Adjusting laptop display brightness doesn't work in Linux Mint 14

I recently installed Linux Mint 14 on my Samsung R580 laptop. The Fn-keys worked out of the box, I can switch screens, adjust volume and so on.

When I try to adjust the brightness (Fn + ↑/↓), the slider shows up and moves to the left and right, but screen brightness does not change at all, it always stays at 100%.

I tried to change the brightness in the system settings and it also doesn't do anything.

When I'm inactive for 5-10 minutes, the energy saving profile kicks in and the screen is dimmed. So the dimming does work, but not when I change it manually.

I found some threads stating that you should change the acpi_background setting to "vendor" in Grub config, but that didn't change anything for me.

What else can I try?

From Samsung R580, Ubuntu 10.04 and Brightness control, not written for Mint but may still work :

Edit the /etc/X11/xorg.conf file and add the RegistryDwords line (only that line) to the Device section:

Section "Device"
  Identifier "Default Device"
  Driver "nvidia"
  Option "NoLogo" "True"
  Option "RegistryDwords" "EnableBrightnessControl=1"
EndSection

Edit this line in /etc/default/grub:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi="

Then:

update-grub
reboot