requirements A perspective function is:
\begin{equation} P\left(x \dots t\right) = \frac{x}{t} \end{equation}
P: \mathbb{R}^{n+1} \to \mathbb{R}^{n}.
\begin{equation} \text{persp}\left(a,b\right) = b f\left(\frac{a}{b}\right) \end{equation}
requirements A perspective function is:
P: \mathbb{R}^{n+1} \to \mathbb{R}^{n}.