Quaternions and Rotations∗ - Iowa State University

3 Quaternion Rotation Operator
How can a quaternion, which lives in R 4 , operate on a vector, which lives in R 3 ? First, we note
that a vector v ∈ R 3 is a pure quaternion whose real part is zero. Let us consider a unit quaternion
q = q 0 + q only. That q 2 0 + ‖q‖2 = 1 implies that there must exist some angle θ such that
cos 2 θ = q 2 0 ,
sin 2 θ = ‖q‖ 2 .
In fact, there exists a unique θ ∈ [0,π] such that cos θ = q 0 and sin θ = ‖q‖. The unit quaternion
can now be written in terms of the angle θ and the unit vector u = q/‖q‖:
q = cos θ + usin θ.
Pure Quaternions
R
3
v
v = 0 + v
R
4
Quaternions
Using the unit quaternion q we define an operator on vectors v ∈ R 3 :
L q (v) = qvq ∗
= (q 2 0 − ‖q‖ 2 )v + 2(q · v)q + 2q 0 (q × v). (3)
Here we make two observations. First, the quaternion operator (3) does not change the length of
the vector v for
‖L q (v)‖
= ‖qvq ∗ ‖
= |q| · ‖v‖ · |q ∗ |
= ‖v‖.
Second, the direction of v, if along q, is left unchanged by the operator L q . To verify this, we let
v = kq and have
qvq ∗
= q(kq)q ∗
= (q0 2 − ‖q‖ 2 )(kq) + 2(q · kq)q + 2q 0 (q × kq)
= k(q0 2 + ‖q‖2 )q
= kq.
The two observations make us guess that the operator L q acts like a rotation about q. This is made
precise by the next theorem.
Before proceeding with the theorem, we remark that the operator L q is linear over R 3 . For any
two vectors v 1 ,v 2 ∈ R 3 and any a 1 ,a 2 ∈ R we can show that
L q (a 1 v 1 + a 2 v 2 ) = a 1 L q (v 1 ) + a 2 L q (v 2 ).
4