Hierarchies and Kinematics

When you animate characters (whether humanoid or otherwise), mechanical assemblies, or complex motion, you can simplify the process by linking objects together to form a hierarchy or chain. In a linked chain, the animation of one member can affect some or all of the others, making it possible to animate a number of objects or bones at once.

The term kinematics describes the movement or animation of the chain. There are two types of kinematics:

• With forward kinematics (FK) , you transform a parent object to move its descendants (its children, their children, etc.).
• With inverse kinematics (IK), you transform a child object to move its ancestors (its parent and so on up the chain). You can also use IK to make an object “stick” to the ground or another surface, while allowing the chain to rotate off the pivot of that object.

Forward kinematics is the most straightforward method for animating hierarchies. Inverse kinematics requires more setup than forward kinematics, but is more intuitive for complex tasks such as character animation or intricate mechanical animation.