# [vtkusers] [vtkUsers] Mapping the real x, y, z points to the volume indices i, j, k using vtkImageData

bcombes benoit.combes at inria.fr
Fri Jan 20 04:57:02 EST 2017

```Hi vtkUsers,

I have a hard time with ComputeStructuredCoordinates.

* Goal:
I have a vtk file and a nii file. I want to recover some scalar values
from the volume to assign it to the surface scalar. So, essentially, I
want to find the mapping from the real x,y,z points to the volume
indices i,j,k.

* Observations:
1) When displaying the surface and the volume in the real frame they fit
perferctly, so data looks not to be the problem.
2) When making the (x,y,z)_Real to (i,j,k)_Volume correspondances using
ComputeStructuredCoordinates or FindPoint, I do not have the expected
results.
In particular, the coordinates given by ComputeStructuredCoordinates
indicates that the points are outside the volume or have wrong scalar
values. In other words, it does not seem to do the job I expect.

* Code:

```

// img is a vtkSmartPointer<vtkImageData> loaded from
// surf is a vtkSmartPointer<vtkPolyData> loaded from

for(int i=0;i<surf->GetNumberOfPoints();++i)
{
std::cout<<i<<","<<std::flush;
double *v=surf->GetPoint(i);

int ijk[] = { -1,-1,-1};
double *foopcoords;
int isOk=img->ComputeStructuredCoordinates(v,ijk,foopcoords) ;

if(isOk && (ijk>0 && ijk>0 && ijk>0))
{float v=img->GetScalarComponentAsFloat(ijk,ijk,ijk,0);
scal->InsertValue(i,v);
}
else
{scal->InsertValue(i,-1);}
}
```

I also try to use FindPoint instead:
```
int id = img->FindPoint(v);
if(id>-1)
{
scal->InsertValue(i,img->GetPointData()->GetScalars()->GetTuple(id));
}
else
{scal->InsertValue(i,-1);}

```
but have the same issue.

* Question

I don't understand how to fix this. Am I wrongly understand the purpose
of ComputeStructuredCoordinates ? Are there any voxels to real frames
that are not accounted for when using this function (or extra used ones) ?

```