/* gcturb.sl
 *
 * shader for viewing gradual, clamped turbulence
 *
 */
#include "rmannotes.sl"

surface gcturb(float turbfreq = 2)
{
  point PP;
  float width, cutoff, fade, f, turb;
  float maxfreq = 16;

  PP = transform("shader", P) * turbfreq;

  width = filterwidth_point(PP);
  cutoff = clamp(0.5 / width, 0, maxfreq);

  turb = 0;
  for (f = 1; f < 0.5 * cutoff; f *= 2) 
    turb += abs(snoise(PP * f)) / f;

  fade = clamp(2 * (cutoff - f) / cutoff, 0, 1);
  turb += fade * abs(snoise(PP * f)) / f;

  Ci = turb;
}