The native Lab path (compose::native_lab) worked a stack out on the
stored L, a and b but kept each adjustment layer's result unrounded, so
the screen showed values between two samples where Photoshop, which
stores every result at the document's depth, has whole bytes (or
1/32768 steps). On saturated colours half a byte of L is a couple of
RGB levels. Each adjustment's result is now rounded to the depth's
samples (adjustment_quantum: 8 and 16 bits; 32-bit stays unrounded),
as the RGB compositor already does.
photoshop lab8 curves.psd goes from 0.0084 to exact (PASS); Photoshop
oracles 185 -> 186, floor 186. lab8 brightness-contrast (modern) stays
DIFF: our modern curve is within half a byte of Photoshop's on L, not
exact, so rounding now shows its misses (0.02 % -> 1.05 % of pixels).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
timothydaly1andClaude Opus 5.5
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