#include "rmannotes.sl"

surface turb (float freq=8)
{

/* DECLARE VARIABLES */
color surface_color, layer_color;
color surface_opac, layer_opac;
float turb, f;

/* ASSIGN VALUES TO VARIABLES */
surface_color = Cs;
surface_opac = Os;
turb = 0;

/* CALCULATE TURBULENCE */
for (f = 1; f < freq; f *= 2) 
  turb += abs(snoise(P * f + 2*PI)) / f;

turb = pow((1 - turb), 8);

/* OUTPUT */
Oi = surface_opac * turb;
Ci = surface_color * turb * surface_opac;
}