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?