[vtkusers] How can I get fixed-size text at a position in world space?

[Wayne Christopher]

I have been trying to accomplish the following with no luck: create a 
text annotation that has a fixed height, in pixels, and a fixed 
orientation (i.e. stays upright, faces the user), that has a fixed 
location in world space.

vtkVectorText + vtkFollower does not work because vtkFollower can only 
adjust the orientation and not the size.  I don't want to zoom in on the 
text when the user zooms in, I want it to stay 20 pixels high.  Also I 
would rather not create a large number of polygons unnecessarily.

vtkTextActor does not work because I can only set a screen position, not 
a world position.  I would like the annotations to move with the data as 
the view transformation changes.

Am I missing something?  Is this impossible without writing a custom actor?

Thanks,

     Wayne