Fig. 1. Acquisition result of valve surface indentation at normal lighting

Fig. 2. (a) Schematic drawing of reflection stripe detection device; (b) diagram of distortion stripes

Fig. 3. (a) Original gray image; (b) median filtered image

Fig. 4. (a) Original image; (b) shadow detail enhancement result

Fig. 5. Result of valve foreground segmentation. (a) Before segmentation; (b) after segmentation

Fig. 6. Graph of gray variation in a line on the stripe image

Fig. 7. Extraction results of fringe-centerlines. (a) Image of gray stripe (the indentation is inside the red circle); (b) image of fringe-centerlines (the indentation is inside the red circle); (c) enlarged image of gray stripe (the indentation is inside the red circle); (d) enlarged image of fringe-centerlines (the indentation is inside the red circle)

Fig. 8. Detection algorithm flow chart of valve indentation based on stripe image

Fig. 9. (a) Noise point (annotated by a red point) and (b) distortion stripe around the hole in the image of the fringe-centerlines

Fig. 10. Test results of indentation on valve surface (the indentation annotated by the red dot). (a) Gray image; (b) image of fringe-centerlines; (c) enlarged image of fringe-centerlines

Fig. 11. Fringe images of valve with (a) tiny indentation and (b) smudginess

Table 1. Experimental results of the cold stamping valve