light:change the overall brightness of the output. this is the reciprocal lambda parameter of the weibull distribution
contrast:contrast of the output. this is the k parameter of the weibull distribution
bias:offset to add to black pixels. use to lift for low contrast renders and to correct elevated raw black level
colour:how to reconstruct colour after applying the curve to the brightness. darktable ucs has in general better colours, but hsl is more robust for edge cases near black
