hist056_TPolyMarker_contour.C

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/// \file
/// \ingroup tutorial_hist
/// \notebook
/// Make a contour plot and get the first contour in a TPolyMarker.
/// This macro generates a color contour plot by selecting entries
/// from an ntuple file.
/// The TGraph object corresponding to the first contour line is
/// accessed and displayed into a separate canvas.
///
/// \macro_code
///
/// \date January 2017
/// \author Rene Brun
 
void hist056_TPolyMarker_contour()
{
   TString dir = gROOT->GetTutorialDir();
   dir.Append("/hsimple.C");
   dir.ReplaceAll("/./", "/");
   if (!gInterpreter->IsLoaded(dir.Data()))
      gInterpreter->LoadMacro(dir.Data());
   TFile *file = (TFile *)gROOT->ProcessLineFast("hsimple(1)");
   if (!file)
      return;
   TTree *ntuple = (TTree *)file->Get("ntuple");
 
   TCanvas *c1 = new TCanvas("c1", "Contours", 10, 10, 800, 600);
   ntuple->Draw("py:px", "px*px+py*py < 20", "contz,list");
 
   // we must call Update to force the canvas to be painted.  When
   // painting the contour plot, the list of contours is generated
   // and a reference to it added to the Root list of special objects
   c1->Update();
 
   TCanvas *c2 = new TCanvas("c2", "First contour", 100, 100, 800, 600);
 
   TObjArray *contours = (TObjArray *)gROOT->GetListOfSpecials()->FindObject("contours");
   if (!contours)
      return;
   TList *lcontour1 = (TList *)contours->At(0);
   if (!lcontour1)
      return;
   TGraph *gc1 = (TGraph *)lcontour1->First();
   if (!gc1)
      return;
   if (gc1->GetN() < 10)
      return;
   gc1->SetMarkerStyle(21);
   gc1->Draw("alp");
 
   // We make a TCutG object with the array obtained from this graph
   TCutG *cutg = new TCutG("cutg", gc1->GetN(), gc1->GetX(), gc1->GetY());
 
   // We create a polymarker object with npmax points.
   const Int_t npmax = 50000;
   TPolyMarker *pm = new TPolyMarker(npmax);
   Int_t np = 0;
   while (1) {
      Double_t x = -4 + 8 * gRandom->Rndm();
      Double_t y = -4 + 8 * gRandom->Rndm();
      if (cutg->IsInside(x, y)) {
         pm->SetPoint(np, x, y);
         np++;
         if (np == npmax)
            break;
      }
   }
   pm->Draw();
}