How do I screencap another Xorg display?

view full story

http://unix.stackexchange.com – I started another x session from my current one like so: xinit xterm -- :128 An x session starts in display :128, xterm gets run in it and everything is fine. I switched back to screen :0 (the default), though, and tried to screencap the new display: $ xwd -root -screen -out screencap.xwd -display :128 and the resulting file was just a big black rectangle. I also tried with imagemagick import, to no avail. What is going wrong? Thanks! (HowTos)