Effect on the spectral line profile of varying the angular quadrature, and of including horizontal velocities (vh) in the RT computation. The figure shows profiles calculated from a single pixel of the simulation. In panel (a) we compare spectra computed assuming a plane-parallel atmosphere, neglecting horizontal velocities, with two different angular quadratures: the Gauß-Legendre (GL) quadrature and the Carlson (1963)-A4 quadrature. In panel (b) we compare the computations from the plane-parallel atmosphere without horizontal velocities with a computation that takes the horizontal velocities into account. In panel (c) we compare the computed spectrum from the 1D plane-parallel atmosphere with horizontal velocities with the spectrum computed in full 3D at the same location.