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# Learning and Proof

# Reading

- Steinert, M., & Leifer, L. J. (2012). ‘Finding One’s Way’: Re-Discovering a Hunter-Gatherer Model based on Wayfaring. International Journal of Engineering Education, 28(2), 251.
- Ma 2004:Ch 5.1

# Learning and Study Technique

## Constructivist Learning Theory

## Learning is Design

## The Hunter-Gatherer Model

# Relative Pose

## Repetition

### 3D Motion

- Rotation by angle \(\theta\) around the vector \(\omega\) is given by \(R=e^{\hat\omega\theta}\) assuming \(\omega\) has unit length.

**Rodrigues’ formula** (2.16)

\[e^{\hat\omega} = I + \frac{\hat\omega}{||\omega||}\sin(||\omega||) + \frac{\hat\omega^2}{||\omega||^2}(1-\cos(||\omega||))\]

See Angular Motion for a more comprehensive summary.

### Basic Result on Relative Pose

- Theorem 5.5

\[E = U\mathsf{diag}\{\sigma,\sigma,0\}V^T,\] where \(U,V\in\mathsf{SO}(3)\)

- Tricky proof. Do not spend too much time on this.

\[ \begin{cases} (\hat T_1,R_1) &= (UR_Z(+\frac\pi2)\Sigma U^T, UR_Z(+\frac\pi2)V^T) \\ (\hat T_2,R_2) &= (UR_Z(-\frac\pi2)\Sigma U^T, UR_Z(-\frac\pi2)V^T) \end{cases} \] where \[R_Z(+\frac\pi2) = \begin{bmatrix} 0 & -1 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 1 \end{bmatrix} \] is a rotation by \(\pi/2\) radians around the \(z\)-axis.

- Note that there are two solutions from one \(U\Sigma V^T\) decomposition.
- Are there more solutions?

There exist exactly two relative poses \((R,T)\) with \(R\in\mathsf{SO}(3)\) and \(T\in\mathbb{R}^3\) corresponding to a nonzero essential matrix \(E\in\mathcal{E}\) Theorem 5.7

## Proofs and Understanding

### Theorem 5.7

**Demo**read the proof (debrief?)

### Lemma 5.6

If \(\hat T\) and \(\hat TR\) are both skew-symmetric for \(R\in\mathrm{SO}(3)\), then \(R\) is a rotation by angle \(\pi\) around \(T\).

**Demo**read the proof (debrief?)- Skew-symmetry gives \((\hat TR)^T=-\hat TR\)
- We also have \((\hat TR)^T=R^T\hat T^T=-R^T\hat T\)
- Hence \(\hat TR = R^T\hat T\),
- and since \(R^T=R^{-1}\), we have \[R\hat TR=\hat T\]
- Write \(R=e^{\hat\omega\theta}\) for some \(\omega\) of unit length and some \(\theta\), to get \[e^{\hat\omega\theta}\hat Te^{\hat\omega\theta}=\hat T\]
multiply by \(\omega\) \[e^{\hat\omega\theta}\hat Te^{\hat\omega\theta}\omega=\hat T\omega\] This represents a stationary rotation of the vector \(\hat T\omega\).

Note that \(\omega\) is stationary under rotation by \(R\), and hence it is an eigenvector associated with eigenvalue 1. Furthermore, it is the only such eigenvector, and \(\hat T\omega\) cannot be such. Hence \(\hat T\omega=T\times\omega=0\). This is only possible if \(T\sim\omega\), and since \(\omega\) has unit length, we get \[\omega = \pm\frac{T}{||T||}\]

We now know that \(R\) has to be a rotation around \(T\), and therefore \(R\) and \(T\) commute. This can be checked in Rodrigues’ formula (Theorem 2.9).

Hence \(R^2\hat T = \hat T\). This looks like two half-round rotations to get back to start. If \(\hat T\) had been a vector or a matrix of full rank, we would have been done. However, with the skew-symmetric \(\hat T\) there is a little more fiddling.