RGB Gamma adjustment, the standard method for control of midtone values, causes large changes in chromaticity. This paper presents an image transform algorithm that allows control of midtone values while producing chromatically-correct results. The exact algorithm requires computations only moderately more complex than those required for gamma adjustment. Approximations to the algorithm are simpler to implement than gamma adjustment, yet produce results which are more chromatically correct.
James M. Kasson, "Efficient, Chromaticity-Preserving Midtone Correction for RGB Images" in Proc. IS&T 2nd Color and Imaging Conf., 1994, pp 78 - 84, https://doi.org/10.2352/CIC.1994.2.1.art00023