[vtkusers] Strange behavior of vtkImageAlgorithm subclass - empty output

[David Doria]

Would someone mind taking a look at this:
http://www.rpi.edu/~doriad/VTK_List/vtkVoxelizePolyData/

I write a file (inside.vti) of the output at the very end of
RequestData. This works properly, the file is correct.

However, as soon as I get the output of the filter, I write another
file, outside.vti. I would expect this to be identical to inside.vti.
However, it is empty (0 cell, 0 points)!

Here is a potential clue:

I write the ImageData called "grid" (this is the part that produces
the correct file)

  vtkSmartPointer<vtkXMLImageDataWriter> writer =
      vtkSmartPointer<vtkXMLImageDataWriter>::New();
  writer->SetFileName("inside.vti");
  writer->SetInput(grid);
  writer->Write();

Then I copy it into the output:
  output->ShallowCopy(grid);

Then, still from inside RequestData, I try to write the file again,
this time using "output" instead of "grid":
  {
  vtkSmartPointer<vtkXMLImageDataWriter> writer =
      vtkSmartPointer<vtkXMLImageDataWriter>::New();
  writer->SetFileName("inside_output.vti");
  writer->SetInput(output);
  writer->Write();
  }

and it segfaults.

Any thoughts on why this might be happening?

Thanks,

David