[vtkusers] Force a 2D actor to be on top?

[Jeremy Winston]

VTKers,

I have two vtkActor2Ds that I want to display in the same
window.  One is an image, the other is a contour (generated
by vtkMarchingSquares) from that image.

At home, when I just add each to the renderer (using Tcl) thusly:

   ren1 AddActor2D vhdImageActor
   ren1 AddActor2D vhdContourActor

all is well, and the contour appears "on top" of the image.
At work, only the image actor appears.  When I comment out
the first line above, the contour appears, so it seems the
image actor is "on top" of the contour actor.

Other than the obvious conclusion that I should only work
from home, what is the best way to force the contour actor
to be "on top?"

Thanks,
-Jeremy