Illustrates how to find peaks in histograms.

This script generates a random number of gaussian peaks on top of a linear background. The position of the peaks is found via TSpectrum and injected as initial values of parameters to make a global fit. The background is computed and drawn on top of the original histogram.

This script can fit "peaks' heights" or "peaks' areas" (comment out or uncomment the line which defines __PEAKS_C_FIT_AREAS__).

To execute this example, do (in ROOT 5 or ROOT 6):

root > .x peaks.C  (generate 10 peaks by default)
 root > .x peaks.C++ (use the compiler)
 root > .x peaks.C++(30) (generates 30 peaks)

To execute only the first part of the script (without fitting) specify a negative value for the number of peaks, eg

root > .x peaks.C(-20)

Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Tuesday, August 09, 2022 at 09:43 AM.

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%%cpp -d
#include "TCanvas.h"
#include "TMath.h"
#include "TH1.h"
#include "TF1.h"
#include "TRandom.h"
#include "TSpectrum.h"
#include "TVirtualFitter.h"

Int_t npeaks = 30;

Comment out the line below, if you want "peaks' heights". Uncomment the line below, if you want "peaks' areas".

efine PEAKS_C_FIT_AREAS 1 /* fit peaks' areas

In [ ]:
%%cpp -d
Double_t fpeaks(Double_t *x, Double_t *par) {
   Double_t result = par[0] + par[1]*x[0];
   for (Int_t p=0;p<npeaks;p++) {
      Double_t norm  = par[3*p+2];  "height" or "area"
      Double_t mean  = par[3*p+3];
      Double_t sigma = par[3*p+4];
#if defined(__PEAKS_C_FIT_AREAS__)
      norm /= sigma * (TMath::Sqrt(TMath::TwoPi()));  "area"
#endif /* defined(__PEAKS_C_FIT_AREAS__) */
      result += norm*TMath::Gaus(x[0],mean,sigma);
   return result;

Arguments are defined.

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Int_t np=10;
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npeaks = TMath::Abs(np);
TH1F *h = new TH1F("h","test",500,0,1000);

Generate n peaks at random

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Double_t par[3000];
par[0] = 0.8;
par[1] = -0.6/1000;
Int_t p;
for (p=0;p<npeaks;p++) {
   par[3*p+2] = 1; // "height"
   par[3*p+3] = 10+gRandom->Rndm()*980; // "mean"
   par[3*p+4] = 3+2*gRandom->Rndm(); // "sigma"
#if defined(__PEAKS_C_FIT_AREAS__)
   par[3*p+2] *= par[3*p+4] * (TMath::Sqrt(TMath::TwoPi())); // "area"
#endif /* defined(__PEAKS_C_FIT_AREAS__) */
TF1 *f = new TF1("f",fpeaks,0,1000,2+3*npeaks);
TCanvas *c1 = new TCanvas("c1","c1",10,10,1000,900);
TH1F *h2 = (TH1F*)h->Clone("h2");

Use TSpectrum to find the peak candidates

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TSpectrum *s = new TSpectrum(2*npeaks);
Int_t nfound = s->Search(h,2,"",0.10);
printf("Found %d candidate peaks to fit\n",nfound);

Estimate background using TSpectrum::Background

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TH1 *hb = s->Background(h,20,"same");
if (hb) c1->Update();
if (np <0) return;

estimate linear background using a fitting method

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TF1 *fline = new TF1("fline","pol1",0,1000);

Loop on all found peaks. Eliminate peaks at the background level

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par[0] = fline->GetParameter(0);
par[1] = fline->GetParameter(1);
npeaks = 0;
Double_t *xpeaks;
xpeaks = s->GetPositionX();
for (p=0;p<nfound;p++) {
   Double_t xp = xpeaks[p];
   Int_t bin = h->GetXaxis()->FindBin(xp);
   Double_t yp = h->GetBinContent(bin);
   if (yp-TMath::Sqrt(yp) < fline->Eval(xp)) continue;
   par[3*npeaks+2] = yp; // "height"
   par[3*npeaks+3] = xp; // "mean"
   par[3*npeaks+4] = 3; // "sigma"
#if defined(__PEAKS_C_FIT_AREAS__)
   par[3*npeaks+2] *= par[3*npeaks+4] * (TMath::Sqrt(TMath::TwoPi())); // "area"
#endif /* defined(__PEAKS_C_FIT_AREAS__) */
printf("Found %d useful peaks to fit\n",npeaks);
printf("Now fitting: Be patient\n");
TF1 *fit = new TF1("fit",fpeaks,0,1000,2+3*npeaks);

We may have more than the default 25 parameters

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Draw all canvases

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