X11: change display resolution with xrandr

1. get current state:

cyril@workstation: xrandr 
Screen 0: minimum 320 x 200, current 3840 x 2160, maximum 16384 x 16384
DFP1 connected primary 1776x1000+0+0 (normal left inverted right x axis y axis) 621mm x 341mm
   3840x2160     29.97 +  24.00    25.00    23.98    29.97  
   2560x1440     59.95  
   2048x1536     24.00    25.00    23.98    29.97    29.97  
   1920x1440     60.00  
   ...
   640x480       72.81    75.00    66.61    59.94  
DFP2 disconnected (normal left inverted right x axis y axis)
DFP3 disconnected (normal left inverted right x axis y axis)

2. change resolution:

cyril@workstation:> xrandr --output DFP1 --mode 3840x2160 --dpi 162
cyril@workstation:> xrandr --output DFP1 --mode 2560x1440
cyril@workstation:> xrandr --output DFP1 --mode 2048x1536 

3. in case of "unkonwn mode":

# compute parameters of new mode

cvt 1440 900 60
#then define and add new mode

xrandr --newmode  "1440x900_60.00"  106.50  1440 1528 1672 1904  900 903 909 934 -hsync +vsync 

xrandr --addmode VNC-0 1440x900_60.00

#and finally set 

xrandr -s 1440x900