[RF] RooDataHist from a variable binning TH1D: different shape, different bin content

[santocch]

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Description

This problem has been already mentioned here [0] but no solution has been proposed. When you compare a RooDataHist obtained from a TH1 with fixed size binning everything looks ok but when you do the same from a variable size binning the comparison fails. See code below to reproduce the comparison.

[0] https://root-forum.cern.ch/t/roodatahist-and-th1d-why-different-shape-bin-content/56971

Reproducer

just run root mytest.cpp

using namespace RooFit;

void mytest() {

  const int nBin=100;
  double xbins[nBin+1];
  const float left_Val = 150;
  const float rightVal = 1150;  
  const float logxmin = TMath::Log10(left_Val);
  const float logxmax = TMath::Log10(rightVal);
  const double dxLog = (logxmax-logxmin)/nBin;
  for (int i=0;i<=nBin;i++) xbins[i] = TMath::Power(10., logxmin + i * dxLog);

  TF1 *f1 = new TF1("f1","expo(0)",left_Val,rightVal);
  f1->SetParameters(1e2,-1e-2);

  TH1D *h1 = new TH1D("h1","",nBin,xbins);
  h1->FillRandom("f1",10000000);
  RooRealVar mInv("mInv","m [GeV]",150,1150);
  RooDataHist hist_test1("hist_test1","hist_test1",mInv,Import(*h1,false));
  TCanvas *cExpo1 = new TCanvas();
  gPad->SetLogx();
  gPad->SetLogy();
  auto plot_test1 = mInv.frame();
  hist_test1.plotOn(plot_test1,DataError(RooAbsData::SumW2));
  plot_test1->Draw();
  h1->Draw("same");

  TH1D *h2 = new TH1D("h2","",nBin,left_Val,rightVal);
  h2->FillRandom("f1",10000000);
  RooDataHist hist_test2("hist_test2","hist_test2",mInv,Import(*h2,false));
  TCanvas *cExpo2 = new TCanvas();
  gPad->SetLogx();
  gPad->SetLogy();
  auto plot_test2 = mInv.frame();
  hist_test2.plotOn(plot_test2,DataError(RooAbsData::SumW2));
  plot_test2->Draw();
  h2->Draw("same");

}

ROOT version

ROOT 6.30/06

Installation method

homebrew

Operating system

OSX Sonoma 14.5

Additional context

No response

[santocch]

Hi
few comments after digging a bit in the code...

• it looks the problem is not in the RooDataHist itself but it's in the plotOn function. If you check the RooDataHist content, bin content is equal to the original histogram. The problem looks coming from this line [0]: if you comment it then the plot looks good.
• the plotting problem occurs also when you get a PDF from the RooDataHist using RooHistPdf. If you compare the PDF with the original histogram (scaled to have integral equal to one), the fixed size bin histogram looks good while the variable size bin histogram doesn't. In this case I didn't find how to fix the problem (the RooHistPdf code is a lot more complicated than the RooDataHist routine) but checking the behaviour of the PDF (i.e fitting the PDF to the RooDataHist), everything looks ok both for the fixed and variable size histogram.

See the attached modified version of the previous code to see the PDF begaviour mentioned above.

Best

Attilio

[0]

root/roofit/roofitcore/src/RooHist.cxx

Line 434 in b8b0a81

scale= _nominalBinWidth/binWidth;

int nEntries = 1000000;

void myTestRooDataHist() {

  const int nBin=100;
  double xbins[nBin+1];
  const float left_Val = 150;
  const float rightVal = 1150;  
  const float logxmin = TMath::Log10(left_Val);
  const float logxmax = TMath::Log10(rightVal);
  const double dxLog = (logxmax-logxmin)/nBin;
  for (int i=0;i<=nBin;i++) xbins[i] = TMath::Power(10., logxmin + i * dxLog);

  TF1 *f1 = new TF1("f1","expo(0)",left_Val,rightVal);
  f1->SetParameters(1e2,-1e-2);

  RooRealVar mInv("mInv","m [GeV]",150,1150);
  RooRealVar expo("expo", "expo", -1.0, -0.00001);
  RooExponential fitExpoFun("background", "background", mInv, expo);

  TH1D *h1 = new TH1D("h1","",nBin,left_Val,rightVal);
  h1->FillRandom("f1",nEntries);
  RooDataHist hist_test1("hist_test1","hist_test1",mInv,h1,1);
  TCanvas *cExpo1 = new TCanvas("cExpo1","",600,600);
  gPad->SetLogx();
  gPad->SetLogy();
  auto plot_test1 = mInv.frame();
  hist_test1.plotOn(plot_test1);
  plot_test1->Draw();
  h1->SetLineColor(2);
  h1->Draw("same");

  TH1D *h2 = new TH1D("h2","",nBin,xbins);
  h2->FillRandom("f1",nEntries);
  RooDataHist hist_test2("hist_test2","hist_test2",mInv,h2,1);
  TCanvas *cExpo2 = new TCanvas("cExpo2","",650,0,600,600);
  gPad->SetLogx();
  gPad->SetLogy();
  auto plot_test2 = mInv.frame();
  hist_test2.plotOn(plot_test2);
  plot_test2->Draw();
  h2->SetLineColor(2);
  h2->Draw("same");

  TCanvas *cExpo4 = new TCanvas("cExpo4","",1300,0,600,600);
  gPad->SetLogx();
  gPad->SetLogy();  
  RooHistPdf h1_PDF("h1_PDF","h1_PDF",mInv,hist_test1,0);
  auto plot_test1_pdf = mInv.frame();
  h1_PDF.plotOn(plot_test1_pdf);
  plot_test1_pdf->Draw();
  TH1D *h1clone = (TH1D*)h1->Clone("h1clone");
  h1clone->Scale(1./h1clone->Integral());
  h1clone->SetMarkerSize(0.5);
  h1clone->SetMarkerStyle(20);
  h1clone->SetMarkerColor(2);
  h1clone->SetLineColor(2);
  h1clone->Draw("same EP");
  auto result1 = fitExpoFun.fitTo(hist_test1,PrintLevel(0),RooFit::Save(true));
  result1->Print();

  TCanvas *cExpo5 = new TCanvas("cExpo5","",0,650,600,600);
  gPad->SetLogx();
  gPad->SetLogy();  
  RooHistPdf h2_PDF("h2_PDF","h2_PDF",mInv,hist_test2,0);
  auto plot_test2_pdf = mInv.frame();
  h2_PDF.plotOn(plot_test2_pdf);
  plot_test2_pdf->Draw();
  TH1D *h2clone = (TH1D*)h2->Clone("h2clone");
  h2clone->Scale(1./h2clone->Integral());
  h2clone->SetMarkerSize(0.5);
  h2clone->SetMarkerStyle(20);
  h2clone->SetMarkerColor(2);
  h2clone->SetLineColor(2);
  h2clone->Draw("same EP");
  auto result2 = fitExpoFun.fitTo(hist_test2,PrintLevel(0),RooFit::Save(true));
  result2->Print();

}