Fig. 1. Image of pantographic slide

Fig. 2. Pixel grey value calculation model with straight edge

Fig. 3. Model of edge calculation by a second order curve

Fig. 4. Sub-pixel edge extraction based on partial area effect

Fig. 5. Upper and lower edge of pantographic slide by using the proposed algorithm

Fig. 6. Edge detection results of pantographic slide by using different methods. (a) Sobel algorithm; (b) Canny algorithm; (c) structured forest algorithm; (d) proposed algorithm