import graph3; import palette; size3(10cm); currentprojection=orthographic(5,4,2); real r(real t) {return 3exp(-0.1*t);} real x(real t) {return r(t)*cos(t);} real y(real t) {return r(t)*sin(t);} real z(real t) {return t;} path3 p=graph(x,y,z,0,6*pi,50,operator ..); tube T=tube(p,2); surface s=T.s; s.colors(palette(s.map(zpart),BWRainbow())); draw(s); draw(T.center,thin());