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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?

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Try using scrot. Tested here, seems to work. – Renan Aug 6 '12 at 1:14
How do you screencap a different display (other than the current) using scrot? I can't find it in the man page – enthdegree Aug 6 '12 at 13:13
DISPLAY=<display> scrot – Renan Aug 6 '12 at 15:08
up vote 2 down vote accepted

Looks like scrot is able to do it, tested with:

xinit thunar -- :128
DISPLAY=:128 scrot

gave me a correct screenshot.

(but unfortunately, scrot has been unmaintained for quite a while)

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It turns out that x displays blank the screen and stop redrawing once their tty loses focus, with the intention of conserving CPU time and RAM. It doesn't look like there is a way to disable this functionality.

Xvfb, 'X Virtual Framebuffer' is a special implementation that does not feature this behavior as part of it's design and purpose.

Instead of initiating a new 'normal' xsession, I ran the client software in a virtual framebuffer:

xvfb-run <client software> --server-args="-screen 0 [width]x[height]x[bitdepth]"

and screencapped like this:

xwd -root -d :99 # (the default for xvfb)

Note that this display may be harder for the user to get at since it's not tied to a terminal.

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