Question about entries in projections

ROOT
1

Hi

I have a TH3D with 1.1x10^10 entries and when I consider ProjectionZ() I find the number of entries is the same, however, if I make a Project3D(“z”) the entries decrease to 8.9x10^9.

I don’t understand this behaviour. Are the two not supposed to return the same thing? Perhaps its a bug fixed in a later version - I use v4-00-04.

Thanks,

Mark

2

Mark,

I cannot reproduce this problem. Could you send me your TH3 object
in a ROOT file?

Rene

3

Hi

Here is some simple code which produces output I don’t understand. I could not upload the file (too large?) so I put it on the web at:

www-pdsf.nersc.gov/~mhorner/test.root

{
 TFile f("test.root");
 TH3D* mainhist =  f.Get("hmassvphi1");
 
 TH1D* xh = mainhist->Project3D("x");
 TH1D* yh = mainhist->Project3D("y");
 TH1D* zh = mainhist->Project3D("z");
 TH1D* zph = mainhist->ProjectionZ("zph");
 TH1D* zph2 = mainhist->ProjectionZ("zph2",0,mainhist->GetNbinsX(),0,mainhist->GetNbinsY(),"");
 
 cout <<"Get Entries\n3D x: "<<xh->GetEntries()<<"\n3D y: "<<yh->GetEntries()<<"\n3D z: "<<zh->GetEntries()<<"\nZ :"<<zph->GetEntries()<<"\nZ with params: "<<zph2->GetEntries() <<endl;
}

Thanks,

Mark

4

Problem now understood and fixed in CVS.
I have added the following note to ProjectionZ and Project3D

// NOTE 2: The number of entries in the projected histogram is set to the
// number of entries of the parent histogram if all bins are selected,
// otherwise it is set to the sum of the bin contents.

Rene

5

I have just run across this problem as well. When I do a Project3D(“xy”), I get fewer entries than the original TH3D.

I know for certain that the original 3D hist has overflows in the z-axis. I was looking at the source code, and when projecting (xy) it ignores the z-axis underflow and overflow bins.
[size=75] Int_t izmin = fZaxis.GetFirst();
Int_t izmax = fZaxis.GetLast();
…
case 4:
// "xy"
if (izbin < izmin || izbin > izmax) continue;[/size]

This will be true whether or not I have “SetRange”.

Is this a correct observation? (I would like to include the z underflow and overflow bins, unless I have used SetRange on the z-axis. Will I have to write a function to add them, after using Project3D(“xy”)?)

6

Please send the shortest possible script reprodicing the problem.
Make sure to use 4.02 or newer.

Rene

7

I’ve run this test script on 4.02 (MacOSX), with the same results on 4.00 (Linux).

The script output is:
[color=darkblue]Entries for small3 = 800
Entries for sma2 = 800
Entries for medium3 = 1000
Entries for med2 = 800
Entries for large3 = 1200
Entries for lar2 = 800[/color]

The difference between the small, medium, and large TH3D histograms, are entries made in the z-axis overflow and underflow bins. I use Project3D(“xy”) to make the TH2D histograms. The 2D hists all have the same number of entries, so the xy-projection ignores the z-axis overflow and underflow bins. The code snipet I listed in the previous post is from TH3::Project3D source code, and shows the limitation (even when SetRange is not being used).
test_file.txt (1.97 KB)

8

Thanks for sending this small script. Thanks to it I have fixed the problem
in the CVS version.

Rene