// the sigmaMap histogram has 3600x1500 bins and contains both positive
// and negative "z" values

void tryme() {
  TFile *f1 = new TFile("sigmaMap.root","READ");
  TH2F *sigmaMap = (TH2F *)f1->Get("sigmaMap");
  sigmaMap->SetTitle("");
  gStyle->SetOptStat(kFALSE);

  TCanvas *c1 = new TCanvas("c1","",800,600);
  c1->ToggleEventStatus();
  c1->SetRightMargin(0.2);
  c1->SetLeftMargin(0.01);
  c1->SetTopMargin(0.01);
  c1->SetBottomMargin(0.01);

  cout << "sig map  " << sigmaMap->GetArray() << endl;
  cout << "bins x   " << sigmaMap->GetNbinsX() << endl;
  cout << "bins y   " << sigmaMap->GetNbinsY() << endl;
  cout << "gHist    " << gHistImagePalette << endl;

  TImage *img = TImage::Create();
  // code crashes on the next line
  img->SetImage((const Double_t *)sigmaMap->GetArray(),sigmaMap->GetNbinsX()+2,sigmaMap->GetNbinsY()+2,gHistImagePalette);
  img->Draw();
  img->StartPaletteEditor();
}