Ubuntu – How to adjust display settings for LightDM greeter on 18.04

displaylightdmmultiple-monitorsxubuntu

I have three monitors. My login does not position the monitors properly — the left and right monitors should be swapped. How do I adjust the resolution and monitor position for the LightDM login screen?

Here is my output from xrandr when logged in. I would like to use the same settings for my login screen and lock screen. 

Screen 0: minimum 8 x 8, current 4800 x 1200, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 1600x1200+3200+0 (normal left inverted right x axis y axis) 367mm x 275mm
   1600x1200     60.00*+
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
HDMI-0 connected 1600x1200+1600+0 (normal left inverted right x axis y axis) 367mm x 275mm
   1600x1200     60.00*+
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
DP-0 connected 1600x1200+0+0 (normal left inverted right x axis y axis) 367mm x 275mm
   1600x1200     60.00*+
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
DP-1 disconnected (normal left inverted right x axis y axis)

Best Answer

I was able to solve this problem by installing arandr:

$ sudo apt install arandr

and running it:

$ arandr (it is also available in the settings manager).

Then I set my monitors the way I like using its GUI. You can try out its configurations on your current desktop by clicking the green checkmark:

enter image description here

Once things were correct, I saved the configuration to my desktop, which generated a .sh file:

#!/bin/sh xrandr --output DP-0 --mode 1600x1200 --pos 0x0 --rotate normal --output DVI-I-1 --primary --mode 1600x1200 --pos 3200x0 --rotate normal --output DVI-I-0 --off --output DP-1 --off --output HDMI-0 --mode 1600x1200 --pos 1600x0 --rotate normal

I then copied that file to /etc/lightdm/monitor-config.sh as root.

Finally, with su I edited /etc/lightdm/lightdm.conf (which did not exist) to include the following lines: 

[Seat:*]
display-setup-script=/etc/lightdm/monitor-config.sh

I did that, logged out, and my login screen had the right monitor positions.

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