[vtkusers] Extracting component of vector field and exporting subset of arrays

[peterm]

Thanks for the response,

The arrayCalculator seems to work using the mentioned modification, when I
export the volume to .vtk using the vtkDataSetWriter the result seems to be
fine.

However saving the selected components to MHD files just won't work! What I
would like to achieve is to save select scalar fields from a structured grid
to a set of MHD files, so that it can be further processed in an external
application. To create the MHD files I use vtkMetaImageWriter. In case I use
the arrayCalculator as the input for the vtkMetaImageWriter than the created
file seems to contain a defaultly selected scalar component. I have tried to
use multiple approaches to select the component I would actually like to
export:

 - use vtkAssignAttribute to specify the component
 - use passArrays to remove everything else, so that the exported component
is the one I would like

I have run into problems using either of the approaches:

 - vtkAssignAttribute:
  I used the filter in the following manner
  
  assignAttribute.SetInputConnection(arrayCalculatorXYZ.GetOutputPort())
  assignAttribute.Assign("U_x", "SCALARS", "POINT_DATA")
  writer.SetInputConnection(assignAttribute.GetOutputPort()) #
vtkMetaImageWriter

  The result is the following error message:
  writer (MHD): vtkMetaImageWriter (0x2d234b0): Unknown scalar type

 - passArrays

  The setup is the following

  passArrays = vtk.vtkPassArrays()
  passArrays.SetInput(arrayCalculatorXYZ.GetOutput())
  passArrays.AddArray(0, "U_x")
  passArrays.UseFieldTypesOn()
  passArrays.AddFieldType(0) # vtkDataObject::POINT
  passArrays.AddFieldType(1) # vtkDataObject::CELL

  In this case I get volumes with zero values (not exactly zero but very
small 10^-20 etc.)

The correctness of the output of the arrayCalculatorXYZ is tested using
saving to the VTK format
Remark: using passArrays with the export to VTK crashes Paraview when the
file contents are displayed

In case there is an alternative solution how to save scalar components of a
structured grid to MHD files, I would really like to hear about it.

Alternatively if anyone has a suggestion, it would be very much welcome.

Thanks a lot,
Peter
   



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