Brightness only works on one display, not two

[Joshua Peisach]

Dell Inspiron 15-3567, Hirsute

My display setup is the laptop and on the right a monitor.

As I change my brightness, lower or higher, on the laptop the brightness will change but on the monitor it won't.

Not the physical monitor brightness-that obviously can't be controlled-but the content of what the monitor shows should be dimming/brightening. (For example if theoretically I turned the brightness to 0, to an all dark screen, it should be all dark on the monitor aswell, even if the monitor brightness is max, the OS is showing nothing)

Yes, I looked through GNOME's settings daemon and this is most likely the package this occurs in.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: gnome-control-center 1:3.38.1-1ubuntu1
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu51
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.455
CurrentDesktop: ubuntu:GNOME
Date: Sun Nov 22 02:03:00 2020
LiveMediaBuild: Ubuntu 21.04 "Hirsute Hippo" - Alpha amd64 (20201120)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-control-center
UpgradeStatus: No upgrade log present (probably fresh install)

[Daniel van Vugt (vanvugt)]

Laptop brightness is unfortunately not an attribute of the screen itself, but of the backlight. So Linux can only easily control the brightness of those screens with known backlight interfaces, which means the laptop monitor only.

If you wish to adjust the brightness of any other monitor then you should do so using the monitor's own buttons so as to not lose colour detail. If you don't mind losing colour detail then you can also use something like:

  xrandr --output NAME --brightness VALUE

Even if interfaces did exist to implement what you want, we could still never know the relative brightness of different models of LCD panel. So choosing one brightness value for two different screens would be kind of useless. Safe to say this will never be implemented, at least not as a single brightness value.