Background interpolation scheme (Sect. 2.6). The central red pixels belong to the source, defining its footprint in this example, whereas the surrounding blue pixels are those of the background. At each pixel of the source, the background is linearly interpolated in the four main directions based on the values of the pixels just outside the footprint (highlighted in darker blue), the resulting four values per pixel being averaged. Such interpolation probes the actual background variations around the sources, and thus the interpolated background is more complex and realistic than just a planar surface.