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I have three unit quaternions,

$$q_1=(1,0,0,0)\\ q_2=(0.9623,0.2578,0.0226,0.0842)\\ q_3=(0.9353,0.2273,0.2708,0.0146)$$

Now, quaternion q, if represented by angle axis convention, will be $q=cos(\theta/2)+sin(\theta/2)\cdot\hat n$, So each quaternion will have its separate angle and axis. Now when we multiply two quaternions say from $q_1$ to $q_2$ they will rotate about an axis and at a certain angle. How are the rotation axis and angle between them related to the individual quaternion axis and angle?

If I want to generate quaternions $q$ rotating from $q_2$ to $q_3$ using spherical interpolation(slerp), $q\in[q_2,q_3]$, What will be the axis of rotation? Will it change for each $q$? Can anyone generate 5 quaternions rotating from $q_2$ to $q_3$ using slerp?

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Complex numbers are couples, they have a real part and an imaginay part. Quaternions are hypercomplex numbers, they are also couples of a real part $w$ and an imaginary vector part $b = (x i, y j, z k)$.

Quaternions are tipically parameterized by two parameters: rotation angle $\theta$ and rotation axis (unit vector) $n$. The real part is $w = \cos(\frac{\theta}{2})$ while the vector part is $b = \sin(\frac{\theta}{2}) n$.

Given two qaternions $q_1 = (w_1, b_1)$ and $q_2 = (w_2, b_2)$ their multiplication in vector form is:

$$q_1 q_2 = (w_1, b_1) (w_2,b_2)$$

$$q_1 q_2 = (w_1 w_2 - b_1 \cdot b_2, w_2 b_1 + w_1 b_2 + b_1 \times b_2)$$

After multiplication the new rotation axis is:

$$b_3 = (w_2 b_1 + w_1 b_2 + b_1 \times b_2)$$

$b_3$ has one component in the plane spanned by $b_1$ and $b_2$ ($w_2 b_1 + w_1 b_2$) and one component in the direction orthogonal to that plane ($b_1 \times b_2$).

The case of the SLERP ia a little bit different. Looking at the SLERP formulae:

$$q(t) = \frac{1}{\sin(\frac{\theta}{2})}( \sin((1-t)\frac{\theta}{2}) q_1 + \sin(t \frac{\theta}{2}) q_2)$$

Or

$$q(t) = \frac{1}{\sin(\frac{\theta}{2})}( \sin((1-t)\frac{\theta}{2}) (w_1, b_1) + \sin(t \frac{\theta}{2}) (w_2, b_2))$$

Where $\theta = 2 \cos^{-1}(n_1 \cdot n_2)$ is the angle formed between the axis of rotation of $q_1$ and $q_2$. So the interpolated axis of rotation is:

$$b_t = \frac{1}{\sin(\frac{\theta}{2})}( \sin((1-t)\frac{\theta}{2}) b_1 + \sin(t \frac{\theta}{2}) b_2)$$

Which is in the plane spanned by $b_1$ and $b_2$ (in between $b_1$ and $b_2$ to be precise), the expected behavior of SLERP.

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  • $\begingroup$ So the axis of rotation and magnitude of bt will change with varying t? $\endgroup$ – Rohan Deshpande Jul 22 '19 at 4:32
  • $\begingroup$ Yes, the axis of rotation and the magnitude of $b_t$ are spherically interpolated values except in the cases when $t = 0$ and $t = 1$. $\endgroup$ – Mauricio Cele Lopez Belon Jul 22 '19 at 5:20
  • $\begingroup$ Also, the axis of rotation won't change in any case if $b_1$ and $b_2$ are collinear. In that case only the angle of rotation will change. $\endgroup$ – Mauricio Cele Lopez Belon Jul 22 '19 at 5:30

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