How to capture a screenshot of the LightDM greeter screen

1. If they are not already installed, install the packages x11-apps/xwd and media-gfx/imagemagick.
   

2. Create the Bash script ~/lightdm-greeter-capture.sh containing the following:

   #!/bin/bash
   sleep 30
   DISPLAY=:0 XAUTHORITY=/var/run/lightdm/root/$DISPLAY xwd -root > /tmp/greeter.xwd
   convert /tmp/greeter.xwd /home/fitzcarraldo/greeter.png
   

   Make sure you make it executable:

   user $ chmod +x lightdm-greeter-capture.sh

3. Log out of the Desktop Environment so that the LightDM greeter screen is displayed.
   

4. Press Ctrl+Alt+F2 to switch to VT2.
   

5. Log in to you user account and enter the following command (do not wait for it to complete):
   

   user $ sudo /home/fitzcarraldo/lightdm-greeter-capture.sh

6. As soon as you have pressed Enter for the above command, press Ctrl+Alt+F7 to switch back to VT7.
   

7. Wait for at least 30 seconds to be sure the Bash script has made a snapshot of the LightDM greeter screen, then log in.
   

8. You should now find the file ~/greeter.png containing a snapshot of your LightDM greeter screen.

If you install media-libs/netpbm instead of (or as well as) media-gfx/imagemagick then you can use a different command to convert in the Bash script:

#!/bin/bash
sleep 30
DISPLAY=:0 XAUTHORITY=/var/run/lightdm/root/$DISPLAY xwd -root | xwdtopnm | pnmtopng > /home/fitzcarraldo/greeter.png

The resulting PNG image looks equally good to my eyes.