TGraph2D Interpolate returns 0

yosse_andrean · March 22, 2023, 9:36am

Dear experts,

I am struggling with the Interpolate() method of TGraph2D.
There are some coordinates that for some reason returns 0.0. These coordinates are completely valid because in the plot I can see that they have non-zero value.

This is how my graph looks like drawn with “colz” option:

To reproduce the problem, please download the root file attached and execute:

root yield_graphs.root
graph_yield_nom->Interpolate(750, 500) // this returns a non-zero value
graph_yield_nom->Interpolate(760, 500) // this returns zero
graph_yield_nom->Interpolate(770, 500) // this returns zero
graph_yield_nom->Interpolate(780, 500) // this returns zero
graph_yield_nom->Interpolate(790, 500) // this returns a non-zero value
graph_yield_nom->Interpolate(800, 500) // this returns zero

I cannot really make sense why some coordinates return zero when they should be interpolatable by Delaunay triangle.
I hope the experts here can help.

Thanks!

Best regards,
Yosse

yield_graphs.root (4.8 KB)

_ROOT Version: v6-24-08

couet · March 22, 2023, 4:41pm

I do not see the same “0” but I see some too with root master on Mac

couet · March 22, 2023, 5:16pm

The computation is done here:

github.com

root-project/root/blob/27b0072582c6dc040d0bc5c50be797f82cd14aa6/math/mathcore/src/Delaunay2D.cxx#L130


      
             // some case the 1d interpolated value
             if (fNdt == 0) {
                return fZout;
             }
          
          
   // Find the z value corresponding to the point (x,y).
             double xx, yy;
             xx = Linear_transform(x, fOffsetX, fScaleFactorX); //xx = xTransformer(x);
             yy = Linear_transform(y, fOffsetY, fScaleFactorY); //yy = yTransformer(y);
             double zz = DoInterpolateNormalized(xx, yy);
          
          
   // Wrong zeros may appear when points sit on a regular grid.
             // The following line try to avoid this problem.
             if (zz==0) zz = DoInterpolateNormalized(xx+0.0001, yy);
          
          
   return zz;
          }
          
          
//______________________________________________________________________________
          void Delaunay2D::FindAllTriangles()
          {

There is an atempt to avoid 0 . It seems a known issue. May Be @moneta has some ideas about it ?

couet · March 22, 2023, 6:07pm

Indeed, as suggested in the code I posted, Interpolate might fail if the point is on/near a edge. Adding an epsilon helps:

graph_yield_nom->Interpolate(800, 500.001)

moneta · March 23, 2023, 11:04am

I think this is a bug and it needs to be fixed. The workaround showed above seems to not work in some cases. Can you please open a GitHub issue on this ?
Thank you,

Lorenzo

yosse_andrean · March 23, 2023, 12:41pm

Thank you for the suggestion! I have open an issue on this problem: TGraph2D's Interpolate() returns 0 · Issue #12535 · root-project/root · GitHub

system · April 6, 2023, 12:42pm

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