Skeletons are the underlying joint and bone hierarchies that let you animate your characters. Every skeleton has several parent joints and child joints, and one root joint. Parent joints are joints that have joints below them in the skeleton’s hierarchy. For example, an elbow is the parent of the wrist and the child of the shoulder. The root joint is the first or top joint in a skeleton’s hierarchy. For more information on setting up skeletons, see Skeletons.
You can use the following methods to animate a skeleton: forward kinematics (FK), inverse kinematics (IK), or IK/FK blending.
With forward kinematics, you transform and key joints directly, rather than using an IK handle to animate a skeleton.
Forward kinematics are useful for creating detailed arcing movements, but not very intuitive for goal-directed movements. For example, you can easily use FK to animate the rotation of an arm at the shoulder joint, but not the arm reaching for a glass. For more information on FK, see Forward kinematics (FK).
With inverse kinematics, you transform and key an IK handle to animate a skeleton. The IK handle is drawn as a straight line between the start and end joints of its IK chain. The effect the IK handle has on the joint chain depends on the type of IK solver used by the IK handle. See IK solvers.
Inverse kinematics are useful for goal-directed movements. For example, you can use IK to animate an arm reaching for a glass of water, but not for specific movements at individual joints. For more information on IK, see Inverse Kinematics (IK).
Instead of using only FK or IK to pose and animate a joint chain, you can use both FK and IK on the same joint chain. The Ik Blend attribute on the ikHandle lets you apply FK and IK animation to the same joints. Ik Blend specifies the amount of influence (weight) that FK or IK have over the animation of the joints.
Blending IK and FK is useful for posing complex characters that have a wide range of movements in their animations. For example, you can use IK to animate the directed motion of a character’s arms, and you can use FK to animate the rotation of the shoulder, elbow, and wrist joints in the arm. For more information, see IK/FK blending.