quats - difference

By adjust, do you mean concatenate? If so, then to do quaternion concatenation, you multiply the the quaternions together.

no. lets say i have an angle of 140 degrees, but its 90 degrees out, i would do 140 - 90 = 50 degrees.
similarly i have a quat thats 90 degrees out, can i just do a difference?

i've tested it and it seems okay, might need to normalise it though after as i get some stretching occuring in my animations :-/

edit: normalizing seems to change the angle to be smaller than expected but stops the stretching (ie. non-normalized gives correct angle, but results in stretching, normalize gives wrong angle, but there is no stretching)

[Edited by - supagu on December 21, 2004 8:23:51 PM]

Quaternion subtraction isn't what you want here (it's not a particularly useful operation in any case).

I would say that you need either quaternion multiplication (as SiCrane suggested), or slerp, depending on what you're trying to do.

Let's say you have a quaternion that represents a 45 degrees rotation about the y axis, and you want to rotate *from that orientation* by an additional 15 degrees. To do so, construct a quaternion representing a 15 degree rotation about the y axis, and multiply it by your original quaternion. The resulting quaternion should represent a rotation of 60 degrees about the y axis.

On the other hand, say you have two quaternions representing rotations of 45 and 90 degrees, respectively, about the y axis. You can get from one to the other, either instantaneously or in increments, using an algorithm known as 'slerp'. If you're not familiar with this algorithm, a google search or search of this forum's archives should help you out.

I hope that helps a little. If you need any further clarification, just ask.