Footprint shrinkage, illustrated in an image with 3″ resolution of a flat-topped source with a peak intensity of 100, half-maximum size of 49″, and S/N of 100 (left), modeled as a 50″ cylinder convolved with a 10″ Gaussian kernel. The initial footprint factor ϕ_n = 3 (Sect. 3.4.4) is too large for the flat-topped source (middle), whose actual footprint relates to the FWHM value by a factor ϕ_n = 1.5. The footprint shrinkage algorithm (Sect. 3.4.6) reduces ϕ_n by a factor of 1.5 (right), which shrinks the footprint and confines it to the pixels belonging to the source alone. This footprint adjustment improves the accuracy of background interpolation and flux measurement on complex backgrounds. Square-root color mapping.