Preserve length, preserve cross product $\Leftrightarrow$ rigid body motion

$\hat{u} \in so(3)$

• $g(v) = |v|$, length preserved
• $g(u) \times g(v) = g(u \times v)$, cross product preserved

$\left<u,v\right> = \frac{1}{4}(|u+v|^2-|u-v|^2)$

$\Rightarrow \left<g(u), g(v) \times g(w)\right> = \left<u, v\times w\right> $, volume preserving

Representation of Rigid-Body Motion

The RBM preserves length and cross product.

$r_i = g_t(e_i)$

$R = (r_1, r_2, r_3)$

$r_i^Tr_j = g_t(e_i)^T g_t(e_j) = e_i^T e_j = \delta_{ij}$

$\quad r_1 \times r_2 = r_3 \Rightarrow \det(R) =1$

$R^T R = RR^T = I$

$R$ is in the Special Orthogonal Group $SO(3)$

Rotation nasty to be represented

$R_1 + R_2 \notin SO(3)$, not a linear space!

$R_1 \cdot R_2 \in SO(3)$, not a linear space!

R has 9 elements, choice is not independent. You have to guarantee: orthogonal and det(R) = 1

Infinitesimal rotation