Comparison of sky-subtraction methods, over a section of the Illustris simulation images. Top-left panel: simulated Illustris Deep Field B section. Top-right panel: same image with a 1 Jy wide Poissonian noise added. Middle-right panel: simulated Illustris Deep Field B with noise masked with SEXtractor (1σ threshold level). Middle-left panel: same image masked with NoiseChisel (default configuration). Bottom panel: estimated sky background as a function of different methods. Black dots: reference methods. Yellow diamond: method applied in the present paper. Notice that all methods overestimate the true sky background level, which is equal to zero in the simulated image. See the legend and axis for details.