Hi rooters,

I have TH2 histo that I call image and I have one more TH2 histo that I call model.

Now I want to fit the model to the image; the shape of the model is determined (but it

is in form of a histo not a function), what can varies it is only the intensty of the TH2.

How can I do that?

Do you know if it’s possible to fit a isto with another histo instead of a function?

Thanks and bye

Marco

Have a look a fithist.C in the tutorial directory

Dear everybody,

I attached the .root file with the data and a simpple macro analysis.C to better

understand the problem.

I used the approach suggested by Eddy, but the shadow visible in the first

histo (whose model is contained in the second (lower) one) cannot be completely removed

with this simple approach.

The approach I am using is to define a function of this kind:

Double_t ftotal(Double_t *x,Double_t *par) {

Double_t xx = x[0];

Double_t yy = x[1];

Int_t binX = bkg_m->GetXaxis()->FindBin(xx);

Int_t binY = bkg_m->GetYaxis()->FindBin(yy);

Double_t br = par[0]*bkg_m->GetBinContent(binX,binY);

return br;

}

where bkg_m is the second histo (the model of the shadow) and I use a parameter

for the bin population.

It seems not to work properly.

Any suggestions or solutions?

Thanks

Marco

analysis.C (1.13 KB)

background.root (13.4 KB)

Hi Marco,

Glancing at the pictures, I notice that the signal has large “walls"

at y=+25 and x=+25 . Make sure that you do NOT use this data

in the fit by specifying the TF1 range smaller than 25 and supplying

"R” to the fit .

Make sure that the errors of your signal are ok . This will influence

the weighting in the flat area vs the “ditch” .

Eddy

Hi Eddy and everybody,

I did what you suggested: limiting the fit between -25 and 23 in both x and y directions.

Things are slightly better even if I cannot remove completely the “ditch”.

What can I do solve this task?

The errors in the signal histogramm are defined as sqrt of the bin counts and I think

this is correct , isn’t it? Since they are really detected counts.

I am attaching again the macro analysis.C to be used with the file backgournd.root

present in my last post.

Can anyone tell me why when I plot the function in the last pad

it doesn’t look like the pad immediately above called “Analitical Pattern”?

They should be the same, except for the scale factor!

If anyone has a good idea how to make the fit and eliminate the ditch, this would

be great.

Thanks

Marco

analysis.C (1.34 KB)

Hi Marco,

One more suggestion, specify the “L” option in the

fit because we are dealing with counting statistics

and chi^2 is not really the proper objective function

to optimize.

On a different note . I have no idea what your data

and simulation mean but it is obvious that if you have a slight

"misalignment", you are toast .

One thing you can do about it:

Bin a bit finer and introduce in the fit 2 more parameters:

shift in the x direction and shift in the y-direction

Eddy

Hi Eddy and Everybody,

here I am again.

Things now look better, but I cannot remove completely the ditch, still.

I was thinkig something like to change the step size and so on.

Any suggestions?

Marco