Comment by JyrkiAlakuijala

One more interesting and substantial difference between Oklab and XYB is that XYB includes biases before the nonlinearity, i.e., one can consider the M1 matrix as a homogeneous matrix. These biases make the receptive model (more) linear close to zero, and the non-linearity ramps up when one goes further into high intensity. The idea there is to model the spontaneous opsin isomerization in the receptors. I believe sRGB approximated this by gluing a small linear ramp into the nonlinearity with if-then-logic.