In this lesson, you will learn how to bridge the gap between two unique skeletal structures to ensure your character animations play correctly regardless of bone size or hierarchy differences.
The Concept: Mapping Motion
IK Retargeting is the process of translating animation data from a "Source" skeleton to a "Target" skeleton. Even if your characters have different limb lengths or bone proportions, the engine uses your previously configured IK Rigs and Retarget Chains to calculate the relative position of the bones.
Think of it like a translator: the source skeleton says "move the hand to this coordinate," but because the target skeleton’s arm is a different length, the retargeter dynamically adjusts the rotation and extension of the elbow and shoulder joints to reach that point without the arm clipping through the body or floating awkwardly.
Executing the Retarget
To perform the transfer, you use an IK Retargeter Asset. Within this asset, you select your Source and Target IKs. The magic happens in the Chain Mapping panel, where you align the chains you defined in your rigs (like "Spine," "LeftArm," or "RightLeg").
Once mapped, you can preview the animation on your target character. If the limbs look bent or misaligned, you refer back to your Retarget Pose settings to ensure the target character is in a "T-pose" or "A-pose" that matches the source character’s reference frame.
To finalize the process for use in your game, select an animation sequence from the source skeleton, right-click it, and choose Retarget Animation Assets > Duplicate and Retarget Animation Assets. This generates a new animation file specifically tailored for the target skeleton:
// Procedural Logic flow during retargeting:
1. Sample source bone rotation/translation at frame T.
2. Calculate IK chain effector goal position.
3. Apply Solver (typically FBIK) to reach goal on target skeleton.
4. Bake result into new animation asset.
By relying on these chains rather than individual bone names, you ensure your animations remain portable across your entire character roster.
Key Takeaway: IK Retargeting creates a mathematical translation layer that allows animations to adapt to unique skeletal dimensions, ensuring limb placement remains accurate even when bone hierarchies differ.