Dump hidden (or minimized) X11 window

I'd like to dump the contents of an X window. Like this:

xwd -id | convert - capture.png

... which works quite well as long as the target window is not hidden (totally or partially behind other windows) or minimized.

Any idea on how to do this?

I guess we somehow have to send a "repaint" signal to the hidden window and then capture its response or something... :)

From stackoverflow

• Do you have an objection to bringing it to the front first?

  If not, you can use a tool like wmctrl to bring the window to the front (and maximize it, etc), use xwd to capture it, then send it back to where it came from with wmctrl.