The computation of the second moment matrix (sometimes also referred to as the structure tensor) formula_33 in the Harris operator, requires the computation of image derivatives formula_73 in the image domain as well as the summation of non-linear combinations of these derivatives over local neighbourhoods. Since the computation of derivatives usually involves a stage of scale-space smoothing, an operational definition of the Harris operator requires two scale parameters: (i) a "local scale" for smoothing prior to the computation of image derivatives, and (ii) an "integration scale" for accumulating the non-linear operations on derivative operators into an integrated image descriptor.