[css-color] Add predefined color space for ECI RGB and few more #13266
Description
CSS Color covers the reference prior medium metric (ROMM) RGB color space specified in ISO 22028-2 as prophoto-rgb. Another part of that international standard, ISO 22028-4, specifies an exchange/working color space originally created by the European Colour Initiative (ECI) as version 2 of its eciRGB. The ICC profile is available for free; I could not find official documentation in human-readable form on their homepage, but the ICC registry is sufficiently informative.
If I understand it correctly from informal references, it is designed to match its gray axis (all i.e. when all component values are equal, R = G = B) linearly with the neutral axis of CIE Lab (i.e. where a = b = 0). This is therefore also known as LStar-RGB.
Glancing at the list at the Three Component Color Encoding Registry, the overall number of RGB color encodings and spaces in common use seems to be below two dozens and mostly coming from four sources:
- ISO 22028 (print, incl. I3A 7466 for Part 3: RIMM),
- IEC 61966-2 (sRGB etc., monitor),
- ITU-R BT.601/709/2020/2100 (TV) and
- SMPTE EG0432 (P3, cinema).
Many of these are already covered by predefined color spaces in Level 4. I assume it would be in everyone’s best interest to make them available out of the box without requiring the download of an ICC profile. Alas, some of them seem so closely related that they can be covered together by the one more suitable for CSS (e.g. ROMM and RIMM or DCI and Display P3), but I don’t feel competent enough to make that call.