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Math & Rigging · 40 min
Skinning: Linear Blend vs Dual Quaternion
IntermediateNeeds: MatricesNeeds: Quaternions
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You will be able to
- Explain the candy-wrapper artefact mathematically.
- Choose between LBS and DQS for a given deformation.
- Diagnose weighting problems from the shape of the artefact.
The concept
Linear blend skinning averages transformation matrices, and the average of two rotations is not a rotation — it shrinks. That is the candy-wrapper collapse on a twisting wrist. Dual quaternion skinning blends in a space where the average stays rigid, fixing volume loss but introducing bulging on bends. Neither is universally correct; production rigs mix them and add corrective shapes.
In your engine
Quality setting selects 1/2/4 bone weights; DQS requires a custom skinning path or Compute.
Practice (4)
Concept check
The candy-wrapper artefact appears because:
Predict the result
Switching a twisting forearm from LBS to DQS most likely:
Tune the value
Weights on a vertex are 0.6, 0.3 and 0.2. What must they sum to for correct skinning?
Find the bug
A shoulder deforms fine in the bind pose but tears at 90° abduction. Best first check?
Knowledge check
01LBS blends:
02DQS main artefact is:
03Vertex weights must:
04Twist joints exist to:
Build challenge
Rig a forearm with and without a twist joint, and render a 180° twist under both LBS and DQS. Produce a 2x2 comparison and name the artefact in each quadrant.
SELF-CHECK RUBRIC