[vtkusers] vtkContourFilter issue

[David Doria]

I create an ImageData:

vtkSmartPointer<vtkImageData> grid = vtkSmartPointer<vtkImageData>::New();
grid->SetOrigin(xmin, ymin, zmin);
grid->SetExtent(0, this->NumberOfCellsX-1, 0, this->NumberOfCellsY-1,
0, this->NumberOfCellsZ-1); //same as grid->SetDimensions(10, 10, 10);
grid->SetSpacing((xmax-xmin)/static_cast<double>(this->NumberOfCellsX),
(ymax-ymin)/static_cast<double>(this->NumberOfCellsY),
(zmax-zmin)/static_cast<double>(this->NumberOfCellsZ));

Then I set a value at each point:

 for (int z = 0; z < dims[2]; z++)
    {
    for (int y = 0; y < dims[1]; y++)
      {
      for (int x = 0; x < dims[0]; x++)
        {
       //compute d ....
        grid->SetScalarComponentFromDouble(x,y,z,0,d);
        }
      }
    }

Then I want to extract the 0 isosurface of this scalar field.

I tried to do:

vtkSmartPointer<vtkContourFilter> contourFilter =
        vtkSmartPointer<vtkContourFilter>::New();

contourFilter->SetInput(grid);

vtkPolyData* surface = contourFilter->GetOutput();

but the result has 0 points and 0 cells. Have I done something wrong
with constructing the ImageData? Or using the ContourFilter?

Thanks,

David