An excerpt from the paper
In any mottled digital image, the Stochastic Frequency Distribution Analysis (SFDA) algorithm measures the two dimensional rate of change in luminance values, or transitions between light and dark, from one are to the next. It is also responsive to the relative number of these transitions. In print applications SFDA mathematically describes the uniformity of ink transfer to the substrate surface by producing a number proportional to the visible, or sub-visible, degree of mottle. It also provides horizontal and vertical mottle orientation measurements. SFDA is used to measure a wide variety of mottled images including ink jet print, toner adhesion, toner quality, paper formation, ink penetration, offset and rotogravure.