volume mgfog (float mindistance = 0, maxdistance = 1; color background = 0) {
  float d;

  d = clamp ((length(I) - mindistance) / (maxdistance - mindistance), 0, 1);
  Ci = mix (Ci, background, d);
  Oi = mix(Oi, color(1,1,1), d);
}