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For a given animation with the set of 3D coordinates of every joint and corresponding global rotations quaternions Q(X, Y, Z, W) in right-hand coordinates (Y-up, X-left, Z-forward) - pic. I want to change axis X to be consistent with Unity (Y-up, X-right, Z-forward).

enter image description here

For coordinates it is obvoius: X' = -X and works as expected.

For rotations in quaternions assign I probed different options from other sources:

  • X, -Y, -Z, W
  • -X, Y, Z, W
  • -X, Y, Z, -W

So, what math behind flipping X axis and how can I achieve this transformation?

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1 Answer 1

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X, -Y, -Z, W <-- This one is correct.

First you replace the axis you have (X) with the axis you want (-X), giving you:

-X, Y, Z, W

Then, since you've flipped an odd number of axes, you're in a mirror image universe with reversed handedness, so the sign of the angle needs to change too. That means negating X, Y, and Z, giving the final result:

X, -Y, -Z, W

I've shown these steps in a few prior answers, so remember to do a search first to find solutions even faster than writing a post and waiting for a reply.

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