Example in README dosen't dim screen on some configurations

[enolan]

You use xrandr | awk '/ primary/{print $1}' to find which display to change the brightness of in the example in the README. This doesn't work on my machine, the display I'm using isn't "primary", even though it's the only one plugged in. Here's my xrandr output.

enolan@chonk ~/m/c/nixpkgs> xrandr 
Screen 0: minimum 8 x 8, current 3840 x 2160, maximum 32767 x 32767
DP-0 disconnected primary (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
DP-4 connected 3840x2160+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
   3840x2160     60.00*+  30.00  
   2560x1440     59.95  
   1920x1080     60.00    59.94  
   1600x900      60.00  
   1280x1024     60.02  
   1280x800      59.81  
   1280x720      60.00    59.94  
   1152x864      59.96  
   1024x768      60.00  
   800x600       60.32  
   720x480       59.94  
   640x480       59.94    59.93  
DP-5 disconnected (normal left inverted right x axis y axis)
USB-C-0 disconnected (normal left inverted right x axis y axis)

AFAIK I didn't do anything weird to make it decide a disabled output was the primary one.

[jD91mZM2]

Then make it primary, xrandr --output DP-4 --primary, or don't use that line to find the display and hardcode it instead.