The second distance of a given pair of distances is subtracted from the first; the distance difference can have negative values. The functional form is similar to the soft-square function (Section 20.3.4), except that the switching region is symmetrically arranged around the ideal distance difference (i.e., the program switches to the soft asymptote for as well as for ) and is defined as

where denotes the distance difference between the two assigned distances. The distances and are obtained by various averaging methods as described in Section 20.2.