Kolyada, A., V. Revinski, and M. Tikhonenko. “Direction Field of Fingerprint Image and Its Optimization With the Help of Relaxation Procedures”. Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, vol. 3, May 2003, pp. 101-5, https://studmath.uken.krakow.pl/article/view/13474.