> 17th March 2010, 02:22       Duss

Calibrating screen's color

This entry applies specifically to Kubuntu 10.04; probably also to later releases and alternate Linux distributions

This setup is only needed if you really care for color accuracy, or if you have more than one monitor, and their models do not match. In this case, a color can look quite different on separate monitors, which can be specially bothering in some cases.

Some monitors have many configuration settings. I focus here on the bare bones: brightness, contrast, and gamma correction (the correlation between a color value and its representation on the screen). Brightness and contrast are usually setup using the monitor settings, while the gamma value usually require specific software setup. More to the point, whatever is the gamma correction defined in the monitor settings, it is usually modified by the software setup: by default, Os X and Windows apply a gamma correction of 2.2 (which is applied over the correction defined by the monitor itself, no overriding here).

I am unsure on how Ubuntu defines the gamma value. There is a command: xgamma that shows the applied gamma, and is, by default, 1.0. However, this does not corresponds to the reality (as far as I can gather, this xgamma 1.0 value corresponds to a 2.0 value). The best help here is using calibration tools, which can be as simple as checking some images.

These pages will show the current gamma value. I personally favor a value of 1.8, which seems to correspond to a xgamma=1.3 value. It is easy to try different values using the command:

xgamma -gamma=x.y

It is better to apply different gamma values and see the results on the black and white levels.

Once the desired xgamma value is calculated, it is needed to update the /etc/X11/xorg.conf file, adding the gamma value into the "monitor" section. In my case, it ends looking up like:

Section "Monitor"
	Identifier 	"Configured Monitor"
	Gamma		1.3

Please note that it is possible to apply different gamma values to the red / green / blue channels. After this change, I would still like to have my configuration automatically updated, so I execute, as detailed on the /etc/X11/xorg.conf file:

sudo dpkg-reconfigure -phigh xserver-org

Unfortunately, this is not enough. If the X server is restarted, the xgamma command will report a 1.3 gamma value. But, if now the command is reissued:

xgamma -gamma=1.3

The gamma correction is then applied: that is, the gamma had not been properly corrected yet. There seems to be some problem with the gamma value applied by the display driver itself. In my case, using NVidia, there is a panel -NVidia X Server settings- that displays, among others, the gamma correction, and which does not match the value reported by xgamma. As far as I have gathered, it is needed to update both values: the nvidia settings and the Xorg.conf value (in fact I am afraid that all is needed is to update the nvidia settings). When this is done, a file is created on the home directory, .nvidia-settings-rc, containing the given gamma values.

So, the right procedure for NVidia drivers is:

  1. Open the NVidia X Server Settings, and set a gamma value of 1.3. Exit the settings; a file will have been created on the home directory, .nvidia-settings-rc, containing the given gamma values
  2. Edit the /etc/X11/xorg.conf file, as detailed above.
  3. Reboot (restarting the X server is enough).