surface 
FHSpiral (
		 float ts = 0,
                       Ks = .5,
		       Kd = .5,
		       Ka = 1,
		       roughness = .1,
                       frequency = 1;
                 color color1 = 1,
                       color2 = color "rgb" (1, 0, 0),
		       specularcolor = 1;
)
{
   point Nf;
   point V = normalize(-I);
   color Ct;
   float ss,tt,msft;

   if (ts == 0) {
      ss = s; tt = t;
   } else {
      ss = 1-t; tt = s;
   }
   msft = mod(ss+frequency*tt, 1);

   Ct = mix(color1, color2,
              smoothstep(.49, .51, msft));
   
   Nf =  faceforward(normalize(N),I);
   Oi=Os;
   Ci=Os*(Ct * (Ka*ambient() + Kd*diffuse(Nf)) 
	  + specularcolor * Ks *specular(Nf, V, roughness)
);
}