void example2dGausContour() {

   // draw 1-sigma and 2 s-gma contour of a 2d correlated gaussian
   // Definition of 1 sigma contour is the contour which has a prob content of 0.708
   // which is normal_cdf(1,1)^2
   // 


   TCanvas *c  = new TCanvas("c","Contours",0,0,800,700);


   TF2 *f2 = new TF2("f1","bigaus");  
   //TF2 *f2 = new TF2("f2","std::log(bigaus)");  
   f2->SetFillStyle(1000);
   f2->SetLineWidth(1);
   f2->SetRange(-5,-5,5,5);
   f2->SetParameters(1,0,1,0,1,0);
   f2->SetNpx(1000);
   f2->SetNpy(1000);
   double contours[2];
   // determine level of gaussian at 1 and 2 sigma using a non-correlated 2d gaussian
   // corr factor needed because the constant term is different in the case of a correlated 2d-gaussian
   double corr = f2->Eval(0,0)/ROOT::Math::bigaussian_pdf(0,0,1,1,0);
   contours[0] = corr*ROOT::Math::bigaussian_pdf(2,2,1,1,0); //0.05;
   contours[1] = corr*ROOT::Math::bigaussian_pdf(1,1,1,1,0);
   f2->SetContour(2,contours);


   Int_t colors[2] = {kYellow, kGreen};
   gStyle->SetPalette(2,colors);

   f2->Draw("cont0 z");
   f2->Draw("cont3 same");

}