Hello,

I've been having some trouble with quaternions and rotations in Unity. I think I must be missing something in the documentation, but I can't seem to figure out what.

I want to rotate 2 unit Vector3s around an axis perpendicular to both of them. This should not change the magnitude of the vector. According to the documentation, quaternions should do the job. But no matter what I do, I don't get the results I want.

Here's an example of what I'm doing:

Debug.Log ("vector1: " + vector1); //start vectors that I want to rotate
Debug.Log ("vector2: " + vector2); 
Debug.Log("angle: " + angle); //rotation angle in radians, arbitrary in this example

Vector3 rotationAxis = Vector3.Cross(vector1, vector2);
Debug.Log ("rotation axis: " + rotationAxis); 

Quaternion rotationQuat = Quaternion.AngleAxis(Mathf.Rad2Deg * startanglelaunch, rotationAxis)
Debug.Log ("rotation quaternion: " + rotationQuat);

Vector3 rotatedVector1 = rotationQuat * vector1;
Vector3 rotatedVector2 = rotationQuat * vector2;

Debug.Log ("rotatedVector1: " + rotatedVector1);
Debug.Log ("rotatedVector2: " + rotatedVector2);

And this is the debug output from this code (slightly edited for clarity):

vector1: (1.0, 0.0, 0.0)
vector2: (0.0, 1.0, 0.0)
angle: -1.792298
rotation axis: (0.0, 0.0, 1.0)
rotation quaternion: (0.0, 0.0, -0.8, 0.6)
rotatedVector1: (-0.2, -1.0, 0.0)
rotatedVector2: (1.0, -0.2, 0.0)

So as you can see, I start with unit vectors, get a perpendicular unit vector with the cross product as expected, but the confusion starts when I build the quaternion. I expected the quaternion to simply rotate the vectors and not change their magnitude, but that's not what happens. I get similar unexpected results no matter how I construct the quaternion.

The Quaternion reference page (http://docs.unity3d.com/Documentation/ScriptReference/Quaternion.html) is very clear and what I am doing here should work. I must be missing something simple, but I've been over it many times and I don't know what I should be doing.

Are negative angles not allowed? Do I also have to somehow specify a point to rotate around, as well as an axis? Is this actually the expected behaviour, and if so what should I be doing? I can't find an answer in the documentation.

Any help is appreciated.