// This macro extracts the contour lines (TGraphs) from
// 2 dim histogram and draws them together with labels
// onto a copy the histogram
// the position of the label is arbitrary choosen to be
// point 0 of the graph

void gen_contours(TH2 *hin = NULL, Int_t nlevels = 5)
{
//	if (!h2 || !h2->InheritsFrom("TH2") return;
	TH2 *h2 = hin;
	if (!h2){
		h2 = new TH2F("h2","h2", 40, -4, 4, 40, -4, 4);
		Double_t x, y;
		Int_t i = 0;
		while(i++ < 100000) {
			gRandom->Rannor(x,y);
			h2->Fill(x,y);
			// try this to make it look more realistic
			//h2->Fill(x-1.5,y+1);
			//if (i%2 == 0) h2->Fill(1.5*x+1,y-1);
		}
		h2->Smooth(); // make contours look smoother/nicer
	}
   auto can = new TCanvas("can","For h2",50,100,600,600);
   h2->SetContour(nlevels);
   h2->Draw("CONT Z LIST");		// generate a list with TGraphs
   can->Update();						// force drawing

   TObjArray *lofconts =
      (TObjArray*)gROOT->GetListOfSpecials()->FindObject("contours");
   if (!lofconts) return;
   
	auto cancon = new TCanvas("cancon","For contours",730,100,600,600);
   TString fname(h2->GetName());
   fname +="_contours.root";
//   auto outfile = new TFile(fname, "RECREATE");
   auto hclone = (TH2*)h2->Clone();
   hclone->Draw("COL Z");
 
   TList *lofgraphs;
   TGraph *graph;
   Int_t nconts = lofconts->GetSize();
   Double_t *clevels = new Double_t[nconts];
   h2->GetContour(clevels);

	Double_t xp, yp;
	TLatex *label;
   // loop on contours
   for (Int_t ic =0; ic < nconts; ic++) {
		// get list of graphs
		lofgraphs = (TList*)lofconts->At(ic);
		if (!lofgraphs) return;
		Int_t ngraphs = lofgraphs->GetSize();
		if (ngraphs == 0) continue;
		// Note: their might be several closed lines on same level
		// loop on graphs
		for (Int_t ig =0; ig < ngraphs; ig++) {
			graph = (TGraph*)lofgraphs->At(ig);
			if (!graph) return;
//			outfile->cd();
//			graph->Write();
			cancon->cd();
			graph->Draw("c");
			// get x, y of 1. point and put label
			graph->GetPoint(0, xp, yp);
			label = new TLatex(xp, yp, Form("%.1f", clevels[ic]));
			label->SetTextSize(0.02);
			label->Draw();
		}
	}
//	if (outfile) outfile->Write();
}