[vtkusers] Global Rotations and Translations

[Jonathan.Bailleul]

Andrew Dolgert wrote:
> 
> Hi John,
> 
> I was trying to get this straight, myself.  vtkTransform is a simple
> object which does not shift the global axes.  I would hope that what you
> want to do requires only that you create a transform which translates
> the object from the origin and then rotates it, R*T, and then poke it
> into vtkActor->SetUserTransform().
> 
> When you lay out the coordinate systems for VTK, they look pretty clear.
> There are three coordinate systems, Screen, World, and Actor, and they
> are connected by transformations
> Screen - s_M_v v_M_w - World - w_M_a - Actor.
> VTK expresses everything from the world's perspective, so the
> UserTransform in vtkActor is a matrix which transforms world points to
> actor points, or a_M_w.  It's that matrix you want to set.  If you are
> doing something funky, like rotating the actor about a world position by
> rotating all of the points, p, in its vtkPolyData, then you have to do
> the traditional linear algebra thing, where R*w_M_a*p = w_M_a*R'*p so
> that R' = a_M_w * R * w_M_a in the actor's coordinate system.
> 
> - Drew
> 

I'm not sure I got all of it, but it seems that I have a problem
_related_ to this one.
I would like to retrieve the transformations (scaling, rotation,
translation) that are applied to an actor when the user moves it with
the mouse.
I finally want to get corresponding  polydata object with point
coordinates modified by these transformations. I hoped I could get it
through Actor->GetMatrix();, but this is always equal to identity.

Ok, I know I seem clueless on this problem, but I don't have decent
documentation at hand (the vtk handbook), and I got pretty confused
running through the online documentation: any help would be really
appreciated!

In advance, thanks for your patience.

-- 
-----------------------------------
Jonathan BAILLEUL, Doctorant
GREYC Image - Université de Caen 
http://www.greyc.ismra.fr/~bailleul