Changing screen brightness
- Backlight - ArchWiki introduces different ways to adjust screen backlight, for both laptops and external monitors.
1. Laptop screen
Use xbacklight from the xorg-xbacklight package to adjust brightness
for laptop screens:
$ sudo pacman -S xorg-xbacklight
Control brightness using -set, -inc, -dec:
$ xbacklight -set 20 # set brightness to 20 $ xbacklight -inc 20 # +20 $ xbacklight -dec 20 # -20
By default, brightness gradually fades into the target value—20 steps
in 200ms. Set -steps to 1 to avoid the fading effect:
$ xbacklight -steps 1 -dec 10
Get current brightness using -get:
$ xbacklight -get 60.000000 $ xbacklight -get | sed 's/\..*//' # remove decimal part 60
i3 config:
bindsym XF86MonBrightnessDown exec --no-startup-id \ xbacklight -steps 1 -dec 10 &&\ notify-send -t 1000 "Laptop brightness $(xbacklight -get | sed 's/\..*//')" bindsym XF86MonBrightnessUp exec --no-startup-id \ xbacklight -steps 1 -inc 10 &&\ notify-send -t 1000 "Laptop brightness $(xbacklight -get | sed 's/\..*//')"
1.1. Problem: No outputs have backlight property
On a fresh installation of Arch Linux, xbacklight may produce error:
$ xbacklight -set 20 No outputs have backlight property
Installing xf86-video-intel will fix the problem:
sudo pacman -S xf86-video-intel
2. External monitors
Display Data Channel (DDC) allows computer to adjust monitor parameters
such as brightness and contrast. ddcutil is a tool that can control
monitors through DDC.
2.1. Install ddcutil
First, install ddcutil and i2c-tools:
$ sudo pacman -S ddcutil i2c-tools
2.1.1. Load I2C module
The i2c-dev module needs to be loaded manually:
$ modprobe i2c-dev
If the module is loaded properly, you should see a list of /dev/i2c-*
devices:
$ ls /dev/i2c-* /dev/i2c-0 /dev/i2c-2 /dev/i2c-4 /dev/i2c-6 /dev/i2c-8 /dev/i2c-1 /dev/i2c-3 /dev/i2c-5 /dev/i2c-7
To load the module at boot, create /etc/modules-load.d/i2c-dev.conf
with:
i2c-dev
2.1.2. Add to user group
Grant RW access to the /dev/i2c-* devices by adding the current user
to the i2c user group:
$ sudo usermod $USER -aG i2c
Reboot to see the effect, or log in to new group for the current shell (the i3 config only works after reboot):
$ newgrp i2c
2.2. Overview
Running ddcutil on my laptop produces the following warnings:
Unable to open directory /sys/bus/i2c/devices/i2c--1: No such file or directory Device /dev/i2c-255 does not exist. Error = ENOENT(2): No such file or directory /sys/bus/i2c buses without /dev/i2c-N devices: /sys/bus/i2c/devices/i2c-255 Driver i2c_dev must be loaded or builtin See https://www.ddcutil.com/kernel_module
But the tool functions properly so far, so they can be ignored. All the results shown below will have the warning removed.
Detect monitors:
$ ddcutil detect
Invalid display
I2C bus: /dev/i2c-4
DRM connector: card0-eDP-1
EDID synopsis:
Mfg id: CMN - Chimei Innolux Corporation
Model:
Product code: 5332 (0x14d4)
Serial number:
Binary serial number: 0 (0x00000000)
Manufacture year: 2017, Week: 48
DDC communication failed
This is an eDP laptop display. Laptop displays do not support DDC/CI.
Display 1
I2C bus: /dev/i2c-5
DRM connector: card0-DP-1
EDID synopsis:
Mfg id: AOC - UNK
Model: Q27P2G5
Product code: 9986 (0x2702)
Serial number: TAUNAHA006059
Binary serial number: 6059 (0x000017ab)
Manufacture year: 2022, Week: 42
VCP version: 2.2
Two monitors are detected:
Invalid display: my laptop's monitor, which, unsurprisingly, does not support DDC/CI.Display 1: external AOC monitor, which supports DDC/CI, and is on bus 5 (/dev/i2c-5). This is the target screen.
Show all VCP Feature Codes that ddcutil understands for display 1:
$ ddcutil -d 1 getvcp known VCP code 0x02 (New control value ): One or more new control values have been saved (0x02) VCP code 0x0b (Color temperature increment ): 100 degree(s) Kelvin VCP code 0x0c (Color temperature request ): 3000 + 35 * (feature 0B color temp increment) degree(s) Kelvin VCP code 0x10 (Brightness ): current value = 50, max value = 100 VCP code 0x12 (Contrast ): current value = 50, max value = 100 VCP code 0x14 (Select color preset ): 6500 K (0x05), Tolerance: Unspecified (0x00) VCP code 0x16 (Video gain: Red ): current value = 50, max value = 100 VCP code 0x18 (Video gain: Green ): current value = 50, max value = 100 VCP code 0x1a (Video gain: Blue ): current value = 50, max value = 100 VCP code 0x1e (Auto setup ): Auto setup not active (sl=0x00) VCP code 0x20 (Horizontal Position (Phase) ): current value = 0, max value = 100 VCP code 0x30 (Vertical Position (Phase) ): current value = 0, max value = 100 VCP code 0x52 (Active control ): Value: 0x00 VCP code 0x60 (Input Source ): DisplayPort-2 (sl=0x10) VCP code 0x62 (Audio speaker volume ): Volume level: 80 (00x50) VCP code 0x6c (Video black level: Red ): current value = 80, max value = 100 VCP code 0x6e (Video black level: Green ): current value = 80, max value = 100 VCP code 0x70 (Video black level: Blue ): current value = 80, max value = 100 VCP code 0x7e (Trapezoid ): Maximum retries exceeded VCP code 0x86 (Display Scaling ): Max image, no aspect ration distortion (sl=0x02) VCP code 0x87 (Sharpness ): current value = 50, max value = 100 VCP code 0xac (Horizontal frequency ): 1093 hz VCP code 0xae (Vertical frequency ): 60.00 hz VCP code 0xb2 (Flat panel sub-pixel layout ): Red/Green/Blue vertical stripe (sl=0x01) VCP code 0xb6 (Display technology type ): LCD (active matrix) (sl=0x03) VCP code 0xc6 (Application enable key ): 0x0040 VCP code 0xc8 (Display controller type ): Mfg: RealTek (sl=0x09), controller number: mh=0x00, ml=0x00, sh=0x00 VCP code 0xc9 (Display firmware level ): 0.1 VCP code 0xca (OSD/Button Control ): OSD disabled, button events enabled (sl=0x01), Host control of power unsupported (sh=0x00) VCP code 0xcc (OSD Language ): Chinese (simplified / Kantai) (sl=0x0d) VCP code 0xd6 (Power mode ): DPM: On, DPMS: Off (sl=0x01) VCP code 0xdc (Display Mode ): Standard/Default mode (sl=0x00) VCP code 0xdf (VCP Version ): 2.2
There are a lot of entries. The one we are interested in is the
brightness (VCP code 0x10), currently at 50, the maximum being 100:
VCP code 0x10 (Brightness ): current value = 50, max value = 100
2.3. Changing & querying brightness
Use setvcp to change brightness in different ways:
$ ddcutil setvcp 10 25 # set brightness to 25 $ ddcutil setvcp 10 + 5 # brightness +5 $ ddcutil setvcp 10 - 5 # brightness -5
Use getvcp (or get) to obtain current brightness:
$ ddcutil getvcp 10 # get current brightness VCP code 0x10 (Brightness ): current value = 25, max value = 100 $ ddcutil getvcp --brief 10 # brief output: VCP <CODE> C <CUR> <MAX> VCP 10 C 25 100 $ ddcutil getvcp --brief 10 | cut -d' ' -f4 # only get current value 25
2.4. Speedup
The above use of ddcutil is simple, yet frustratingly slow:
$ time sh -c "ddcutil setvcp 10 25 && ddcutil get --brief 10"
VCP 10 C 25 100
real 0m1.079s
user 0m0.061s
sys 0m0.105s
Simply setting and getting brightness takes a whole 1 second.
According to rockowitz's comment:
ddcutilexamines each/dev/i2c-*device on startup, which is a slow process. Specifying the monitor bus number with--bus(or-b) reduces ~180ms:$ time ddcutil setvcp 10 25 real 0m0.580s user 0m0.031s sys 0m0.068s $ time ddcutil --bus=5 setvcp 10 25 real 0m0.404s user 0m0.011s sys 0m0.017s
However, simply specifying a display number does not reduce time, as I2C devices are still examined to determine which one is used:
$ time ddcutil --display=1 setvcp 10 25 real 0m0.579s user 0m0.028s sys 0m0.066s
Most of the elapsed time is spent in pauses mandated by the DDC spec. Use
--sleep-multiplierto adjust the length of time spent in mandated sleep. For example,--sleep-multiplier=0.2multiplies the sleep time by0.2:$ time ddcutil -b 5 --sleep-multiplier=0.2 --brief get 10 VCP 10 C 25 100 real 0m0.109s user 0m0.018s sys 0m0.008s
This significantly reduces the overall time, reaching 0.1s. However, reducing sleep time may incur DDC errors, see the original comment for detail.
- When using multiple monitors, using
--asyncto add some parallelism.
To summarize, specifying bus number and reducing sleep time together
reduce the overall time to ~270ms, a 3.7x speedup (the redirection &
tail command is used to filter out error messages mentioned earlier):
$ time sh -c \ "ddcutil --bus=5 --sleep-multiplier=0.2 setvcp 10 50 >/dev/null &&\ ddcutil --bus=5 --sleep-multiplier=0.2 --brief getvcp 10 | tail -n1" VCP 10 C 50 100 real 0m0.267s user 0m0.053s sys 0m0.038s
Finally, the i3 config to ±10 brightness:
bindsym Shift+XF86MonBrightnessDown exec --no-startup-id \
ddcutil --bus=5 --sleep-multiplier=0.2 setvcp 10 - 10 &&\
notify-send -t 1000 "External brightness $(ddcutil --bus=5 --sleep-multiplier=0.2 --brief getvcp 10 | tail -n1 | cut -d' ' -f4)"
bindsym Shift+XF86MonBrightnessUp exec --no-startup-id \
ddcutil --bus=5 --sleep-multiplier=0.2 setvcp 10 + 10 &&\
notify-send -t 1000 "External brightness $(ddcutil --bus=5 --sleep-multiplier=0.2 --brief getvcp 10 | tail -n1 | cut -d' ' -f4)"