IK / FK
Inverse vs Forward Kinematics
Also known as: Inverse kinematics · Forward kinematics
Definition
IK (Inverse Kinematics) and FK (Forward Kinematics) are the two modes of joint animation. FK rotates each joint independently — animate the shoulder, then elbow, then wrist, then fingers — giving precise arc control. IK works backwards: place the hand where you want it and the rig solves where the elbow and shoulder need to be. IK is fast for placing limbs (foot on the ground, hand on a sword); FK is better for arcs in mid-air motion.
In production
Production rigs offer an IK/FK switch per limb so animators choose the right mode per shot. The switch must "snap" cleanly so the limb doesn't jump position when toggled — a hallmark of good rigging, usually achieved by matching the FK joint chain's rotation values to the IK solution at the moment of the switch via a "snap" script button. Maya's Human IK and Blender's Rigify both build this switching logic automatically, exposing an IK/FK blend attribute (0-1) on a custom control rather than a hard toggle, so animators can blend gradually across a few frames. Games commonly use IK at runtime too: a foot-IK system in Unreal Engine or Unity's Animation Rigging package adjusts foot placement per-frame so characters plant correctly on uneven terrain instead of clipping into or floating above slopes. A common rigging bug is "IK pole vector flipping," where the elbow or knee snaps to the wrong side because the pole vector control lost its reference direction during a fast rotation.
