Color Balance added a share of each slider weighted by the pixel's
lightness, then (Preserve Luminosity) rescaled to the old luma. On the
corpus's Photoshop-made file (photoshop/adjustments/rgb8/color-balance.psd,
all three tones moved, Preserve Luminosity on) that was up to 43 levels
off on 99% of the pixels.
Measured on Photoshop 27.11 (scripted: a gray ramp and 16 pure colours
through each slider at ±25, ±50 and ±100, with Preserve Luminosity on
and off, plus two and three tones and two sliders at once), every
slider is a levels-and-gamma curve on each channel:
- Preserve Luminosity off: midtones is gamma 2^-a on the slider's own
channel; shadows gamma 2^(-a/2), after a black point at 0.39·|a| when
lowered; highlights gamma 2^(-a/2), after a white point at 1 - 0.39·a
when raised.
- Preserve Luminosity on: midtones gamma 2^(-a/2) on its channel and
2^(a/2) on the other two; shadows and highlights go by the differences
between a tone's sliders: a black point at 0.39·(max - own) and a
white point at 1 - 0.39·(own - min) per channel.
- Tones apply highlights, shadows, then midtones.
All 88 measured cases match within 1 level, except midtones +100 below
about 28 of 255 (Photoshop flattens the steepest gamma near black, by
up to 9 levels). The corpus file is now within 4 levels (mean 1.4, was
20.9).
The GPU builds the same curves into per-channel tables and uses the
Levels/Curves kernel; the old Color Balance shader case is gone.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
3794ad471f