Chapter 10. Skeletons and Inverse Kinematics

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

In general, nature serves as the best template for placing joints: Use pictures of skeletons, human or animal (Figure 10.1). 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.

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

Joints are hierarchicalthat is, the joints at the top of the hierarchy will move those beneath them. The first joint you place will be at the top of the hierarchy and is often referred to as the root joint. This is the joint that 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. Animating in this fashion is called forward kinematics (FK) (Figure 10.2).

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

Inverse kinematics, or IK, refers to animating from the bottom of the hierarchy up. In this type of animation, if you moved the foot around, the knee and hip would rotate accordingly (Figure 10.3).

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 and mechanical motion but require more effort from the animator because each joint in a chain has to be animated.

Inverse kinematics is great for bipedal and quad pedal creatures like humans or dogs because you can place the hand, foot, or paw of the character by animating the handle rather than adjusting each joint in the skeleton. However, IK handles can flip and getting precise movement out of the intermediate joints can be difficult.

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

A complex character will employ all of these methods to give the animator the greatest amount of freedom and control.