“…Dual-threshold algorithm applies 2 thresholds T � and T � to non-maximum suppression image, and T � � 2T � , so 2 threshold edge image N � �i, j� and N � �i, j� are obtained. Because N � �i, j� is obtained using higher threshold, it contains very few false edge, but contains discontinuity (non-closure) [5]. Dual-threshold algorithm will connect edges to contour in N � �i, j�, when arriving at end points of the contour, the algorithm will locate the edge able to connect the contour at the location of 8 neighbouring points of N � �i, j�, as such, the algorithm continuously collects edge in N � �i, j� till connecting with N � �i, j� .…”