4759d12b57
The non-legacy B/C was a linear scale that hard-clipped; Photoshop's modern
B/C is a pair of smooth curves that pin pure black and white. Reverse-
engineered from Photoshop 2026 ground truth (a 0..255 ramp pushed through the
app via executeAction BrgC, useLegacy=false):
- Contrast: symmetric cubic-Hermite S pivoting at 0.5 (end slope 1-c/128,
centre 1+c/128) -- near-exact, <=0.5/255 vs Photoshop.
- Brightness: line of slope 1.375^(b/50) rolled off to (1,1) by a v^P white
anchor -- close, approximate at the extremes (the real roll-off is a spline).
Applied brightness-then-contrast; matched in the wgsl compositor (mbright/
mcontrast) so GPU == CPU oracle (gpu parity 18/18). Golden tests pin both
curves to the captured Photoshop samples.
Corpus oracle: brightnesscontrast_grayscale.psd max error 0.478 -> 0.086
(bad px 91% -> 29%); 113 pass / 30 differ unchanged (no regressions).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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