prox passo simulare e toccare erba virtuale
$\phi (\theta )=\frac{\pi }{2}{e}^{-\theta /8\pi }$

$X(\theta )=1-\frac{1}{2}{\left(\frac{5}{4}{\left(1-\frac{\theta \mathrm{mod}2\pi }{\pi }\right)}^2-\frac{1}{4}\right)}^2$

$(x,y,z)=\left(r\sin \theta ,r\cos \theta ,X(x\cos \phi -y\sin \phi )\right)$