Fig. 1. Grayscale distribution of interference fringe. (a) Tangential direction; (b) normal direction

Fig. 2. Interference fringe patterns. (a) Original image; (b) R-value image; (c) Gaussian filter image

Fig. 3. Grayscale along normal direction. (a) Original image; (b) after Gaussian filtering

Fig. 4. Schematic of interval fitting

Fig. 5. Results of curve fitting. (a) f₁(x); (b) f₂(x)

Fig. 6. Images of f₁(x) and f₂(x) and filtered image

Fig. 7. Image of f(x)=f₂(x)-f₁(x)

Fig. 8. Extraction results. (a) Interference fringe pattern; (b) centerline extraction result

Fig. 9. Interference fringe patterns. (a) Image 1; (b) image 2; (c) image 3; (d) image 4; (e) image 5

Fig. 10. Known centerline images. (a) Image 6; (b) image 7; (c) image 8; (d) image 9; (e) image 10