Fig. 1. (a) New sagger bottom; (b) defect-free sagger after usage for a few times; (c) sagger with cracks after usage for a few times

Fig. 2. (a) Original image of sagger bottom crack; (b) result of median filtering; (c) detailed image after subtraction and phase inversion; (d) effect of image subtraction process

Fig. 3. (a) Original image of sagger bottom crack; (b) result of image subtraction process; (c) result of threshold segmentation; (d) gray histogram of (b); (e) effect before close operation; (f) effect after close operation

Fig. 4. (a) Two types of crack pixels and their corresponding sector neighborhood difference histograms; (b) two types of noise pixels and their corresponding sector neighborhood difference histograms

Fig. 5. (a) G'(x,y,σ) function and sector filter acquisition process; (b) sector filters in the first eight directions

Fig. 6. (a) Crack pixel sector neighborhood difference histogram (left) and its transforming result (right); (b) noise pixel sector neighborhood difference histogram (left) and its transforming result (right)

Fig. 7. (a) Original image (top) and pretreatment result (bottom); (b) crack extraction results (1st-, 2nd-, and 3rd-lines corresponding to the segmentation probability thresholds of 0.3, 0.4, 0.5, respectively; 1st-, 2nd-, and 3rd-columns corresponding to the number of iterations of 0, 2, 5, respectively)

Fig. 8. Crack extraction results. (a)-(c) Sagger bottom crack images; (d) wall crack images

Fig. 9. Performance evaluation of the crack extraction

Fig. 10. Crack severity assessment results. (a) E=0.96, HH; (b) E=0.83, HH; (c) E=0.65, H