Get coordinates of a point from a TGraph2D

ROOT
1

Hi,

I am interested in finding the coordinates of the minimum point (Z-direction) of at TGraph2D. I know how to get the value of z:

gr -> GetZmin();

I’d like to know the x and y coordinates corresponding to that point.

Is that possible without fitting the graph with a function?

Thanks in advance,
Estela.

2

You can try the following, assuming a TGraph2D* g

int ix,iy,iz; g->GetHistogram()->GetMinimumBin(ix,iy,iz); double xmin = g->GetHistogram()->GetXaxis()->GetBinCenter(ix); double ymin = g->GetHistogram()->GetYaxis()->GetBinCenter(iy);
Rene

3

I’m sorry, but there is something that I don’t understand in this.

Why are the values that I obtain in ix and iy and iz not the same? If I understand correctly, these are indeces of the arrays (x,y,z) that I use to produce my TGraph2D, like:

According to this iz should be the index of the element that is the minimum value of the array z. The corresponding elements of x and y should be in the same position, as they are the other two coordinates of the same point.

Or are ix, iy and iz something else?

Thanks,
Estela

4

ix, iy, iz returned by g->GetHistogram()->GetMinimumBin(ix,iy,iz); are histogram bins in the underlying histogram build by GetHistogram(). They are not indexes in the x, y and z arrays. But the coordinates returned by :

double xmin = g->GetHistogram()->GetXaxis()->GetBinCenter(ix);
double ymin = g->GetHistogram()->GetYaxis()->GetBinCenter(iy);

should be correct.