xcalib display number / screen number [duplicate]

Question

I am trying to reduce the contrast of my monitor by using xcalib. This is the output of my xrandr.

Screen 0: minimum 8 x 8, current 3840 x 1200, maximum 32767 x 32767
DVI-D-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 527mm x 296mm
   1920x1080     60.00*+
   1680x1050     59.95  
   1600x900      60.00  
   1440x900      59.89  
   1280x1024     75.02    60.02  
   1280x720      60.00  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
HDMI-0 connected 1920x1200+1920+0 (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200     59.95*+
   1920x1080     60.00  
   1680x1050     59.95  
   1600x1200     60.00  
   1280x1024     60.02  
   1280x960      60.00  
   1024x768      60.00  
   800x600       60.32  
   640x480       59.94  
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 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 disconnected (normal left inverted right x axis y axis)
DP-5 disconnected (normal left inverted right x axis y axis)

I could till now proportionally change the contrast of both the monitors by using this command -

xcalib -co 80 -a #reduces contrast by 80 percent.

How do I specify the monitor name?

xcalib -co -80 -d DVI-D-0

throws an error - Can't open display

Answer 1

From trial and error xcalib's -screen parameter takes the zero indexed number of your monitor according to your xrandr output, ie, in your case xcalib -s 0 (the default) will control DVI-D-0 and xcalib -s 1 will control HDMI-0.