[vtkusers] Setting the center of rotation

[Wayne Christopher]

I have seen this question asked a few times with no solutions - if it's 
not possible to do, could somebody confirm this?  Do I need to write my 
own interactor?

I am using a trackball interactor to manipulate a scene with a number of 
actors.  I zoom in to one area of one actor, and when I rotate the 
scene, I want the center of rotation to be a point that I can select on 
the visible actor - that is, not the center of the entire scene, which 
would cause what I'm looking at to move out of the screen.  How can I do 
that?

Setting the focal point of the camera causes the scene to pan and move 
this point to the center of the screen, which is not what I want.  Also 
after panning with the middle button, the center of rotation does not 
stay where it was set.

If I were doing this in OpenGL I would apply a transform to translate 
the center point to the origin, then apply rotation and scaling 
operations, then translate the origin back to the center point.

Thanks,

     Wayne