Improved calculation of DNG white balance from AsShotWhiteXY

[kmilos]

Attempt to address darktable-org/darktable#14805 @jenshannoschwalm @mcogoni

Not sure if an additional chromatic adaptation step is needed on the as shot XYZ values (relative to D65) before using the D65 matrix on them...

[LebedevRI]

Hmm yeah, hardcoding D65 here clearly looks suspicious.

[codecov]

Codecov Report

Patch coverage: 6.66% and project coverage change: -0.03% ⚠️

Comparison is base (060cb05) 59.12% compared to head (92ce5be) 59.10%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #515      +/-   ##
===========================================
- Coverage    59.12%   59.10%   -0.03%     
===========================================
  Files          232      232              
  Lines        13892    13898       +6     
  Branches      1937     1938       +1     
===========================================
  Hits          8214     8214              
- Misses        5545     5551       +6     
  Partials       133      133              

Flag	Coverage Δ
benchmarks	8.18% <0.00%> (-0.01%)	⬇️
integration	47.51% <7.14%> (-0.02%)	⬇️
linux	57.02% <7.14%> (-0.03%)	⬇️
macOS	18.68% <0.00%> (-0.01%)	⬇️
rpu_u	47.51% <7.14%> (-0.02%)	⬇️
unittests	18.22% <0.00%> (-0.01%)	⬇️
windows	∅ <ø> (∅)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/librawspeed/decoders/DngDecoder.cpp	52.68% <6.66%> (-0.67%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[LebedevRI]

Ok, i agree that this is an improvement, it very obviously fixes an existing CRW_8531.DNG sample.
This roughly matches what Translating White Balance xy Coordinates to Camera Neutral Coordinates says in the DNG spec, other than the rest of advanced steps mentioned there.

This "regresses" things in case where CalibrationIlluminant* is missing (e.g. Nikon LS-5000 sample),
or where we don't have a color matrix w/ CalibrationIlluminant = D65
(since that is what DngDecoder::parseColorMatrix() requires).

Thank you!