Add labels to peaks found by TSpectrum on histogram


I would like to know how to add labels to peaks identified by the TSpectrum class.
Something similar to what I found here:

See also screenshot attached.



_ROOT Version: ROOT 6.12/06

_Platform: OS X Yosemite 10.10.5

_Compiler: Apple LLVM version 7.0.0 (clang-700.1.76)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

Hi Laura,

there is no built-in functionality to do this. You can either trigger the drawing of the bin content for each bin (with the text drawing option for your histogram) or (which I think is better) you can draw it as a text object next to every identified peak. Since you fit the peaks a gaussian anyway, you can just draw the “constant” at positions returned by s->GetPositionX().

Could you be more explicit and tell me how to do this?

Int_t npeaks = 30;
double fpeaks(double *x, double *par) {
   double result = par[0] + par[1]*x[0];
   for (Int_t p=0;p<npeaks;p++) {
      double norm  = par[3*p+2]; // "height" or "area"
      double mean  = par[3*p+3];
      double sigma = par[3*p+4];
      result += norm*TMath::Gaus(x[0],mean,sigma);
   return result;

void peakstext(Int_t np=10) {
   npeaks = TMath::Abs(np);
   TH1F *h = new TH1F("h","test",500,0,1000);
   // Generate n peaks at random
   double par[3000];
   par[0] = 0.8;
   par[1] = -0.6/1000;

   for (int 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"

   TF1 *f = new TF1("f",fpeaks,0,1000,2+3*npeaks);
   auto *c1 = new TCanvas();


   TSpectrum *s = new TSpectrum(2*npeaks);
   Int_t nfound = s->Search(h,2,"",0.10);

   TList *functions = h->GetListOfFunctions();
   TPolyMarker *pm = (TPolyMarker*)functions->FindObject("TPolyMarker");
   double *xp = pm->GetX();
   double *yp = pm->GetY();
   int      n = pm->GetN();
   for (int i = 1; i<n; i++) {
      auto t = new TText(xp[i],yp[i],Form("%g", yp[i]));

1 Like

Great! Thanks a lot.

Of course some polishing is needed to nicely display de text (font, angle, size position, size …) , and choose the right text. But you have the idea.

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