TGraph2D - rebinning axis

ROOT
1

Dear ROOTers,

I am trying to rebin the axis of a TGraph2D plot. I need this class because I have an x,y,z dataset, but I need to display it as a 2D map.

The code is the following

b1_map[j-3] = new TGraph2D(4);
b1_map[j-3]->GetXaxis()->Set(4, -180., 180.);
b1_map[j-3]->GetYaxis()->Set(4, -1., 1.);
b1_map[j-3]->SetTitle("; phi [DEG]; cos(theta) [adm]; b1 [adm]");
//filling b1_maps
for (int k = 0; k < 16; k++) //k = all phi and cos(theta) = 4*4
     b1_map[j-3]->SetPoint(k, phi[k], cos_theta[k], b1[k+(j-3)*16]);
b1_map[j-3]->Draw("COLZ");

but I have 40 bins in each axis. What’s the problem??

Thank you for your time!!

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ROOT Version: 5.34/36
Platform: Windows10
Compiler: VS2015

2

There is no problem. That’s the Default value. See SetNpx and SetNpy

3

I don’t know how happened that I haven’t seen that method.
Thank you very much for your time!!

4

ROOT classes have a lot of methods.
It is easy to miss something.

1 Like
5

Hi!
A question arose doing some modifications to the code.
The data I want to plot is 4x4. How does TGraph2D behave if I set a greater binning, perhaps 8x8 or 16x16?
It seems to me it smoothes the results, but I want to ask it to experts.

Thank you again for your patience!

LG, Giammarco

6

First of all TGraph2D deals with unbinned data. Like TGraph. But it is possible to project these unbinned data on a 2D histogram. The number of bins of this histogram is defined (along X and Y) with SetNpx and SetNpy.

7

I guess this is the answer to my question: a TGraph2D INTERPOLATES the z axis, therefore if your axis binning is higher than the actual content of the x,y,z array (i.e. > n), interpolation of z is called.

// TGraph2D linearly interpolate a Z value for any (X,Y) point given some

// existing (X,Y,Z) points. The existing (X,Y,Z) points can be randomly

// scattered. The algorithm works by joining the existing points to make

// Delaunay triangles in (X,Y). These are then used to define flat planes

// in (X,Y,Z) over which to interpolate. The interpolated surface thus takes

// the form of tessellating triangles at various angles. Output can take the

// form of a 2D histogram or a vector. The triangles found can be drawn in 3D

from https://root.cern/root/html530/TGraph2D.html

8

The explanation is better in more recent versions of the ROOT reference guide.

9

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