just going with 45deg slope in overbrights, just shifted by the last value in LUT
This. Though kind of hard to think of it that way since it's not a 1D LUT. I would imagine using the same slope as the last segment of the LUT would cause unintuitive effects if the slope is negative or very steeply positive.
Negative values are clamped to 0.0, there's no extrapolation for those.
