[1] T. Crimmins. The geometric filter for speckle reduction. Applied Optics, 1985.
[2] R.C. Gonzalez and R.E. Woods. Digital image processing. Prentice-Hall Inc., 2002.
[3] N. Otsu. A threshold selection method from gray-level histograms. IEEE Transactions Systems, Man. and Cybernetics, 1979.
[4] P. Perona and J. Malik. Scale-space and edge detection using anisotropic diffusion. IEEE Trans. Pattern Anal. Machine Intell., 1990.
[5] J. Shen and S. Castan. Further results on drf method for edge detection. In 9th I.C.P.R.
[6] J. Shen and S. Castan. An optimal linear operator for step edge detection. CVGIP: Graphical Models and Understanding, 1992.