I have an image with a default colour temperature of 4327K, CCT 4340 Planckian. When I switch to custom, this becomes 35.5° and 16.9%.
It was my recollection that if I were to change these custom figures, either manually or through colour picking the image, and then switch back to Planckian, CC would attempt to produce the nearest Planckian equivalent to the amended custom values.
Instead, it simply changes back to the original 4327K CCT 4340.
In other words, CC always reverts to the image’s default temperature, whenever Planckian or Daylight are selected.
I am sure this never used to happen?
I am using darktable 5.4.1 in Windows, and I can repeat the issue with a clean copy of the image in an independent portable copy of darktable.
It makes no difference whether I click on the picker to resample the colour, or manually change the hue and chroma. The moment I click on the drop down and select Planckian or daylight, the image immediately reverts to 4327K (even if I change custom to 0° hue and 100% chroma).
I’ve noticed this too, and agree it is different behaviour from when the module was first implemented. Happens not just when using ‘custom’, but all illuminants. Used to be able to change the temperature slider in ‘planckian’, then switch to ‘daylight’ and have a near match.
Has been that way for some time though, not sure when the change occurred.
Another quirk that I’ve found, is that if I reset CC to achieve the default 4327K Planckian, and immediately click custom I get 30.4° hue , 17.2% chroma and CCT 4327, which is wrong for 4327K Planckian.
If however I reset CC, to achieve the default 4327K Planckian, then move the temperature slider to another value before restoring it to 4327K, and then click custom, I get 35.5° hue, 16.9% chroma and CCT 4340, which is correct for 4327K Plankian.
In other words, going immediately from default to custom, will lead to incorrect custom values. To achieve the correct custom values, adjust and restore the default temperature, prior to clicking custom.
I noticed something similar, too. Scene referred defauld gives me a slight green tint while picking “as shot” and then “custom” leads to slightly different values.
I would have to dissect this and see if indeed there is a bug from looking at the code but CCT can have several hue /chroma combinations that will statisfy the same CCT value and the CCT value is not a reversable calculation so some of the behaviour may reflect how that is being handled when you go back and forth…
Also this comes from AP and the DT manual is similar I think…
" When switching from one illuminant to another, the module attempts to translate the previous settings to the new illumninant as accurately as possible. Switching from any illuminant to custom preserves your settings entirely, since the custom illuminant is a general case. Switching between other modes, or from custom to any other mode, will not precisely preserve your settings from the previous mode due to rounding errors.
Other hard-coded illuminants are available (see below). Their values come from standard CIE illuminants and are absolute. You can use them directly if you know exactly what kind of light bulb was used to illuminate the scene and if you trust your camera’s input profile and reference (D65) coefficients to be accurate. Otherwise, see caveats below."