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

10 10 10 setlightsrc
.1 setlinewidth
.6 setgray
/f {
2 dict begin
   /y exch def
   /x exch def
#rpn# (x^2 + y^2)/4
end
} def

-4 4 -4 4 [.5 .5] {f} newsurface
dup [.3 .5] solidputinouthuecolors
drawsolid**