Perlin Turbulence
•double turbulence(double x, double y, double z)
•{
•  double value = 0;
• double f;
• 
• for(f = MINFREQ; f < MAXFREQ; f *= 2)
• value +=
•     fabs(gnoise(x * f, y * f, z * f) / f);
• return(value);
•}