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---
title: Representations of 3D Motion
categories: lectures 3D mathematics
---
# Representation of Rotations
# Homogenous Co-ordinates
## Six degrees of Freedom
+ Translation - add $T=[y_1,y_2,y_3]$
+ Rotation - multiply by $\exp(\hat{[\omega_1,\omega_2,\omega_3]})$
+ $x\mapsto xR+T$ is affine, not linear
## Points in Homogenous Co-ordinates
+ Point $\textbf{X}=[X_1,X_2,X_3]^\mathrm{T}\in\mathbb{R}^3$
+ Embed in $\mathbb{R}^4$ as
$\mathbf{\tilde X}=[X_1,X_2,X_3,1]^\mathrm{T}\in\mathbb{R}^4$
+ Vector $\vec{pq}$ is represented as
$$\mathbf{\tilde X}(q)-\mathbf{\tilde X}(p) =
\begin{bmatrix} \mathbf{ X}(q) \\ 1 \end{bmatrix}
- \begin{bmatrix} \mathbf{ X}(p) \\ 1 \end{bmatrix}
=
\begin{bmatrix} \mathbf{X}(q) - \mathbf{X}(p) \\ 0 \end{bmatrix}$$
+ In homogenous co-ordinates,
- points have 1 in last position
- vectors have 0 in last position
+ Arithmetics
+ Point + Point is undefined
+ Vector + Vector is a Vector
+ Point + Vector is a Point
## Rotation
Let $R$ be a $3\times3$ rotation matrix.
$$
R\cot\vec{x}=
R
\cdot
\begin{bmatrix} x\\y\\z \end{bmatrix}
=
\begin{bmatrix} x'\\y'\\z' \end{bmatrix}
$$
$$
\begin{bmatrix}
R & 0 \\
0 & 1
\end{bmatrix}
\cdot
\begin{bmatrix} x\\y\\z\\1 \end{bmatrix}
=
\begin{bmatrix} x'\\y'\\z'\\1 \end{bmatrix}
$$
## Arbitrary motion
What happens if we change some of the zeroes?
$$
\begin{bmatrix}
R & \vec{t} \\
0 & 1
\end{bmatrix}
\cdot
\begin{bmatrix} x\\y\\z\\1 \end{bmatrix}
=
\begin{bmatrix} x'\\y'\\z'\\0 \end{bmatrix}
+
\begin{bmatrix} \vec{t}\\1 \end{bmatrix}
=R\vec{x}+\vec{t}
$$
We have rotated and translated!