Yes, please do the HSV series as well. If you can, please record the HSV values of the sample pixels in both renders.

The formula for this one seems to be:

y(x) = x * (1-exp(-eta * x / 255)),  0 <= x and x <= 255

where
x = the input pixel channel value [R, G, or B],
y = corresponding output value,
eta = the "exponent" parameter, and
exp() = the exponential function for the base of natural logarithms.

This is a pretty standard saturation-type expression.

Whichway

(Of course, I realize that as soon as somebody else checks my figures, I'll be wrong. )