tinyusdz/doc/skinning.md

Skinnig in usdSkel

https://graphics.pixar.com/usd/dev/api/_usd_skel__intro.html

Evaluation of skinning

Terminology

• bindTransforms : World matrices of each joint
• restTransforms : Local matrices of each joint. Need to evaluate in bone(joint) hierarchy. Used when there is no corresponding transform from SkelAnimation
• SkelAnimation : (local?) matrices for each joint
• Sparse : When SkelAnimation supplies(maps?) animation transform to subset of joints
• Non-Sparse : When SkelAnimation supplies(maps?) animation transform to all of joints

restTransforms could be optional when SkelAnimation supply transforms for all joints, but it seems usdview and Houdini does not allow omitting restTransforms(restTransforms is not automatically calculated from bindTransforms)

Also, without bindTransforms skinning does not work well(it is not automatically caculuated from restTransforms)

So, both bindTransforms and restTransforms must exist in USD.

Skinning matrix

skinM = inv(jointWorldSpaceBindTransform) x jointSkelSpaceTransform

jointSkelSpaceTransform = jointLocalSpaceTransform * parentJointSkelSpaceTransform

//

jointWorldSpaceTransform = jointLocalSpaceTransform *
    parentJointSkelSpaceTransform * skelLocalToWorldTransform

Skinning a point

# geomBindTransform = `primvars:skel:geomBindTransform` 

skelSpacePoint = geomBindTransform.Transform(localSpacePoint)
p = (0,0,0)
for jointIndex,jointWeight in jointInfluencesForPoint:
    p += skinningTransforms[jointIndex].Transform(skelSpacePoint)*jointWeight

EoL.