load("docdraw.mac");

docdraw3d("figures/draw_parametric3",
	title = "Sea shell",
	xu_grid = 100,
	yv_grid = 25,
	view = [100,20],
	surface_hide = true,
	parametric_surface(0.5*u*cos(u)*(cos(v)+1),
		0.5*u*sin(u)*(cos(v)+1),
		u*sin(v) - ((u+3)/8*%pi)^2 - 20,
		u, 0, 13*%pi, v, -%pi, %pi) )$