Disable brightness controls for systems using external displays

[ayoungethan]

As per elementary/wingpanel-indicator-power#163

on 5.1.7

The brightness control settings are irrelevant for systems using only external displays. Suggest that both the display brightness slider and the "automatically adjust brightness" button be dimmed out/disabled when only external displays are detected, to avoid the appearance of hardware control capacity that does not exist.

This is similar, eg, to behavior that disables software control of bluetooth when no bluetooth device is connected.

[cassidyjames]

Some external displays may actually support brightness controls; we should check if brightness control is supported on any enabled displays before disabling stuff.

[ayoungethan]

https://en.wikipedia.org/wiki/Display_Data_Channel#DDC/CI

a means for a computer to send commands to the monitor, as well as receive sensor data from the monitor, over a bidirectional link.
Despite its ubiquity in post-2016 displays, DDC/CI is not generally used by the operating system by default for brightness control on external displays.[9]

I wonder if it is a long-standing myth amongst computer users and developers that external displays do not support bi-directional communication and OS-sourced commands? Most of my searching has come up with people saying, "External displays don't support this." But if the above is true, in fact most displays now support this; it is the OS that simply doesn't make use of it!

[Maccer1]

Coming back to this:

The tool: ddccontrol

Short explanation: how to change brightness

To add support for new models: LANG= LC_ALL= ddccontrol -p -c -d and share it with ddccontrol-users@lists.sourceforge.net

This tool works but I'm unsure if they are actively developing it. Anyhow, I could change the brightness easily first time I opened it with a generic profile.

There is also ddcutil but I still need to understand it.

[Maccer1]

To change brightness with ddcuitil from cli:

sudo ddcutil detect --> will provide list of displays and specify the ones supported.
sudo ddcutil getvcp known --display Y --> will provide VCP codes of display Y.

In my case:

VCP code 0x10 (Brightness ): current value = 30, max value = 100

NOTE: According to standard page 17. Brightness always has 0x10 code.
To change brightness you need to change the VCP code value with:

sudo ddcutil setvcp 10 XXwhere XX is the brightness value and 10 comes from 0x10

---

Extra info:

• You can install i2c-tools and add your user to the group i2c to do this without root

• Open Issue: Check if brightness is always VCP 0x10.

• Nvidia specific fix: https://www.ddcutil.com/nvidia/

[Maccer1]

Related to elementary/wingpanel-indicator-power#41