Animation retargeting: transfer motion between different skeletons

Transfer a movement, not just matching bone names

Animation retargeting adapts motion from a source skeleton to a target skeleton. Matching names alone does not establish matching local axes, reference poses or proportions. Godot's documentation explicitly distinguishes bone names from rest transforms; Unreal's IK Retargeter maps skeletal chains and provides editable retarget poses. Establish this correspondence before judging the motion. [1][2]

Define what should stay invariant

Decide which properties matter: the character's overall travel, a planted foot, a hand reaching a handle, or the timing of a gesture. These goals can conflict when body proportions differ. Align reference poses, check bone orientation and identify the root and pelvis roles. A source in a T-pose and a target in an A-pose need an intentional reference alignment, not compensating rotations scattered through every animation. [1]

Worked example: stride scaling changes travel speed

Consider an illustrative source walk cycle lasting 1 second and advancing 1.5 meters. Suppose the target has a uniform limb-length ratio of 0.8, and the chosen policy scales travel by that ratio. The target's cycle advances 1.5 * 0.8 = 1.2 meters.

At the unchanged cycle duration, its implied speed is 1.2 meters per second. If the character controller still advances 1.5 meters in that second, the discrepancy is 1.5 - 1.2 = 0.3 meters per cycle. Contact sliding is a predictable consequence of the mismatch. Changing playback speed, movement speed or the retarget policy are different design choices. This simplified proportional model is not a rule that every retargeter applies automatically.

Validate a small motion set first

Start with the neutral pose, a walk, a turn, a crouch and a reaching motion. Compare source and target side by side. Display chain mappings, root trajectories and end-effector paths. Check whether motion meant for the root was accidentally assigned to the pelvis, or whether an unmatched helper bone changes the chain interpretation.

Inspect planted contacts over time rather than judging one appealing frame. Verify that foot correction and root-motion consumption do not fight the transferred animation. Test both small and large target characters, and inspect hands near props where proportion errors become obvious. Save the mapping and reference-pose configuration as versioned import data so a model reimport does not silently change the result.

Know when an animation needs authoring

A skeleton missing an important joint cannot reproduce every source gesture faithfully. Very different proportions can demand target-specific poses or motion edits even when the mapping is technically valid. Retargeting also does not repair an unsuitable source performance or guarantee collision-free contact with props. Keep visual acceptance criteria for each animation family, and measure runtime retargeting cost separately from selecting or playing the resulting clips.

Sources

• [1] IK Rig Animation Retargeting — Epic Games, Unreal Engine documentation.
• [2] Retargeting 3D Skeletons — Godot Engine contributors.