Table 2: Crack width statistics for detected cracks at different times: the top two rows are from the level set-based approximate method, and the bottom two rows are from the explicit searching method (numbers in parentheses are true values of crack width considering the image resolution).

Crack width statisticsCrack images at

Approximate Maximum 11.8 ( ) 11.9 ( ) 16.7 ( )
Average 4.3 ( ) 7.5 ( )9.9 ( )

Exact Maximum 12.6 ( ) 11.1 ( ) 16.0 ( )
Average 4.3 ( ) 7.3 ( ) 9.8 ( )