In this lesson, you will learn how to map specific bone hierarchies between different characters so that your animations transfer correctly during retargeting.
Understanding Retarget Chains
When you want to share animations between two different characters, Unreal Engine needs to know exactly which bones in your source skeleton correspond to the bones in your target skeleton. Even if two characters share the same "Humanoid" naming convention, their skeletons might have different proportions or missing bones. Retarget Chains solve this by grouping bones into logical segments—like the Spine, Left Arm, or Right Leg—so the animation data can be intelligently mapped from one to the other.
Defining the Chains in the IK Rig
To start, open your IK Rig asset. Within the Chain panel, you define a chain by selecting a start bone and an end bone in the hierarchy. For example, if you are setting up the LeftArm chain:
- Select the shoulder or clavicle as the Start Bone.
- Select the hand or wrist as the End Bone.
- Right-click these selected bones and choose New Retarget Chain.
Once created, you define the chain’s "Goal" (like LeftArm). You repeat this process for all major segments: Spine, Head, LeftLeg, RightLeg, and RightArm.
Why this Mapping Matters
Think of these chains as "translation bridges." By grouping bones into these chains, you are telling the animation system: "Whatever movement happens to this entire arm segment in the source skeleton, apply it to the corresponding arm segment in the target skeleton."
Even if the target's arm is shorter or its spine is curved differently, the retargeter uses these chains to calculate the transformation difference between the skeletons. Without these chains, the animation would not know how to distribute motion across the hierarchy, often resulting in "broken" or distorted mesh deformations where limbs appear to twist unnaturally or detach from the body.
By consistently defining these chains in both your source and target IK Rigs, you create a common language that allows the engine to transfer complex movement data seamlessly across your character roster.
Key Takeaway: Retarget Chains allow you to map animation data accurately between different skeletal structures by grouping bones into logical, reusable segments that the retargeting system can interpret and scale.