troublesome for posing because each joint must be rotated into position. As such, FK is best suited for posing freely moving objects, because it's difficult to maintain the end of an FK chain in one location.

Inverse kinematics is best used when you need precise control over the end of a chain, because you animate the handle rather than adjusting each joint in the skeleton. It's easy to put the joints exactly where you want, so you might use IK when placing the hand or foot of a character on an immobile object. However, there are drawbacks: IK handles have a tendency to flip, create floaty or disconnected animation, and leave flat arcs. Plus, positioning the intermediate joints can be difficult.

Spline IK (discussed later in this chapter) is good for tentacles and spinal columns or any skeleton that has many joints that need to move in a fluid, organic manner.

Full Body IK, new in Maya 7.0, is a quick and powerful rigging option. Full Body IK connects the whole skeleton together, allowing you to pose the character from the end of any chain. It suffers from the same drawbacks of any IK rig, but it has a lot more flexibility than the standard IK options.

A complex character employs all of these methods to give you the greatest amount of freedom and control.