This is an interpolation plug-in for the GIMP. It works by doing plane/curve fitting and calculating weighted averages. It takes samples from a grid of pixels from 'known' image areas, trying to fit a plane or a curve to each, and then uses those samples to estimate the 'unknown' image areas.
Upper-left: original image, upper-right: result after interpolation,
lower left: selection (a.k.a. output mask), lower right: input mask