Fitting 2D Histograms

ROOT
1

Hello,
I have some problems in fitting a 2D histogram. I attached an example of the kind of histogram I’m dealing with.
Can you help me?
Thank you in advance,
Valentina
angolivsalignedarray.pdf (59.6 KB)

2

which kind of fit you would like to to ? If you want to see a trend like an angle vs some other variable, it is maybe better to make a profile (TProfile class) and fit it with a 1D function.

Cheers, Lorenzo

3

Lorenzo,

Note that I recently implemented (SVN trunk) the possibility to fit directly a TH2 with a TF1 without producing an intermediate TProfile.

Rene

4

Rene,
Can yo be more specific about how to do the fitting, or at least point us out to where we can learn more about this.
Regards.
Aous

5

see example below

void f12() {
   Double_t angle = TMath::Pi()/4.;
   Double_t cosphi = TMath::Cos(angle);
   Double_t sinphi = TMath::Sin(angle);
   
   TH2F *h2 = new TH2F("h2","h2",100,0,0.8,100,0,0.8);
   TRandom r;     
   for (Int_t i=0;i<1000;i++) {
      Double_t x = r.Uniform(0,1);
      Double_t y = r.Gaus(0,0.02);
      Double_t u = cosphi*x -sinphi*y;
      Double_t v = sinphi*x +cosphi*y;
      h2->Fill(u,v);
   }
   TF1 *f1 = new TF1("f1","[0]+[1]*x",0,1);
   f1->SetParameters(0.,1.);
   f1->SetLineColor(kRed);
   h2->Fit(f1);
   h2->Draw();
   f1->Draw("same");
}

Rene

6

Rene.
I ran the code you attached and I keep getting this message:
Error in TH2F::Fit: function f1 dimension, 1, does not match histogram dimension, 2

Another thing, when I chnage the histogram, say by multiplying the y value by 2 (v in the code) teh fit function does not seem to match the histogram. I attach a plot showing what I got and the modified code.
c1.ps (19.5 KB)
temp.C (550 Bytes)

7

It is likely that you are using an old version of ROOT. With 5.18 or newer, I get the following picture when executing your script

Rene