Binning in root file

JunKai · February 5, 2022, 11:07am

Hi everyone,
I am curious that when viewing the histogram of a single branch in a TTree (I opened up a root file and clicked on the branch to view the histogram), why does in some of the case, the rebin option is less flexible than the other (as in figure b). I dont think that this is limited by entries number, because the root file in figures (a) and (b) have 1M and 5M entries respectively.
Is is something to do with the saving process? If so, how do I take care of this when saving root files ?

Thanks in advance.

_ROOT Version: 6.24 (via cvmfs)
Platform: Ubuntu20.0LTS on WSL 2
Compiler: gcc9

couet · February 7, 2022, 7:47am

Seems to me the left scale from -5 tp 5 is wrong. It should be a scale changing the number of bins. I see the same when I access and histogram produced by TTree::Draw. When I access an other histogram it is ok:

auto h = new*TH1F("h","h",500,0,1)
h->Draw()
h->Fill(0.5)
h->Draw()

Note that it seems only the slider scale is wrong. When you move it changes the number of bins correctly. Same when you enter directly the number of bins in the text box.

couet · February 7, 2022, 8:56am

Actually looking at the code, I see there is two different cases depending if the histogram comes from a tree or not. In the “tree case” the code is here:

github.com

root-project/root/blob/61e89ed8235f418e3721e576396759ed80ab75b8/gui/ged/src/TH1Editor.cxx#L403

      
        
            fBinSlider1->Resize(107,20);
            fBinSlider1->SetRange(1,9);
            fBinSlider1->SetScale(12);
            fBinSlider1->SetPosition(5);
            f21->AddFrame(fBinSlider1, new TGLayoutHints(kLHintsLeft, 3,0,0,3));
            fBinCont1->AddFrame(f21, new TGLayoutHints(kLHintsTop, 15, 7, 5, 0));
            
            
//  Lettering of the Rebin Slider
            TGCompositeFrame *f24 = new TGCompositeFrame(fBinCont1, 80, 20,
                                                                    kHorizontalFrame);
            TGLabel *l1 = new TGLabel(f24, "-5");
            f24->AddFrame(l1, new TGLayoutHints(kLHintsLeft, 18, 1, -1, 0));
            TGLabel *l2 = new TGLabel(f24, "-2");
            f24->AddFrame(l2, new TGLayoutHints(kLHintsLeft, 26, 2, -1, 0));
            TGLabel *l3 = new TGLabel(f24, "2");
            f24->AddFrame(l3, new TGLayoutHints(kLHintsLeft, 17, 2, -1, 0));
            TGLabel *l4 = new TGLabel(f24, "5");
            f24->AddFrame(l4, new TGLayoutHints(kLHintsLeft, 32, 3, -1, 0));
            fBinCont1->AddFrame(f24, new TGLayoutHints(kLHintsTop, 0, 0, 0, 0));
            
            
TGCompositeFrame *f22 = new TGCompositeFrame(fBinCont1, 140, 20,

I have no idea it was done that way …

couet · February 7, 2022, 9:24am

The doc is here:

github.com

root-project/root/blob/61e89ed8235f418e3721e576396759ed80ab75b8/gui/ged/src/TH1Editor.cxx#L65-L96

      
        
              This Tab has two different layouts. One is for a histogram which
              is not drawn from an ntuple. The other one is available for a
              histogram which is drawn from an ntuple. In this case the rebin
              algorithm can create a rebinned histogram from the original data
              i.e. the ntuple.
              To see te differences do:
                 TFile f("hsimple.root");
                 hpx->Draw("BAR1");        // non ntuple histogram
                 ntuple->Draw("px");       // ntuple histogram
            Non ntuple histogram:
               'Rebin': with the Slider the number of bins (shown in the field
                        below the Slider) can be changed to any number which
                        divides the number of bins of the original histogram.
                        Pushing 'Apply' will delete the origin histogram and
                        replace it by the rebinned one on the screen
                        Pushing 'Ignore' the origin histogram will be restored
            Histogram drawn from an ntuple:
               'Rebin'  with the slider the number of bins can be enlarged by
                        a factor of 2,3,4,5 (moving to the right) or reduced
                        by a factor of 1/2, 1/3, 1/4, 1/5

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