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Revision as of 15:33, 4 June 2011 by Twl (talk | contribs) (Using Xrandr with VNC)
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Note: credit to and retrieved from the Ubuntu wiki

Dynamically testing different resolutions

Template:Codeline 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 Template:Codeline only last through the current session. xrandr has a lot more capabilities - see Template:Codeline for details.

Adding undetected resolutions

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

If the mode already exists, but just isn't associated for the particular output, you can add it like this:

 xrandr --addmode S-video 800x600

If the mode doesn't yet exist, you'll need to create it first by specifying a modeline:

 xrandr --newmode <Mode Line>

You may create a modeline using the Template:Codeline or Template:Codeline utility. For example, if you want to add a mode with resolution 800x600, you can enter the following command: (The output is shown following.)

 $ cvt 800 600
 # 800x600 59.86 Hz (CVT 0.48M3) hsync: 37.35 kHz; pclk: 38.25 MHz
 Modeline "800x600_60.00"   38.25  800 832 912 1024  600 603 607 624 -hsync +vsync

Then copy the information after the word "Modeline" into the xrandr command:

 xrandr --newmode "800x600_60.00"   38.25  800 832 912 1024  600 603 607 624 -hsync +vsync

Making xrandr changes persistent

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

Setting resolution changes in xorg.conf (Preferred)

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


See Template:Codeline for full details on how to craft an Template:Filename 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 Template:Filename , while for KDM this is done at Template:Filename.

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 Template:File

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.



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"