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Dynamically testing different resolutions

xrandr shows you the names of different outputs available on your system (LVDS, VGA-0, etc.) and resolutions available on each:

Screen 0: minimum 320 x 200, current 1400 x 1050, maximum 1400 x 1400
VGA disconnected (normal left inverted right x axis y axis)
LVDS connected 1400x1050+0+0 (normal left inverted right x axis y axis) 286mm x 214mm
   1400x1050      60.0*+   50.0  

You can direct xrandr to set a different resolution like this:

 xrandr --output LVDS --mode 1024x768

The refresh rate may also be changed, either at the same time or independently:

 xrandr --output LVDS --mode 1024x768 --rate 75

Note that changes you make using xrandr only last through the current session. xrandr has a lot more capabilities - see man xrandr for details.

Adding undetected resolutions

Due to buggy hardware or drivers, your monitor's correct resolutions may not always be detected by xrandr. For example, the EDID data block queried from the monitor may be incorrect.

First we run gtf or cvt to get the Modeline for the resolution we want:

 $ cvt 1280 1024
 # 1280x1024 59.89 Hz (CVT 1.31M4) hsync: 63.67 kHz; pclk: 109.00 MHz
 Modeline "1280x1024_60.00"  109.00  1280 1368 1496 1712  1024 1027 1034 1063 -hsync +vsync

Then we create a new xrandr mode. Note that the Modeline keyword needs to be ommited.

   xrandr --newmode "1280x1024_60.00"  109.00  1280 1368 1496 1712  1024 1027 1034 1063 -hsync +vsync

After creating it we need an extra step to add this new mode to our current output (VGA1). We use just the name of the mode, since the parameters have been set previously.

   xrandr --addmode VGA1 "1280x1024_60.00"

Now we change the resolution of the screen to the one we just added:

   xrandr --output VGA1 --mode "1280x1024_60.00"

Note that these settings only take effect during this session.

Making xrandr changes persistent

There are several ways to make xrandr customizations permanent from session to session:

  • xorg.conf ( Preferred)
  • .xprofile
  • kdm/gdm

Setting resolution changes in xorg.conf (Preferred)

While xorg.conf is largely empty these days, it can still be used for setting up resolutions. For example:

Section "Monitor"
    Identifier      "External DVI"
    Modeline        "1280x1024_60.00"  108.88  1280 1360 1496 1712  1024 1025 1028 1060  -HSync +Vsync
    Option          "PreferredMode" "1280x1024_60.00"
Section "Device"
    Identifier      "ATI Technologies, Inc. M22 [Radeon Mobility M300]"
    Driver          "ati"
    Option          "Monitor-DVI-0" "External DVI"
Section "Screen"
    Identifier      "Primary Screen"
    Device          "ATI Technologies, Inc. M22 [Radeon Mobility M300]"
    DefaultDepth    24
    SubSection "Display"
        Depth           24
        Modes   "1280x1024" "1024x768" "640x480"

Section "ServerLayout"
        Identifier      "Default Layout"
        Screen          "Primary Screen"

See man xorg.conf for full details on how to craft an xorg.conf file.

Setting xrandr commands in xprofile

See Execute commands after X start.

This method has the disadvantage of occurring fairly late in the startup process thus it will not alter the resolution of the Display Manager if you use one.

Setting xrandr commands in kdm/gdm startup scripts

Both KDM and GDM have startup scripts that are executed when X is initiated. For GDM, these are in /etc/gdm/ , while for KDM this is done at /usr/share/config/kdm/Xsetup.

This process requires root access and mucking around in system config files, but will take effect earlier in the startup process than using xprofile.


Resolution lower than expected

Try this first

If you video card is recognized but the resolution is lower than you expect, you may try this.

Background: ATI X1550 based video card and two LCD monitors DELL 2408(up to 1920x1200) and Samsung 206BW(up to 1680x1050). Upon first login after installation, the resolution default to 1152x864. xrandr does not list any resolution higher than 1152x864. You may want to try editing /etc/X11/xorg.conf, add a section about virtual screen, logout, login and see if this helps. If not then read on.

Change xorg.conf

Section "Screen"
        SubSection "Display"
                Virtual 3600 1200

About the numbers: DELL on the left and Samsung on the right. So the virtual width is of sum of both LCD width 3600=1920+1680; Height then is figured as the max of them, which is max(1200,1050)=1200. If you put one LCD above the other, use this calculation instead: (max(width1, width2), height1+height2).

Use cvt/xrandr tool to add the highest mode the LCD can do

The actual order was different, as I tried to add new mode to one LCD at a time. Below is the combined/all-in-one quote

$ cvt 1920 1200 60
# 1920x1200 59.88 Hz (CVT 2.30MA) hsync: 74.56 kHz; pclk: 193.25 MHz
Modeline "1920x1200_60.00"  193.25  1920 2056 2256 2592  1200 1203 1209 1245 -hsync +vsync
$ cvt 1680 1050 60
# 1680x1050 59.95 Hz (CVT 1.76MA) hsync: 65.29 kHz; pclk: 146.25 MHz
Modeline "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync
$ xrandr --newmode "1920x1200_60.00"  193.25  1920 2056 2256 2592  1200 1203 1209 1245 -hsync +vsync
$ xrandr --newmode "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync
$ xrandr --addmode DVI-1 "1920x1200_60.00"
$ xrandr --addmode DVI-0 "1680x1050_60.00"

Obtaining modelines from Windows program PowerStrip

Obtaining modelines from Windows program PowerStrip.


# This script toggles the extended monitor outputs if something is connected

# your notebook monitor

# outputs to toggle if connected

# get info from xrandr


        if [[ $XRANDR == *$CURRENT\ connected*  ]] # is connected
                if [[ $XRANDR == *$CURRENT\ connected\ \(* ]] # is disabled
                        EXECUTE+="--output $CURRENT --auto --above $DEFAULT_OUTPUT "
                        EXECUTE+="--output $CURRENT --off "
        else # make sure disconnected outputs are off 
                EXECUTE+="--output $CURRENT --off "

xrandr --output $DEFAULT_OUTPUT --auto $EXECUTE

Using xrandr with VNC

If you are using a VNC server that supports xrandr you can change the vnc resolution on the fly by using "xrandr -s <width>x<height>". tigervnc is an example of a client that supports xrandr


xrandr -s 1920x1200

After you VNC in, if you open a console and type "xrandr" you will get a list of currently configured modes. Each of these modes can be activated with the xrandr -s option; however, if the mode you want does not exist in the list, you can add it by doing the following:

Example: Say I want to add 1024x600 (a common netbook resolution)

First run CVT to get the correct modeline for the resolution you want to add

$ cvt 1024 600

You will get something like the following output

# 1024x600 59.85 Hz (CVT) hsync: 37.35 kHz; pclk: 49.00 MHz
Modeline "1024x600_60.00"   49.00  1024 1072 1168 1312  600 603 613 624 -hsync +vsync

Use that modeline output to run the commands below

xrandr --newmode "1024x600"   49.00  1024 1072 1168 1312  600 603 613 624 -hsync +vsync
xrandr --addmode default "1024x600"

Doing the above will give you the ability to change to 1024x600 by typing xrandr -s 1024x600, but it will only last for the current x session. To insure that you can use the newly added resolution each time you start vncserver, add the following to ~/.vnc/xstartup

xrandr --newmode "1024x600"   49.00  1024 1072 1168 1312  600 603 613 624 -hsync +vsync
xrandr --addmode default "1024x600"