The bottom boundary is obtained using canny edges and a rough ROI is selected heuristically. The convex direction code for each canny edge pixel is computed to represent its local direction. The direction can either be convex leftward (+1) or convex rightward (−1). The pixels those are not on any curve are assigned value (0). The convexity direction measures the strength of dominant direction locally. The convex direction codes of all canny edge pixels over a vertical scan-line are added to obtain the convex direction magnitude for that scan-line. The curves along small area around phalangeal joint do not have any dominant convex direction. Hence, the vertical scan-line for which minimum convex direction magnitude is obtained is considered as the y-axis. The various steps involved in knuckleprint ROI extraction are shown in Fig. 4.