Screen

Some times in System > Preferences > Display > Resolution does not display all the desired resolutions.

So if we want to get those resolution , we need to add them mannnulaly.

following I have done for Ubuntu for setting resolution to 1024x768

to know the supported resolution

$ xrandr

Screen 0: minimum 320 x 200, current 1024 x 768, maximum 4096 x 4096

VGA1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 312mm x 234mm

1024x768 85.0 + 75.1*

1280x1024 75.0

800x600 75.0

640x480 75.0 60.0

720x400 70.1

1024x768_60.00 59.9

Find Modeline deatails for 1024x768

$ cvt 1024 768

# 1024x768 59.92 Hz (CVT 0.79M3) hsync: 47.82 kHz; pclk: 63.50 MHz

Modeline "1024x768_60.00" 63.50 1024 1072 1176 1328 768 771 775 798 -hsync +vsync

Now use modeline config with xrandr to add resolution mode and

edit /etc/gdm/PreSession/Default

and add following in the file-

xrandr --newmode "1024x768_60.00" 63.50 1024 1072 1176 1328 768 771 775 798 -hsync +vsync

xrandr --addmode VGA1 1024x768_60.00

reboot and U will get the 1024x768 value in System > Preferences > Display > Resolution

and u can choose .

Same can be done for other supported resolution values.