Diagram illustrating the basic relationship of the rest-frame frequencies of spectral lines (ν_CMF) to observer’s frame frequency (ν_obs) for one particular (non-core) p-ray in the spherically symmetric geometry (p,z geometry). Shown are two spectral lines of different opacity χ which get shifted across the observer’s frame frequency by the velocity field in the wind. The dots represent the stepping points of the adaptive microgrid used in solving the transfer equation in the radiative line transfer⁵.