-5 5 setxrange
-4 6 setyrange
25 setxunit
10 10 12 SetCamPos
0 0 0 SetCamView
qplanxy
2 setlinejoin 

GetCamPos setlightsrc
.1 setlinewidth
.6 setgray

/f {
2 dict begin
   /z exch def
   /y exch def
   /x exch def
   x y
#rpn# (x^2 + y^2)/4
end
} def


5.5 [6 18]  newgrillecirculaire
dup .8 solidaffine
{f} solidtransform
dup videsolid
dup [.3 .5] solidputinouthuecolors
drawsolid**