How is a bone’s roll defined

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FORUMS FORUMS Rigging How is a bone’s roll defined

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      I have a number of bones defined as positions + rotation quaternions.

      In blender I understand bones are defined as a head, a tail and a roll. With the head -> tail vector being the localY axis

      Its easy to get the head (position) and the tail (position + localY * arbitraryDistance) but it’s harder to get the roll. The roll is obviously around the localY axis, but beyond that I’m not sure how it’s defined. I have been finding the localZ from the quaternion and then fiddling with the roll until it matches but that has been painful, and doesn’t suit itself to the programmatic workflow I want.


      How can I get the blender bone roll from a Quaternion? (Or from a Local Z axis, which I can also calculate from the Quaternion)

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