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color: st2084_pq dictates min/max_lum

Pekka Paalanen requested to merge pq/wayland-protocols:mr/pqlumi into color

ST 2084 a.k.a perceptual quantizer is an absolute luminance encoding dealing in cd/m². This sets it apart for most other transfer characteristics that are relative to the target display dynamic range.

Therefore the primary color volume minimum and maximum are fixed by the ST 2084 specification, and cannot be changed. The reference white is not fixed and remains settable.

ST 2084 allows displays to deviate from the specified EOTF due to hardware limitations. How that works is not defined. Therefore code point zero may produce more than 0 cd/m², but accoding to BT.2100 should be less or equal to 0.005 cd/m². The difference between decoding with the exact EOTF and then tone-mapping to display capability, and decoding straight to display capability is mostly philosophical. The ideal presentation is defined to be exactly as the ST 2084 specified EOTF.

I believe the image description should reflect this ideal presentation.

This is the minimal change. We could also split it into two requests: min + max, and reference level.

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