fbm3D

//! requires noise3D
  fn fbm3D(x: vec3f, seed: f32) -> f32 {
    var p = x + seed;
    var f = 0.0;
    var w = 0.5;
    for (var i = 0; i < 5; i++) {
      f += w * noise3D(p);
      p *= 2.0;
      w *= 0.5;
    }
    return f;
  }