SaveBehind field of NeWS canvases

[Ramani Pichumani]

I have been trying in vain to build a popup window in 4sight that will
save the contents of a DGL window underneath it.  I really need this
feature because the time to redraw the images underneath can be
unacceptably long.  I've tried the following PostScript code with no
success:

/win framebuffer newcanvas def   % create a canvas
win /SaveBehind true put         % enable SaveBehind
framebuffer setcanvas            % shape the canvas
100 100 translate
0 0 100 0 360 arc                % make the canvas circular
win reshapecanvas
win setcanvas 
erasepage clippath .5 setgray fill    % fill the canvas
win /Mapped true put             % map it
win /Mapped false put            % unmap

After unmapping the canvas, all I'm left with is a white hole where
the canvas used to be.  The only thing I could think of is maybe I
should pass the DGL canvas as the parent canvas but I don't know where
to get it's handle.  The code works fine on my Sun3 but doesn't work
as expected on a 4D.

Ramani Pichumani
Department of Computer Science, Stanford University
ramani at patience.stanford.edu