Subtraction of surfaces of average slope

It is possible to subtract the surfaces of two types -- surface of the first order (plane), and surface of the second order. The corresponding Commands are called by Planefit (button ) and Paraboloic Fit (button ). If you have select a surface area in advance, then there are two possible ways to fulfill an operation -- to process only the selected area (In selection), or to process the whole image by using the slope coefficients derived from the selected area (By selection). If the area is not selected, then the both variants are equivalent.