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Thread: Collada skeleton hierarchy

  1. #1

    Collada skeleton hierarchy

    Hello everyone. The collada skinning equation is the following:

    v +={[(v * BSM)*IBMi*JMi]* JW}

    • n : The number of joints that influence vertex v
    • BSM: Bind-shape matrix
    • IBMi: Inverse bind-pose matrix of joint i
    • JMi: Transformation matrix of joint i
    • JW: Weight of the influence of joint i on vertex v

    So far I understand. What I do not understand is how this applies when a particular bone is son of another bone.

    Should I do this?


    v
    +={[(v *ParentBSM* BSM)*ParentIBMi*IBMi*ParentJMi*JMi]*ParentJW* JW}

    Or, what do I have to do so that the transformations of the children are done well in relation to the parents?

    Thank you very much for your help.

  2. #2
    Senior Member
    Join Date
    Mar 2016
    Posts
    119
    If I'm not mistaken there is code here (https://sourceforge.net/p/collada-do...Controller.cpp) where I've implemented this for arbitrarily parented skeletons and morph targets.

    I think normally you roll IBMi and JMi together somehow, so they are one matrix. Then you multiply as a chain just like with static geometry. Depending on how things are set up I think IBMi may be optional. BSM is probably optional too. This is not really a COLLADA problem. But there you are

    P.S. These are all conveniences for artists. An efficient animation isn't parented, and doesn't require bind-poses, etc. You want to get rid of all those things if your application is not arts software.

    EDITED: In that link... don't read the following comment... It's driving me mad now trying to think of what I had to say!
    //COLLADA uses the concept of a "base mesh" but it
    //is really no different from another target. Most
    //systems won't make this distinction, since there
    //is a need to blend key frames when all are equal.
    Last edited by Mick P.; 01-11-2019 at 08:49 PM.
    This account can Moderator spam in the COLLADA forum.
    ColladaDOM 3 (COLLADA-DOM 2016)
    https://sourceforge.net/p/collada-do...ussion/531263/

  3. #3
    I am going to try. Thank you very much for your response, friend

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