[Feature request] Support DDC/CI backlight setting

[isf63]

I believe currently lxqt-config-brightness only supports setting monitors' backlights via ACPI (/sys/class/backlight/). Many external monitors do not use ACPI but instead DDC/CI (https://wiki.archlinux.org/title/Backlight#External_monitors). It'd be wonderful if software backlight setting for DDC/CI monitors could be incorporated into LXQt.

Current Behavior

'Backlight' slider is greyed out in lxqt-config-brightness due to the external monitor not having ACPI backlight support.

Possible Solution

Look at ddcutil and see how it sets backlights. (Note: ddcutil requires the i2c-dev kernel module)

Context

Software setting of backlight is very convenient, especially so when not on laptops with dedicated keyboard keys for setting screen brightness.

System Information

• Distribution & Version: Arch Linux
• Kernel: Linux 6.3.2
• Package version: 1.2.0.14.g36868e68

[stefonarch]

Went trough the issues searching for one specific and stumpled upon that, didn't know that it was possible (actually only by command line here as the driver won't build: ddcutil setvcp 10 70)

"Brightness" under X11 shows one backlight and the gamma brightness for each/both monitor here.

[isf63]

"Brightness" in XRandr I believe sets the pixel values (lower brightness - more black ...) Gamma also I believe sets pixel values.

For embedded screens, brightness has a simple driver /sys/class/backlight/, but for external monitors you have to send a DDC/CI command.