Please continue referring to the embodiment of FIG. 3(b). After obtaining the boundary line BL, the computing device can obtain another boundary line which is symmetrical to the boundary line BL and on the other side of the first center line CL1 by using the first center line CL as a center line. The computing device can use the area enclosed by the two boundary lines BL and the two reference lines L1 and L2 as the detection zone DZ. Similarly, the computing device can also use the second center line CL2 as the center line to obtain the detection zone DZ. It is worth noting that, when respectively using the first center line CL1 and the second center line CL2 as the center lines to obtain three boundary lines BL, the computing device can move the two boundary lines BL located on the both sides of the first center line CL1 towards the first center line CL1 by a preset distance as the updated two boundary lines BL, and move the two boundary lines BL located on the both sides of the second center line CL2 towards the second center line CL2 by the preset distance as the updated two boundary lines BL. The preset distance can be 20% of the distance between the two first center points CP1 and CP1′, or 20% of the distance between the two second center points CP2 and CP2′, the present disclosure is not limited thereto. Accordingly, the two detection zones DZ connecting to each other can be separated apart.