According to a further aspect of the invention, the constant value is determined by solving ??? 2 ???? ??? - 1 / 2 ??? S ( c ) exp ( - 1 2 ( x - u ) t ??? - 1 ( x - u ) ) ??? x = ??? S ( c ) ??? 2 ??? 2 ???? ??? 1 / 2 , wherein ?? is said spread, c is said constant value, S(c) is said window, x is a point in said window, and u is the center of said target structure, and ??? S ( c ) ??? = ??? S ( c