Oh yeah, this is the legacy computer from heck: ancient generic whitebox with ancient nVidia GeForce2 MX 400, CentOS, KDE, and the above 1920x1080 pixel which is kind of pushing the MX400 as far as it was designed to go. (I do have the current nVidia legacy driver installed, which they graciously still support.)
Now the fun starts: There are three different places where display gamma gets set in CentOS + KDE. First, on X startup, /etc/X11/Xorg.conf gets read. This sets up the settings for the login screen only, so just leave this alone unless the gamma is intolerably bad.
That in turn will be overridden by KDE, which appears to run the "xgamma" command to source in gammas it finds in ~/.kde/share/config/kgammarc. This file is updated by the Monitor Gamma tab in the KDE Display Setting widget.
This is turn is overridden only when you happen to run the utility nVidia provides with the driver, /usr/bin/nvidia-settings, which is invoked by the nVidia driver settings widget that gets installed in the KDE System menu. This utility stores its gamma settings in ~/.nvidia-settings-rc.
So to keep these three stooges of apps from messing up your settings:
- Remove the nVidia driver utility from the KDE menu so you are never tempted to run it, and delete the .nvidia-settings-rc file. The settings aren't used unless you run the nvidia-setting utility.
- Use only the the Monitor Gamma tab in the KDE Display Setting widget to set gammas / color correction.
- If you want to make the login screen look nice, edit /etc/X11/xorg.conf.
- You could "lock in" the values by running either xgamma or nvidia-settings from your .bashrc file. Or both, if you are not easilty confused.
And don't forget - with LCDs, "brightness" is really "contrast" and vice versa. http://www.poynton.com/notes/brightness_and_contrast/ has a pretty good sum-up of that.
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