[vtkusers] scaling vtkpolydata to the extents of a vtkimagedata set ?

[afyfe]

Hello,
I'm a new vtk user with two questions.

(1) I would to like to overlay a volumetric rendering of an imagedata 
set (electron density) with polydata-derived actors that display 
characteristics of the data (molecular model and meshes). Overall, this 
seems to work but the polydata-derived actors are not displayed at the 
expected location. Whereas  a STRUCTURED_POINTS dataset specifies the 
extent/spacing/origin of the grid a POLYDATA dataset does not. Is there 
a way of ensuring two such data sets share the same coordinate system  
(that is a particular  x,yz, coordinate refers to the same location)?

(2) I assume this is a fairly straightforward question but could find no 
answer in the User's Guide or the mailing-list archives- are there other 
good resources to check?

thanks.