set pm3d depth lighting primary 0.6 specular 0.6
set object rect from sc 0,0 to sc 1,1 behind fc "#114400"
set palette rgb 8,8,7
set samples 128
set isosamples 128
unset border
unset tics
unset colorbox
unset key
set xyplane 0
set view 320,35, 1.8, 
set term pngcairo size 360,360
set out "shell004.png"
set parametric
set urange [-pi:pi]
set vrange [0:2*pi]
c = 300
set xrange [-0.2*c:1.8*c]
set yrange [-c:c]
set zrange [-c:c]
a = 0.9
b = 0.87

x(u,v) = (exp(a*v)*cos(u) + exp(b*v))*cos(v)
y(u,v) = (exp(a*v)*cos(u) + exp(b*v))*sin(v)
z(u,v) = exp(a*v)*sin(u)

splot x(u,v), y(u,v), z(u,v) w pm3d
!display "shell004.png"