[vtkusers] GetOutput & GetOuputPort

[B.J.Prosser at cs.cardiff.ac.uk]

This may seem like a simple question but ive been reading through the vtk book
and the online documentation but i dont really understand the difference
between GetOutput and GetOutputPort. Specifically i am trying to use the ICP
algorithm to match two data sets and i need to transform the second to get it
closer.

im transforming an obj file data set with:

vtkTransformPolyDataFilter tpdf1
   	tpdf1 SetInputConnection [obj1 GetOutputPort]
   	tpdf1 SetTransform t1

then i need to use this transformed data in the ICP algorithm which i need to
use:
   	icp1 SetTarger [somedata GetOutput]

but when i use
   	icp1 SetTarget [tpdf1 GetOutput]

i am told that 'Can't execute with NULL or empty target' but i dont understand
why it would be null ?