Inverse Kinematics (IK) & Retargeting

Inverse Kinematics (IK) & Retargeting

For humanoid characters, Unity includes advanced tools to adjust animations to match physical environments.

Animator Retargeting

Retargeting is the ability to share animations between different humanoid 3D models.

• Set the Rig Type on both models to Humanoid in the import settings.
• Unity maps both models' skeletons to a standard avatar profile.
• You can then use the same Animator Controller and clips on both characters, regardless of bone size differences.

Inverse Kinematics (IK)

Standard animations use Forward Kinematics (FK): rotation values propagate down the skeleton (e.g. rotating the shoulder moves the elbow, which moves the hand).

Inverse Kinematics (IK) works in reverse: you define a target position (e.g., a coffee cup), and Unity calculates the rotations of the elbow and shoulder to place the hand exactly at that position.

IK Scripts in Unity:

To use IK, check IK Pass in your Animator Controller layer settings:

void OnAnimatorIK(int layerIndex)
{
    // Enable and set hand IK target
    anim.SetIKPositionWeight(AvatarIKGoal.RightHand, 1.0f);
    anim.SetIKRotationWeight(AvatarIKGoal.RightHand, 1.0f);
    anim.SetIKPosition(AvatarIKGoal.RightHand, lookTarget.position);
}

Common goals include placing feet flat on uneven terrain or making a character look toward an object.

TL;DR

• Humanoid Rig settings enable animator retargeting between skeletons.
• Forward Kinematics rotates bones; Inverse Kinematics targets endpoints.
• Check the 'IK Pass' option on the animator layer settings to enable IK callbacks.
• Implement OnAnimatorIK in scripts to set goal positions and weights.