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I'm working on a Roll animation for my character and it isn't behaving porperly. On frame 12 the "W" component of the "hip" bone becomes negative (using quaternions). Then between the frame 20 and 32 it tries to interpolate towards a positive number, but it flips around the wrong direction. How do I fix this? If that is not possible then what is the best way to handle this kind of animation?

P.S. I'm using version 2.77.

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Your issue comes from the quaternion rotation of the hip bones flipping in the W value, as you guessed. The fix here is to negate the W value on frame 40 so that the hip doesn't have to do a full rotation. Setting the Rotation keyframe to [-1, 0, 0, 0] on frame 40 fixes the problem because the W value on frame 20 is -0.961.

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  • $\begingroup$ Negating the W causes the right leg to flip around. This is a very weird problem, because quaternions should not behave this way. Blender should know how to rotate it properly. Isn't this a bug? $\endgroup$
    – Cássio Eudardo
    Jul 26, 2016 at 22:31
  • $\begingroup$ I'm not certain what's happening internally, but it looks like negating the W rotation for both of the foot IK targets fixes the problem as well. $\endgroup$
    – Poyo
    Jul 26, 2016 at 22:38

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