verify-capture has said for weeks that the RMSE "has a floor" because the capture
carries the game's tone ramp. Nobody measured the floor. Aligned luma RMSE is
10.60; one fitted monotone transfer curve takes it to 4.77.
A fitted curve always removes something, so: the same fit against an unrelated
capture (EXTRAS) removes 27 %, against the matched pair 55 %. The tone difference
is pair-specific, not the fit absorbing variance.
Nothing in the port changes. I cannot say which side's tone is wrong and I am not
tuning to match. What is new is a bound: 4.77 of the headline number is not tonal.
Also recorded: my first pass at this compared a 1280x720 render against a 1279x675
capture and reported a uniform 75.6 % difference. verify-capture corrects exactly
that, in a comment I had read, and I reached past it.
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