In Lab documents Photoshop evaluates these adjustments on the Lab channels,
not on the screen RGB colour:
- Invert and Posterize act on L, a and b alike (normalised L/100,
(a+128)/255, (b+128)/255; Invert is n -> 1 - n).
- Brightness/Contrast (modern and legacy) applies its tone curve to L only;
a and b are unchanged.
- Threshold splits on L alone and outputs neutral black or white.
- Gradient Map is driven by L (Reverse uses 1 - L).
Adjustment layers (CPU compositor) and destructive Image > Adjustments both
go through the new `adjust::apply_in`; the GPU compositor defers these
adjustments to the CPU in Lab documents. Levels and Curves keep their Lab
records; other adjustments stay on the screen colour.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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