There are $2$ rectangles $R_a$ and $R_b$ in space.
$R_a$ can be defined by $x_{ai}, y_{ai}, z_{ai}$ for $i = 1...4$.
$R_b$ can be defined by $x_{bi}, y_{bi}, z_{bi}$ for $i = 1...4$.
How can I create a function $f(x_a, y_a, z_a)$ that maps a point inside $R_a$ to its corresponding point in $R_b$?
$R_a$ and $R_b$ have the same aspect ratio
$x_{ai}$ corresponds to $x_{bi}$
$R_a$ and $R_b$ are randomly placed in the world and may not have the same size