void read_TH2D() {
  TFile *f = TFile::Open("TH2D.root");
  if (!f || f->IsZombie()) { delete f; return; } // just a precaution
  TCanvas *c; f->GetObject("PDM", c);
  if (!c) { delete f; return; } // just a precaution
  c->Draw();
  TH2D *h2 = (TH2D *)c->GetPrimitive("Simulated PDM");
  if (!h2) { delete f; return; } // just a precaution
  Int_t x, y;
  Double_t photons;
  for (x = 1; x <= h2->GetNbinsX(); x++) {
    cout<<"----------" <<endl;
    cout<<"new x" <<endl;
    for (y = 1; y <= h2->GetNbinsY(); y++) {
      cout<<"new y" <<endl;
      photons = h2->GetBinContent(x,y);
      cout<<"bin("<<x <<"," <<y <<") = " <<photons <<endl;
      if(photons>0)
        cout<<"bin("<<x <<"," <<y <<") with content =! 0 " <<endl;
    }
  }
}