supports an arbitrary number of displays!

Brightness Controller changes gamma and not hardware setting and as such is equivalent to xrandr.

see also

• How to improve your WFH lighting to reduce eye strain - When it comes to light brightness, too much is just as problematic as too little
  • An even, diffused lighting environment is best for the eyes.
  • Even the perfect lighting environment will fatigue you — take breaks, and think beyond lighting!

Display Data Channel

Enable the display to communicate its supported display modes to the adapter and that enable the computer host to adjust monitor parameters, such as brightness and contrast.

Need to be member of i2c group to be usefull (or sudo)

ddcutil / Doc - a Linux program for querying and changing monitor settings, such as brightness and color levels. - Display Brightness Slider for Gnome Shell - Brightness slider for all the monitors detected by ddcutil

DDCcontrol

DC/CI kernel module allows any tool which can use the kernel’s backlight system (/sys/class/backlight) to drive the backlight on a DDC/CI monitor. - xbacklight

MacOS

• MonitorControl - Controls your external display brightness and volume and shows native OSD.

see also

• ACPI