# Showing the Correctness of Quaternion Rotation


## Quaternion Product

As a stepping stone, a formula for the product of two quaternions is necessary. The product of two quaternions is \begin{align*} (a + b \mq{i} + c\mq{j} + d\mq{k})(e + f \mq{i} + g\mq{j} +h\mq{k})& =\\ &(ae + af\mq{i} + ag\mq{j} + ah\mq{k}) + \\ &(be\mq{i} +bf \mq{i}\mq{i} + bg \mq{i}\mq{j} + bh \mq{i}\mq{k} + \\ &(ce\mq{j} +cf \mq{j}\mq{i} + cg \mq{j}\mq{j} + ch \mq{j}\mq{k} + \\ &(de\mq{k} +df \mq{k}\mq{i} + dg \mq{k}\mq{j} + dh \mq{k}\mq{k} \\ \end{align*} It is possible to simplify $\mq{i}\mq{i}, \mq{i}\mq{j}, \mq{i}\mq{k},\dots$. Recall the famous fact $\mq{i}\mq{i} = \mq{j}\mq{j} = \mq{k}\mq{k} = \mq{i}\mq{j}\mq{k} = -1$. So the simplifications for $\mq{i}\mq{i}$, $\mq{j}\mq{j}$, and $\mq{k}\mq{k}$ are already obvious. What about $\mq{i}\mq{j}$? We have \begin{align*} \mq{i}\mq{j}\mq{k} &= -1 \\ \mq{i}\mq{j}\mq{k}\mq{k} &= -\mq{k} \\ -\mq{i}\mq{j} &= -\mq{k} \\ \mq{i}\mq{j} &= \mq{k}. \\ \end{align*} Similarly, \begin{align*} \mq{i}\mq{j}\mq{k} &= -1 \\ \mq{i}\mq{i}\mq{j}\mq{k} &= -\mq{i} \\ \mq{j}\mq{k} &= \mq{i}. \\ \end{align*} Using the above fact, it follows that \begin{align*} \mq{i} &= \mq{j}\mq{k} \\ \mq{j}\mq{i} &= \mq{j}\mq{j}\mq{k} \\ \mq{j}\mq{i} &= -\mq{k}. \\ \end{align*} In particular, note that $\mq{i}\mq{j} \neq \mq{j}\mq{i}$. It is left as an exercise to the reader to prove the remaining three identities: $\mq{k}\mq{j} = -\mq{i}$, $\mq{i}\mq{k} = -\mq{j}$, $\mq{k}\mq{i} = \mq{j}$. With these identities in our toolbox, the quaternion product can be significantly simplified \begin{align*} &(ae + af\mq{i} + ag\mq{j} + ah\mq{k}) + \\ &(be\mq{i} +bf \mq{i}\mq{i} + bg \mq{i}\mq{j} + bh \mq{i}\mq{k} + \\ &(ce\mq{j} +cf \mq{j}\mq{i} + cg \mq{j}\mq{j} + ch \mq{j}\mq{k} + \\ &(de\mq{k} +df \mq{k}\mq{i} + dg \mq{k}\mq{j} + dh \mq{k}\mq{k} = \\ &ae - (bf + cg + dh) + \\ &(be + af + ch - dg)\mq{i} + \\ &(ce + ag + df - bh)\mq{j} + \\ &(de + ah + bg - cf)\mq{k}. \\ \end{align*} It is useful to consider a quaternion to be the sum of a scalar and a vector. For instance, the quaternion $a + b \mq{i} + c\mq{j} + d\mq{k}$ is simply the scalar $a$ plus the three-dimensional vector $b\mq{i} + c\mq{j} + d\mq{k}$. This notation means that $b$ is the $x$-component of the vector, $c$ is the $y$-component, and so on. We shall denote that vector $\vec{v} = b\mq{i} + c\mq{j} + d\mq{k}$, so that we can write $a + b \mq{i} + c\mq{j} + d\mq{k} = a + \vec{v}$. Similarly, we write $e + f \mq{i} + g\mq{j} +h\mq{k} = e + \vec{w}$. Now observe that \begin{align*} &ae - (bf + cg + dh) + \\ &(be + af + ch - dg)\mq{i} + \\ &(ce + ag + df - bh)\mq{j} + \\ &(de + ah + bg - cf)\mq{k} = \\ &ae - (bf + cg + dh) + \begin{bmatrix} be + af + ch - dg \\ ce + ag + df - bh \\ de + ah + bg - cf \\ \end{bmatrix} = \\ &ae - (bf + cg + dh) + e\begin{bmatrix} b \\ c \\ d \end{bmatrix} + a\begin{bmatrix} f \\ g \\ h \end{bmatrix} + \begin{bmatrix} ch - dg \\ df - bh \\ bg - cf \end{bmatrix} = ae - \vec{v} \cdot \vec{w} + e\vec{v} + a \vec{w} + \vec{v}\times\vec{w} \end{align*} Thus, if a quaternion is viewed as the sum of a scalar and a vector, then an elegant result emerges: $$(a + \vec{v})(e + \vec{w}) = (ae - \vec{v} \cdot \vec{w}) + (e\vec{v} + a \vec{w} + \vec{v}\times\vec{w}) \label{eq:quatprod}$$ So the scalar part of the quaternion product is $(ae - \vec{v} \cdot \vec{w})$, and the vector part is $(e\vec{v} + a \vec{w} + \vec{v}\times\vec{w})$. As expected, the product of two quaternions yields another quaternion. This formula shall be applied extensively in the next section.

## Simplifying Quaternion Rotation

We start with the formula for quaternion rotation \begin{align*} \mq{q} \vec{v} \mq{q}^{-1} = \left(\cos\left(\frac{\theta}{2}\right) + \vec{u}\sin\left(\frac{\theta}{2}\right)\right) \vec{v} \left(\cos\left(\frac{\theta}{2}\right) - \vec{u}\sin\left(\frac{\theta}{2}\right)\right) \end{align*} Equation $\eqref{eq:quatprod}$ is utilized twice to expand the product: \begin{align*} \mq{q} \vec{v} \mq{q}^{-1} = &\left(\cos\left(\frac{\theta}{2}\right) + \vec{u}\sin\left(\frac{\theta}{2}\right)\right) \vec{v} \left(\cos\left(\frac{\theta}{2}\right) - \vec{u}\sin\left(\frac{\theta}{2}\right)\right) &= \\ &\left(\left(-\sin\thetatwo{} \uv\cdot\vv \right) + \cos\thetatwo{}\vv + \sin\thetatwo{}(\uv\times\vv) \right) \left(\cos\left(\frac{\theta}{2}\right) - \vec{u}\sin\left(\frac{\theta}{2}\right)\right) &= \\ &\color{zerocol}{-\sin\thetatwo{}\cos\thetatwo{} \uv \cdot \vv + \left( \cos\thetatwo{}\vv + \sin\thetatwo{}(\uv\times\vv)\right) \cdot \left(\uv\sin\thetatwo{}\right)+} \\ &\color{black}{}\left(-\sin\thetatwo{}\uv\cdot\vv\right)\left(-\uv\sin\thetatwo{}\right)+ \\ &\cos\thetatwo{}\left(\cos\thetatwo{}\vv + \sin\thetatwo(\uv\times\vv)\right) + \\ &\left(\cos\thetatwo{}\vv+ \sin\thetatwo{}(\uv \times \vv)\right) \times \left(-\uv \sin\thetatwo{}\right) \end{align*} Observe that the purple part ends up evaluating to zero. This is because the dot product is distributive under addition($a \cdot(b +c) = a \cdot b + a \cdot c$), and since $(\uv \times \vv) \cdot \uv = 0$. It is easy to see why: the vector $\uv \times \vv$ is a vector perpendicular to both $\uv$ and $\vv$. But then $(\uv \times \vv) \cdot \uv$ is simply the dot product of two vectors that are perpendicular. Ergo, this dot product is zero. The geometric situation is illustrated in the below image.

So we obtain the simplified expression: \begin{align*} &\sin^2\thetatwo{} (\uv \cdot \vv)\uv + \\ &\cos^2\thetatwo{}\vv + \sin\thetatwo\cos\thetatwo(\uv\times\vv) \\ &+\sin\thetatwo{}\cos\thetatwo{} (\uv \times \vv) - \sin^2\thetatwo{} (\uv\times\vv) \times \uv \end{align*} Where we have applied the distributive property of the cross product: $(\vec{a} + \vec{b})\times\vec{c} = \vec{a}\times\vec{c} + \vec{b}\times\vec{c}$. From the double angle formula $\sin(x) = 2\sin\left(\frac{x}{2}\right)\cos\left(\frac{x}{2}\right)$, it simplifies to \begin{align*} &\sin^2\thetatwo{} (\uv \cdot \vv)\uv + \cos^2\thetatwo \vv + \sin(\theta)(\uv \times \vv) -\sin^2\thetatwo{} (\uv\times\vv) \times \uv \end{align*}

Some more simplifications are then performed using the vector triple product($\vec{a}\times(\vec{b}\times\vec{c}) = \vec{b}(\vec{a}\cdot\vec{c}) - \vec{c}(\vec{a}\cdot\vec{b})$) and a couple more double angle formulas. This is not shown here, but left as a small exercise to the reader. After the simplifications are done, what remains is \begin{align*} \vv\cos(\theta) + (\uv\times\vv)\sin(\theta) + (\uv\cdot\vv)\uv(1 - \cos(\theta)). \end{align*} But this is simply equation $\eqref{eq:rodri}$. And with that, the proof is done. It has been shown that quaternion rotation is equivalent to Rodrigues' formula, and this means that quaternion rotation does indeed do what we expect it to.