Solved [Laptop] Setting display brightness

After a way to much home office, I got myself out of that cave and are blinded by the brightness of the world. At least in comparison to my screen.
How can I manually set the display brightness?

When I plug into a power cord the display jumps to a high brightness.
When I start with a power cord plugged in, the display is dimmed.
When I replug a power cord after starting with it plugged in or not, the display jumps to a higher brightness.

This happens with and with out an X server.
I guess it is not supprising that `xrandr --output eDP-1 --brightness ${B}`
Oversaturates the whites for `B` > 1 and goes more dim for `B` < 1.

sysctl Ahw.i915kms.enable_dpcd_backlight=1
did also not change anything apparent.

While the manual has `acpi_video(4)`, I find no such entry with
sysctl hw.acpi | grep video
On a thinkpad T470p with FreeBSD 13.0-RELEASE FreeBSD 13.0-RELEASE #21 r369549M.
With some power savings one as in `/etc/rc.conf`:
powerd_flags="-a hiadaptive -b adaptive"

Any suggestions are very welcome!

Just realized I wort this all with markdown. Will convert to the forum syntax as soon as I am in a dark enough place again! Have mercy!
On some laptops. the function keys will work to dim brightness. There's also a command that is escaping me at the moment, and be a Linux only thing, but I cannot remember it nor find it with a web search. But maybe, someone reading this will know of it. I remember using it and it working, but I don't remember if it was Linux or FreeBSD. It was, I am almost sure, a Lenovo yoga2 laptop.
Sorry to be of so little help, but the hope is that someone will see this and remember the little program that I am forgetting.
After kldload acpi_video those sysctls should appear.

I would try sysctl -a |grep backlight, or sysctl -a | grep brightness.

There is also x11/xbacklight, which uses Xorgs RandR extension (whatever that is..).

Powerd does not change the brightness. It only throttles the CPU down/up.

I'd also look at /etc/devd.conf, and search for acpi events, to see what script actually changes the brightness levels on your system.