this post was submitted on 01 Apr 2025
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I should elaborate on why the "Peak white" stuff is wrong, they give this math here for mapping linear luminance. This can be really confusing, "what do we map the references to" well if PQ "graphics white" is 203, should we map sRGB to 203? clearly not, at least not always as implied by BT.2408.
the question as to what we map SDR content to in an HDR space is complex, and in many cases almost certainly not some number that we can do 1:1 mapping with, which is why specifications for inverse tonemapping exist. for instance BT.2446 defines multiple tone mapping algorithms to go from SDR->HDR->SDR or HDR->SDR->HDR or any step inbetween with minimal content loss and fidelity loss.
we cannot do a simple one size fits all function and expect everything to be hunky dory
Again, the reference luminance mapping is all about how applications should use the Wayland protocol.
How to map SDR to HDR can indeed be made much more complicated, from simple gamma adjustments to some full blown ITM meant for images or videos, like what BT.2446 suggests, but as far as applications are concerned, those are edge cases that they don't really need to be prepared for.
It's not like they have a different choice - unless the compositor supports custom reference luminance levels (which KWin does, but not all others do), then they need some logic to calculate peak luminance levels. If the compositor steps outside of those common expectations for reference luminance mapping, then the result may not be ideal, but there is no way for the application to do better.