P: Custom Whitepoint adaption when changing Camera Profiles

When switching Camera profiles in LrC and “As Shot” white balance is selected, LrC will change the Temperature/Tint so the visual result is the same before and after the profile switch. Yet, when the whitepoint is set to “Custom” the numbers are not changed so the visual result is different. Since this is hard to explain here is a small screencast: notice that Temperature changes when white blaance is “As Shot” (“Wie Aufnahme”, sorry for German), but does not change when it is set to “Custom” (“Benutzerdefiniert” in German).

The user effect is the follows:

• With as shot white balance: the image is visually kept identical (apart from the different camera profile), with the same temperature and tint even though numbers change
• With custom white balance: the image visually changes, the perceived temperature and tint is different in addition to the different camera profile
• When using the white balance picker tool a neutral gray patch that was selected with one profile is not neutral gray with another profile
  This sounds clearly unexpected to the user.

Now I can understand this decision: with custom white balance the user gave us exact numbers, so they are never changed. With “As shot” the user doesn’t care about the numbers, but the photo should look the same as shot, so updating the numbers is okay. 

Yet, I’d argue that even with custom WB, the user is more interested in keeping the photo visually similar with the same visual tint and temperature and not numerical. This is a feature request to change the behavior and always adapt white balance to the color profile and not only for “As shot”.

Technical details, this is just my understanding of DNG/DCP, you can skip the rest if not interested. 

I worked with DNG/DCP profiles before, So I understand what is happening. “As shot” white balance is usually encoded in camera space (in AsShotNeutral) whereas temperature/tint give xy color space (camera independent) numbers. The conversion from camera color space to xyz is basically the camera profile. White balance always happens in camera color space. 

So

• As shot always does:
  • AsShotNeutral is already camera color space, so it can be directly used for WB correction
  • For display we need temperature/tint, so we do:
    AsShotNeutral → xy → temperature/tint
  • when the camera profile changes, the xy conversion changes, so numbers change
  • Yet, AsShotNeutral DOES NOT change, so the actual WB does not change
• With custom WB: 
  • We ignore AsShotNetural for Whitebalance instead we do 
    temperature/tint → xy → whitepoint in camera space
  • we use this white point for correction
  • Since the xy → camera space conversion changes with the profile, the WB correction is now different, even though numbers are the same

What this feature request technically asks to do is to adapt the xy white point on profile change. Let’s say we change from profile A to profile B. Then the white point is updated such that we do 

• old WP xy → profile_A → WP in camera space → profile B → new WP xy

So to make sure that the white point correct in camera color space stays the same, we transform the custom whitepoint to camera color space and back.