shader sheet_occlusion (
  normal Normal=N,
  float Radius=0.05,

  output float Fac=1
){
  // calculate angle of incidence
  float alpha = acos(dot(I,Normal));
  // treat front and back the same
  alpha = alpha > M_PI_2 ? M_PI - alpha : alpha;
  //printf("%.2f %.2f\n",alpha,tan(alpha));
  // calculate the non occluded fraction 
  Fac = 1 - Radius - Radius * tan(alpha);
}