HDR and color management in KWin, part 6: Fixing night light

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2025-01-07 21:30:05

Most operating systems nowadays provide a feature like night light: Colors are adjusted over the course of the day to remove blue light in the evening, to potentially help you sleep1 and make your eyes more comfortable.

Linux is no different; there’s Redshift to apply this on X11 desktops, and since many years ago desktop environments also ship it built in. However, there’s always been a rather annoying problem with these implementations: None of them were color managed!

On a low level, these implementations set the video cards gamma curves to multiply the red, green and blue channels of the image that’s sent to the screen.

The actual intention behind the feature though is to change the white point of the image. The white point is, as the name implies, the color that we consider to be “white”. With displays, this usually means that red, green and blue are all at full intensity. By reducing the intensity of green and blue we can change that white point to some other color.

The scenario I described only concerns itself with white, but there are more colors on the screen… If you multiply the color channels with some factors, then they get changed as well. To show how much and why it matters, I measured how this naive implementation behaves on my laptop.