Written: 2020-08-01 20:00 +0000
A short post detailing the many trials and tribulations of setting brightness on multiple monitors in tandem.
As regular readers might know, I have a multi-screen setup, which accounts for having touch enabled on my primary laptop screen (detailed here). A failing of this setup was that I was not able to control the brightness of both monitors at the same time.
Since I use
i3, my brightness control is simply done with
bindsym lines as follows1:
bindsym XF86MonBrightnessDown exec light -U 10 bindsym XF86MonBrightnessUp exec light -A 10
Note that to get the right
bindsym I use screenkey with the
keysyms preference. My software of choice was
Unfortunately, my existing setup (with
light, since that is in the Arch
community repo) did not actually allow dimming external screens arbitarily. To be more exact,
Usage: light [OPTIONS] [VALUE] Commands: -H, -h Show this help and exit -V Show program version and exit -L List available devices -A Increase brightness by value -U Decrease brightness by value -T Multiply brightness by value (can be a non-whole number, ignores raw mode) -S Set brightness to value -G Get brightness -N Set minimum brightness to value -P Get minimum brightness -O Save the current brightness -I Restore the previously saved brightness Options: -r Interpret input and output values in raw mode (ignored for -T) -s Specify device target path to use, use -L to list available -v Specify the verbosity level (default 0) 0: Values only 1: Values, Errors. 2: Values, Errors, Warnings. 3: Values, Errors, Warnings, Notices. Copyright (C) 2012 - 2018 Fredrik Haikarainen This is free software, see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE
Clearly it is possible to target specific devices, but for arbitrary additions this is quite tough. Additionally, the project has been more or less been stuck in maintainence mode for a while now.
The first hurdle faced in o was actually getting the external monitor to expose the right controls. This is accomplished neatly with
ddci-driver found here. As demonstrated (courtesy of the ArchWiki):
# Load module sudo modprobe ddcci_backlight # Check that it worked sudo ddcutil capabilities | grep "Feature: 10" sudo ddcutil getvcp 10 # Set brightness sudo ddcutil setvcp 10 70
One of the obvious caveats of this technique is that
sudo access or a dedicated
polkit agent is required. My preferred method of loading this comes from a comment on the
ddcci-driver-linux-dkms page of the AUR:
# Put in /etc/systemd/system/ddcci-backlight.service: # https://aur.archlinux.org/packages/ddcci-driver-linux-dkms/ # Placing "ddcci_backlight" into /etc/modules-load.d # leads to hang on boot with external (HDMI) monitor # connected to the laptop, so we need to add the module later. # And ddcci_backlight can't detect monitor during sddm.service startup. [Unit] After=multi-user.target Before=sddm.service [Service] Type=oneshot ExecStart=/usr/bin/modprobe ddcci_backlight ExecStop=/usr/bin/modprobe --remove ddcci_backlight RemainAfterExit=yes [Install] WantedBy=multi-user.target
This is then activated with a standard
systemctl enable ddcci_backlight.service command. At this point, the device interface should be exposed to most backlight controllers.
This is the most obvious of all methods, and does not even require the
ddcci-driver. We will simply tweak the brightness with
# Get devices xrandr | grep " connected" # Tweak xrandr --output DP1 --brightness 0.2
Note that this is an in-exact method, since it actually adjusts the gamma value instead, and it effectively tints your screen rather than modifying the brightness.
clight is an excellent, highly performant alternative to redshift, but it tends to force the main screen brightness to zero. Nothing which can’t be configured away, but in practice, I work late and tend to turn off the tint anyway. This requires a daemon to be run, as well as needing to be turned on manually for
i3. A very elegant additional feature gained by using
clight is that external monitors are turned off automagically when lockscreens are activated.
brillo is one of the newer controllers, and is pretty actively developed. The interface is almost exactly like
light, and unlike
clight there is no need to use a daemon. It meshes perfectly with a key-press based system like
i3 and also has controls for keyboard LEDS as well as for smoothed ramping up and down of the brightness. Most importantly, it features an
-e flag which sets the brightness across all output devices. Essentially this means my configuration is simply modified to:
bindsym XF86MonBrightnessDown exec brillo -e -U 10 bindsym XF86MonBrightnessUp exec brillo -e -A 10
tl;dr moving from
ddcci-driver-linux-dkms was a fantastic idea.