Effect of reduced horizontal resolution on the 3D-NLTE spectral line profile of the middle component of the O i IR triplet. Top panels: synthetic spectral lines computed with full resolution (black line) and with resolution reduced by a factor 3 in each horizontal direction (red dots). Bottom panels: difference in residual intensity between synthetic spectral lines computed with full and reduced horizontal resolution. A negative difference means that the spectral line computed with full resolution is stronger at the particular wavelength. Left and right panels show the results at μ = 0.2 and μ = 1.0, respectively.