Masking and inpainting effects. Each of the top panels shows a small () high Galactic latitude patch cut from Planck’s 857 GHz map. The brightness-colour mapping is the same for both panels. The left panel is from the foreground map and the right from the background. For this low spatial density field, the “mask+inpaint” method recovers the background brightness map very accurately. The two lower panels, with foreground above and background below, show a very bright low Galactic latitude region (). In this region of high spatial density of sources the “mask+inpaint” process is much less accurate and some degradation of the background can be seen.