HDR and color management in KWin, part 5: HDR on SDR laptops
Quoting: HDR and color management in KWin, part 5: HDR on SDR laptops | Xaver’s blog —
A big part of what a desktop compositor needs to get right with HDR content is to show SDR and HDR content properly side by side. KWin 6.0 added an SDR brightness slider for that purpose, but that’s only half the equation - what about the brightness of HDR content?
When we say “HDR”, usually that refers to a colorspace with the rec.2020 primaries and the perceptual quantizer (PQ) transfer function. A transfer function describes how to calculate a real brightness value from the “electrical” signal encoded in the content - PQ specifically has encoded values from 0 to 1 and brightness values from 0 to 10000 nits. For reference, your typical office monitor does around 300 or 400 nits at maximum brightness setting, and many newer phones can go a bit above 1000 nits.
Now if we want to show HDR content on an HDR screen, the most straight forward thing to do would be to just calculate the brightness values, write them to the screen and be done with it, right? That’s what KWin did up to Plasma 6.1, but it’s far from ideal. Even if your display can show the full range of requested brightness values, you might want to adjust the brightness to match your environment - be it brighter or darker than the room the content was optimized for - and when there’s SDR things in HDR content, like subtitles in a video, that should ideally match other SDR content on the screen as well.
Luckily, there is a preexisting relationship between HDR and SDR that we can use: The reference luminance. It defines how bright SDR white is - which is why another name for it is simply “SDR white”.