Color management

[lifthrasiir]

Color management in JPEG XL is a complicated story. Ideally it's the best possible design: every JPEG XL codestream has a defined color profile (either ICC profile or abbreviated color space parameters) and it should be converted to the display color space. Unfortunately the status quo is not that ideal.

• ICCv2 and v4 are yet another 100-plus-page standards.
• The full conversion process is a many-to-many relation; not only the display color space is defined, but the image color space can vary.
• There is no public domain color management system (CMS) to my knowledge.
• There is no single-file CMS to my knowledge either. The closest is probably skcms, a part of Skia, which consists of two source files and two header files, and it is already ~4000 lines of C++.

As such, I don't think J40 will ever implement the full ICC profile by itself (at least in C). This is also the case in libjxl, where either skcms or Little CMS 2 can be used, but it is a bit baffling to require an external library to implement the full format. Any other ideas are welcomed.

Anyway, this decision leads to the following subtasks:

• Fully implement the ICC codec
• Handle abbreviated color space parameters
• Implement and expose API functions for external CMSes (e.g. J40_USE_LCMS2)