[vtkusers] Extract VOI and reposition to the origin

[Michel-Alexandre Cardin]

Hi,

I have a vtkImageData object representing a certain volume in space (this
volume has some arbitrary shape). The original volume is centered at
(0,0,0). I wish to extract a sub-volume or VOI from this original volume
using a function such as vtkExtractVOI. The center of this sub-volume is not
positioned at (0,0,0) within the original volume, so it has some relative
position with respect to (0,0,0). I want to have the center of this newly
extracted subvolume positioned at (0,0,0).

My problem is exemplified when I extract such VOI and start rotating it
using RotateX, RotateY, RotateZ, and SetOrientation. It will only rotate
around axes defined at the origin at (0,0,0), and I want it to rotate around
axes passing through its own center, which center I want to position at
(0,0,0).

In other words, I do not want to keep the original 3D position of the center
of this new sub-volume, and would like to position the center on the origin
at (0,0,0). I don't need any other information about the original volume
apart for that VOI. I could extract the sub-volume using vtkExtractVOI and
translate it to the origin, but I can't (don't know how to) measure the
coordinates of such sub-volume with respect to the original one. What are
the possibilities? Should I copy the VOI to another vtkImageData object? How
can I get rid of the original position of each voxel of the sub-volume with
respect to the origin (0,0,0)?

I'm new to VTK and may not be aware of all the tricks and trades. Thanks for
your help,

MIKE