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Overview

Two Bone IK attaches a limb, usually an arm or leg, to a world-space target or another bone.
  • Tip — this bone will be aligned with a target. Usually, it is a hand or a foot.
  • Target — bone to reach. It can be an IK bone or a custom Game Object in the hierarchy.
  • Use World Target — whether to use world position and rotation as a target.
  • IK Target Transform — world-space target transform.
  • Hint Weight — joint or pole target alpha.
  • Pole Target — pole/hint bone, which is typically an elbow or a knee.
  • Hint Offset — hint or pole target position offset.
  • Max Limb Length Scale — max allowed limb length multiplier.
Note: Hint or pole target is usually a knee or elbow.

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Use-cases

Two-Bone IK is generally used in combination with other modifiers, like Foot or Step IK. It is also used to attach hands to props, like barrel or a lever.

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IK Setup

Most Two-Bone IK setups rely on proxy, empty Game Objects that work as IK bones:

IK Bones.

In Procedural Animation Settings, poses are copied from real (e.g., right hand or left foot) to IK bones using Copy Bone Modifiers:

Copy right foot example.

Then, procedural logic is applied (e.g., Foot and Step IK):

IK Setup.

Finally, Two Bone IK modifiers are applied to animate hands or feet based on their targets. Thanks to this workflow, it’s possible to cache hands or feet animation, and then alter it procedurally without loosing the original character pose.