Problem with Quaternions

I'm seeing some things I don't understand while converting a G8 armature from DAZ Studio to Blender:

I think I finally have understood what orientation is, and how to preserve it in Blender. If I take a G8 and set Left Thigh Bend to -90 in Daz Studio, and take the same exported G8 in Blender and rotate the Left Thigh Bone with rxx-90, the models look identical. So that much is good.

If I switch the rotation to Quaternion (WXYZ) in Blender, it correctly shows (0.707, -0.707, 0, -0) which is exactly what I would expect, cos(90/2) being .707 .

But if in DAZ Studio, I call getLocalRot() on the same bone, it instead gives me w x y z = (0.707107 0.697806 0.0973892 -0.0598513) and so DAZ does not even agree with itself... this is not a rotation of -90 degrees about the local x axis.

Can anyone explain the discrepancy?

Thank you...

 

Comments

  • With a little further experimentation, I find that if, in Blender, I instead rotate the Left Thigh Bend bone around the global x axis with rx-90 (instead of rxx-90), the quaternions values match the Daz Studio values much, much more closely. Could it really be that when you express the rotations as quaternions, the bone's orientation is ignored?

  • OK, this is very wierd, but it appears to be how DS actually behaves:

    If you rotate a bone in the DS GUI with bend twist, and side-to-side, and presumably programmatically with Euler angles, the bone rotates around its oriented axes.

    But if you rotate the bone by calling node->setLocalRot( qRot ); where qRot is a DzQuat, the bone rotates around its unoriented axes.

    More experimentation is warranted, but as soon as I stopped orienting the exported bone in Blender, applying the WXYZ that DS reports to the bone in Blender ( y-up vs z-up notwithstanding, of course) causes the position to match quite perfectly.

     

  • Seven193Seven193 Posts: 945
    edited October 2

    I've came to the conclusion many months ago that Daz's euler-to-quat functions do not give the same results as other 3D modellers.  So, I don't trust any euler-to-quat functions from Daz Studio's interface.  If there's a function to set them directly without conversion, use it.

    Post edited by Seven193 on
  • @Seven193 It may even have been your thread that I remembered that convinced me to not use Euler angles at all, only for orientation.

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