The White-Point Preserving Least Squares (WPPLS) algorithm is a method for colour correction that constrains the white point to be exactly mapped into its correct XYZ equivalent. For printers, however, the mapping is from device coordinates to colorimetric densities: the devicewhite is thus mapped into a zero vector and the WPPLS method cannot go forward. Here we use a polynomial regression model and specify that both white (the zero vector) and an average grey be exactly mapped. Moreover we extend the method to accurately but approximately map a subspace of the entire achromatic curve, thus reproducing the neutral tones with far greater accuracy.
Mark S. Drew, Graham D. Finlayson, M. Ronnier Luoy, Ján Morovič, "Colour Printer Characterization by Regression with Greyspace Constraint" in Proc. IS&T 6th Color and Imaging Conf., 1998, pp 193 - 196, https://doi.org/10.2352/CIC.1998.6.1.art00041