[kwin] [Bug 524038] HDR ICC profile in KWin 6.7.4 crushes PQ blacks and compresses midtones

https://bugs.kde.org/show_bug.cgi?id=524038

--- Comment #7 from [email protected] ---
One more relevant profile-format detail: the profile attached to this report is
ICC v2.2, so it cannot contain the CICP tag added in ICC.1:2022 profile version
4.4.

ICC v4.4 CICP can identify a display profile's data encoding as Rec.2020 PQ RGB
and specify full or narrow range. For example, 9-16-0-1 identifies full-range
Rec.2020 PQ RGB. CICP identifies primaries, transfer characteristics, matrix
coefficients and range. It does not define display peak luminance or replace
the profile transform.

I searched KWin 6.7.4 and current master and found no CICP parsing. The HDR ICC
path currently constructs the profile-side transfer function as gamma 2.2 in
DrmOutput::createColorDescription(). This means that generating a valid v4.4
profile with a PQ CICP tag would not currently help because KWin would ignore
the tag.

This looks like an additional missing part of HDR display-profile support,
separate from the three bugs described above. KWin likely needs to read CICP
for v4.4 display profiles and use it to describe the profile's device encoding.
We can generate a v4.4 Rec.2020 PQ test profile for validation once there is a
code path that consumes the tag.

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