“…The Sobel operator 22 adds weights on the basis of the Prewitt operator and considers that the closer adjacent pixels have a greater impact on the current pixel. Niu et al 13 . proposed the eight-direction Sobel operator, extending the Sobel operator to eight directions, i.e., the x-axis direction, 45 deg, y-axis direction, 135 deg, 180 deg, 225 deg, 270 deg, and 315 deg.…”