cot(x)=1/tan(x)
x(u,v)=D*(A*sin(b)*cos(u)+R*cos(v+phi)*cos(u+W)-R*sin(m)*sin(v+phi)*sin(u+W))*exp(u*cot(a))
y(u,v)=(-A*sin(b)*sin(u)-R*cos(v+phi)*sin(u+W)-R*sin(m)*sin(v+phi)*cos(u+W))*exp(u*cot(a))
z(u,v)=(-A*cos(b)+R*sin(v+phi)*cos(m))*exp(u*cot(a))
D=1;a=pi/180*88.9;b=pi/45;phi=11*pi/36;m=pi/180;W=-pi/90;A=22.2;R=1.2
L=20
set xrange[-L:L]
set yrange[-L:L]
set zrange[-1.7*L:0.3*L]
set pm3d explicit
set palette rgb 9,9,1
unset colorbox
set cbrange[-2*L:0]
set parametric
set hidden3d
set ticslevel 0
set size square
set view 65,30,3.6,1
set vrange[-7*pi/4:pi/4]
unset key
unset border
unset xtics
unset ytics
unset ztics
set isosamples 256,18
set urange[-26*pi:0]
set term png transparent xffffff
set out "vs_turritella.png"
set multiplot
splot x(u,v),y(u,v),z(u,v) w pm3d 
splot x(u,v),y(u,v),z(u,v) lt 1
unset multiplot
!convert -geometry 50% -colors 256 vs_turritella.png vs_turritella_s.png
!display vs_turritella_s.png