From \$ROOTSYS/tutorials/fit/fit2a.C

```#include "TF2.h"
#include "TH2.h"
#include "TCutG.h"
#include "TMath.h"
#include "TCanvas.h"
#include "TStyle.h"

//+ Fitting a 2-D histogram (a variant)
// This tutorial illustrates :
//  - how to create a 2-d function
//  - fill a 2-d histogram randomly from this function
//  - fit the histogram
//  - display the fitted function on top of the histogram (lego-plot)
//    using a surface plot in a sub-range of the histogram.
//
// This example can be executed via the interpreter or/and the compiler
//   root > .x fit2a.C
//   root > .x fit2a.C++
//Author: Rene Brun

Double_t g2(Double_t *x, Double_t *par) {
Double_t r1 = Double_t((x[0]-par[1])/par[2]);
Double_t r2 = Double_t((x[1]-par[3])/par[4]);
return par[0]*TMath::Exp(-0.5*(r1*r1+r2*r2));
}
Double_t fun2(Double_t *x, Double_t *par) {
Double_t *p1 = &par[0];
Double_t *p2 = &par[5];
Double_t *p3 = &par[10];
Double_t result = g2(x,p1) + g2(x,p2) + g2(x,p3);
return result;
}

TCanvas *fit2a() {
TCanvas *c = new TCanvas();
gStyle->SetOptStat(kTRUE);
gStyle->SetPalette(57);
const Int_t npar = 15;
Double_t f2params[npar] = {100,-3,3,-3,3,160,0,0.8,0,0.9,40,4,0.7,4,0.7};
TF2 *f2 = new TF2("f2",fun2,-10,10,-10,10, npar);
f2->SetParameters(f2params);

//Create an histogram and fill it randomly with f2
TH2F *h2 = new TH2F("h2","From f2",40,-10,10,40,-10,10);
Int_t nentries = 100000;
h2->FillRandom("f2",nentries);
//Fit h2 with original function f2
Float_t ratio = 4*nentries/100000;
f2params[ 0] *= ratio;
f2params[ 5] *= ratio;
f2params[10] *= ratio;
f2->SetParameters(f2params);
h2->Fit("f2","N");
TCutG *cutg = new TCutG("cutg",5);
cutg->SetPoint(0,-7,-7);
cutg->SetPoint(1, 2,-7);
cutg->SetPoint(2, 2, 2);
cutg->SetPoint(3,-7, 2);
cutg->SetPoint(4,-7,-7);
h2->Draw("lego2 0");
h2->SetFillColor(38);
f2->SetNpx(80);
f2->SetNpy(80);
f2->Draw("surf1 same bb [cutg]");
return c;
}
```
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