displacement NatachaGccTestDisplacement (uniform float K = 0.01, F = 1) {
  float d;
  d = K*(noise(s*F, t*F)+noise(s*2*F, t*2*F)/2+noise(s*4*F, t*4*F)/4);
  P -= d*normalize(N);
  N = calculatenormal (P); }