[vtkusers] 3D, constant view size, billboard object?

[BURRELL Benjamin]

Hi,

I am wondering if anyone has any suggestions on how to solve the following:
What I require is 3D object (which might be some simple line drawn symbols or a 3D model) that always appears the same size on screen, even when I move the camera close to it. I need this to happen in real-time, eg. I am probably going to use the vtkInteractorStyleTrackballCamera interactor and use the mouse. This interactor 'dollies' the camera (moves the camera closer and further away) compared to the std. interactor style which zooms in and out.
I am guessing I will need to overwrite the dolly callback function but I am not sure how to do it, and what to implement to get the desired result of having the 3D object stay the same size on screen.

I might also want the 3D model to actually be a billboard object, eg. face the camera all the time with a 2D image. 
Does anyone have a suggested vtkObject to do this with. It will also need to stay the same size on the screen no matter how close/far away the camera gets.

Thank you for your help.

Regards,
Benjamin Burrell