color calibration CAT16 blackened areas - bug or feature?

kofa · May 26, 2024, 6:53am

Given the following colour wheel:

Applying the preset swap G and B:

Then setting adaptation to CAT16:

OpenCL on/off does not matter. Darktable master version.

@Pascal_Obry , @hannoschwalm , is this a bug or a feature?

clip negative rgb does not fix it:

Not even a 2nd instance does (basic channel mixer + clipping option):

It is unaffected by gamut compression:

But can be avoided by adding another instance before the channel swap, and applying gamut compression there:

The black area is present with working space set to default (Rec 2020), sRGB (web-safe), Rec 709, linear Rec 709, HLG P3 RGB, Display P3 RGB, linear ProPhoto RGB and many others, but not with Adobe RGB (compatible):

Also not with sRGB-elle-V2-g22.

hannoschwalm · May 26, 2024, 7:34am

Looks strange on first thinking.

kofa · May 26, 2024, 7:42am

Thanks. Opened color calibration CAT16 blackened areas · Issue #16875 · darktable-org/darktable · GitHub.

I assume the fact that the blues basically disappear (are replaced by purples) is due to Rec 2020 ‘blue’ being something we don’t really perceive as ‘blue’? No such problem with sRGB / Rec 709, so after a simple swap (no messing with the adaptation, leaving it as none):

Swapping B and G in linear Rec 2020: