\begin{pspicture*}(-3,-3)(4.5,5)
\psset{lightsrc=viewpoint,viewpoint=5 -5 3,Decran=10}
\defFunction{G}(t)
   {t}
   {t dup mul .25 mul}
   {-2}
\psframe(-3,-3)(4.5,5)
\psSolid[object=cone,
   r=1, h=1](-2,10,0)
\psSolid[object=cone,
   function=G,
   range=-3 2,
   ngrid=3 10,
   origine=0 0 0, %% le sommet du cone
]
\end{pspicture*}