# Are there two conflicting definitions of conversion between quaternion and axis-angle representation?

I have some trouble understanding (in my opinion) two conflicting definitions of the quaternion logarithm or the conversion between quaternions and axis-angle representation.

In the second paragraph of this Wikipedia article, a rotation / orientation quaternion is defined as

$$[a, b, c, d] = [\cos(\theta/2),e_x \sin(\theta/2),e_y \sin(\theta/2),e_z \sin(\theta/2)]$$

representing a rotation of $$\theta$$ about a unit axis $$[e_x, e_y, e_z]$$ (changed notation). If we want to obtain the axis-angle representation from a quaternion, we can determine the angle as $$\theta = 2\arccos(a)$$.

I also often see this definition of the quaternion logarithm:

$$\log([a, b, c, d]) = \log ([\cos \theta, e \sin \theta]) \equiv [0, \theta e]$$

Which does not imply that there is a factor 2 involved in the conversion. You would compute $$\theta = \arccos(a)$$.

Now, the definition in this paper (Equation 19, page 4) is similar and is used to obtain an axis-angle representation from the quaternion. However, to obtain angular velocity from the logarithm of the difference of two quaternions, they multiply the logarithm by 2 again:

$$\omega = 2 \log(q_1 * \overline{q}_2)$$

So it seems like you could as well define

$$\log([a, b, c, d]) = \log ([\cos \theta / 2, e \sin \theta / 2]) \equiv [0, \theta e]$$

to obtain $$\theta = 2\arccos(a)$$ and then define

$$\omega = \log(q_1 * \overline{q}_2).$$

Are these actually two conflicting definitions of this conversion (or the quaternion representation of rotations)? What is the reason and how can this be resolved?

It's only a notation issue. Some use $$\alpha = \theta / 2$$, with $$\theta$$ as the actual rotation angle, i.e. $$\mathbf{q} = \cos\left(\frac{\theta}{2}\right) + \mathbf{n}\sin\left(\frac{\theta}{2}\right) = \cos \alpha + \mathbf{n}\sin \alpha$$
Indeed, 3D Math Primer for Graphics and Game Development – the reference used in the linked to other definition source – explicitly states $$\alpha = \theta / 2$$ with $$\theta$$ being the angle of rotation, before equation 10.15, $$\log \mathbf{q} = \log\left(\left[\begin{matrix}\cos\alpha & \mathbf{n}\sin\alpha]\end{matrix}\right]\right) \equiv \left[\begin{matrix} 0 & \alpha\mathbf{n}\end{matrix}\right]$$
• @alfa: Well, I meant in the literal sense: the $\alpha$ form has fewer terms, thus it is the simpler form. Oct 28, 2020 at 21:58