Accessing an openGL viewport's pixels on multiple X servers

[Jerome Guilmette]

Hi everyone,

I hope this is the good list to send my question to for it is a rather a 
specific matter. All my tests were done using an NVIDIA GeForce 8600M, 
driver version 180.29, under the Fedora 10 OS.

Here is my issue: I have a program that takes a few screenshots of an 
application (window) and saves it into a file.  I discovered that, while 
taking a snapshot of an application running an openGL viewport, i.e. 
glxgears, and switching to another X server, I was not able to save the 
images in their integrity, the openGL viewport wasn't displayed. So the 
raw content of an openGL viewport isn't available while we are in 
another X Server than the one the program is running into. So would it 
mean that only one openGL context per X server is possible?
 
In a multiple X sessions context, is there a way to have access to the 
rendered openGL viewport pixels while being currently on a different X 
server using standard X functions? In any case, what exactly are the 
mechanisms in place?

Any inputs would be very much appreciated, might it be a link to more 
documentation or a direct answer.

Thank you very much,

Jerome