Configuring IK Rigs for Retargeting

In this lesson, you will learn how to configure an IK Rig asset to define the specific bone chains required to map motion accurately between different skeletal meshes.

When retargeting animations from one character to another, Unreal Engine needs a "map" that tells it which bones correspond to limbs like the arms or legs. An IK Rig provides this map by grouping individual bones into named Chains. Without these chains, the engine wouldn't know that the "UpperArm_L" and "LowerArm_L" of your character should be treated as a single kinematic unit during retargeting.

Creating the IK Rig

To start, right-click in your Content Browser, go to Animation > IK Rig, and select your character's Skeleton. Once opened, you will see your character's bone hierarchy on the left.

Defining Bone Chains

Think of a chain as a biological link; you are telling the engine, "This specific set of bones represents the left leg."

1. Create New Chain: In the IK Rig panel, click the + icon in the Chains tab. Name it something descriptive, like LeftLeg.
2. Assign Bones: Select the Thigh_L bone in your hierarchy, right-click it, and choose Set Start Bone. Then, select the Foot_L bone, right-click, and choose Set End Bone.
3. Automatic Filling: The IK Rig will automatically identify and include the bones between your Start and End selections, such as the Calf_L.

Repeat this for your RightLeg, LeftArm, and RightArm. These chains act as the structural "connective tissue" that the IK Retargeter uses to move one character’s limbs in sync with another, even if their limb lengths or proportions differ.

By clearly defining these chains, you ensure that when an animation plays, the character's feet stay planted on the ground rather than floating or clipping through the floor during the transition.

Key Takeaway: Defining bone chains in an IK Rig creates the logical groupings the engine requires to intelligently map movement between skeletons with different proportions.