//
//
//				B O R R O W E D   F R O M  T U T O R I A L S / F I T 2 . C
//
//         
TEllipse *pEllipse=0;
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;
}

void Start() {
   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",100,-10,10,100,-10,10);
   Int_t nentries = 100000;
   h2->FillRandom("f2",nentries);
   h2->Draw("colz");
   gPad->AddExec("ex1","event()");
   gStyle->SetPalette(1,0);
}
void event()
{
	static Bool_t bDown=false;
	static Double_t xc,yc,rc;
	static TH2F *h;
	static Double_t x0,y0;
	Double_t x,y;
	int ix,iy;
	int event = gPad->GetEvent();
	if ( (kButton1Down!=event) && (kButton1Up!=event) && (kButton1Motion!=event) && (kMouseMotion!=event)) return;
	TObject *select = gPad->GetSelected();
	if (!select) return;
	if (select->InheritsFrom("TEllipse")) goto here; // what a kludge!
	//printf("%s %d\n",select->GetName(),event);
	if (!select->InheritsFrom("TH2")) return;
	h = (TH2F *)select;
	ix = gPad->GetEventX();
	iy = gPad->GetEventY();
	x = gPad->AbsPixeltoX(ix);
	y = gPad->AbsPixeltoY(iy);
	//printf("event x,y = %d %f %f\n",event,x,y);
	if (kButton1Down==event || ((kButton1Motion==event) & !bDown))
	{
		bDown=true;
		x0 = x;
		y0 = y;
	}
	else if (kButton1Motion==event || bDown)
	{
		//printf("x0,y0=%f %f\n",x0,y0);
		xc = 0.5*(x+x0);
		yc = 0.5*(y+y0);
		rc = 0.5*sqrt((x-x0)*(x-x0)+(y-y0)*(y-y0));
		if (pEllipse) delete pEllipse;
		pEllipse = new TEllipse(xc,yc,rc,rc);
		pEllipse->SetLineColor(2);
		pEllipse->SetLineWidth(3);
		pEllipse->Draw();
		gPad->Update();
	}
here:
	if (kButton1Up==event || (kMouseMotion==event&&bDown))
	{
		bDown=false;
		printf(" xc, yc, rc = %7.2f %7.2f %7.2f\n",xc,yc,rc);
		gPad->DeleteExec("ex1");
	}
}