Drawing into backplanes

[Andre Leblanc]

Is it possible to open a gl window and give it priority 
back bitplane access over any other window?  

That is, I would like to render an image into the backplanes, 
with z-buffering, then retrive content of the back color planes
and save it into an image file.  While the image is being rendered
(which takes quite a bit of time, even if its a VGX system),
I would like to continue working in other text windows
(wsh, text editor,...).  The problem occurs when I bring
text windows in front of the gl window.  It seems that gl clips
the backplane drawing according to visibility of windows.
Am I right?  

If not, what am I doing wrong?  
If so, do you have any suggestions to assure that the image 
that's being rendered in the backplanes is not affected by
text windows in front?

Thanks,
Andre LeBlanc
Swiss Federal Institute of Technology
Lausanne, Switzerland.