Screen brightness for non-Laptop displays

Yesterday in the office, we were using a projector to collaborate on something. We had turned off the lights and lowered the blinds in order to improve the projector's readability. My screens are of course configured for a brightly-lit room, so I was now staring into two enormously bright screens which hurt my eyes incredibly.

On a laptop, I would just turn the brightness down without really thinking about it. But with standard desktop displays, this involves digging through the display's on-screen menu, fiddling with knobs that only work part of the time, and the prospect of having to repeat this step when we're done working to get back to normal. This sucks. So I looked for a way to set the brightness in software.

The first Google hit hinted at xgamma, which sucks because it just messes up the colors and looks ugly as hell. The manpage even states "meh, you shouldn't be using this tool anymore because it's probably broken. Use xrandr." So I did, and xrandr works:

xrandr --output HDMI1 --brightness 0.5 --output HDMI2 --brightness 0.5

Packed into a bash function:

function b() {
    xrandr --output HDMI1 --brightness "$1" --output HDMI2 --brightness "$1"
}

Now I can adjust the brightness using b 0.5 in a shell.