Arch Linux

mcloaked

Member

From: Yorkshire, UK

Registered: 2012-02-02

Posts: 1,240

Changing screen brightness of HDMI connected monitor

I have a machine which has a single monitor connected with an HDMI cable. The system works - but I have a puzzle with setting the screen brightness that I would like to understand.

If I query the system with xrandr to find the name of the monitor and then use xrandr to set the screen brightness then all is well as follows:

$ xrandr -q | grep -w connected
eDP1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
HDMI2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 477mm x 268mm
$ xrandr --output HDMI2  --brightness 0.83
$ xrandr --output HDMI2  --brightness 0.93

Then the screen brightness dims as requested and then brightens as requested when the last two lines above are executed.

Yet if I do:

xbacklight -get

After each of the commands the output remains as 93.000000 even when the brightness has been changed to 0.83 with the xrandr command.

On the other hand if I do

$ xbacklight -dec 10
$ xbacklight -inc 10

I get a nice informative popup in KDE saying that the brightness is 83% after the first of these, and back to 93% after the second but the screen does not change brightness at all during either command!

I also noticed that if I set the screen to dim after some period using the KDE power settings then I get the KDE popup informing of the change of brightness but the screen remains unchanged. So it is only the xrandr commands in the first section of this post that actually makes the screen change brightness.

So I am confused - and maybe it is my total ignorance about how to set screen brightness but I would appreciate any input on understanding how to get KDE to dim the screen under the power settings in this situation. The machine is a small fanless desktop based on the Intel DQ77KB motherboard with an i3 CPU - and the screen is an Asus LED connected via a single HDMI cable for both graphics and audio.

---

Mike C