# Analysis of Fine Crack Images Using Image Processing Technique and High-Resolution Camera
## 5 Width Measurement: Real Crack (Concrete Bridge)  
In this chapter, we investigate the effect of the measured pixels when measuring an actual crack. The cracks at the abutment of a pedestrian bridge were selected as the ground truth due to safety issues during visual inspection. Crack measurement was carried out through visual inspection, and a scale loupe with a magnifying glass of 10X magnification was used to check the width of fine concrete cracks, as shown in Figure 16. Concrete crack images were obtained from a minimum working distance to 2000 mm, at 200 mm increments. For the analysis of the acquired crack images, the same image processing method as the method used in the indoor experiment was implemented. However, unlike the linear virtual images, the crack images had bad conditions and a texture on the target surface, so preprocessing was performed to remove noise. The preprocessing procedure was to separate the foreground (cracks and noise) from the background by applying adjacency and connectivity conditions on neighboring pixels. The area of the crack and noise was computed and indexed in descending order. As shown in Figure 17, the noise regions were removed, excluding those with the largest area. Figure 16: Concrete crack measurement apparatus and method. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **WD** \\ **(mm)** \\ \end{tabular}  } & \multicolumn{2}{c}{ \begin{tabular}{c} **GSD** \\ **(mm/Fixed)** \\ \end{tabular}  } & \multicolumn{2}{c}{ \begin{tabular}{c} **Real** \\ **Pixels (ea)** \\ \end{tabular}  } & \multicolumn{2}{c}{ \begin{tabular}{c} **Measured** \\ **Fixed (ea)** \\ \end{tabular}  } & \multicolumn{2}{c}{ \begin{tabular}{c} **Measured** \\ **Width (mm)** \\ \end{tabular}  } & \multicolumn{2}{c}{\ \begin{tabular}{c} **Error** \\ **(\%)** \\ \end{tabular}  } & \multicolumn{2}{c}{ \begin{tabular}{c} **Ground** \\ **Truth** \\ **(mm)** \\ \end{tabular}  } \\ \cline{2-11}  & \multicolumn{6}{c}{**Focal Length (mm)**} \\ \cline{2-11}  & **50** & **100** & **50** & **100** & **50** & **100** & **50** & **100** & **50** & **100** \\ \hline
400 & 0.0265 & -  
Table 9, and Figures 18 and 19 indicate the number of pixels (Pm), measured crack value (Wm), and errors (Em) according to the width value of the target crack through image processing. In addition, the estimated pixels in Table 9 were derived using the approximation curve in Figure 18 and crack measurement using the estimated pixels illustrated in Figure 19. Regression analysis using the power law was carried out, and the coefficient of determination was 0.9829. The approximated pixels (Pa), crack width (Wa), and error (Ea) were the calculated crack width and errors according to the working distance, respectively. Here, Pa showed a similar tendency to the pixel Pr corresponding to the actual crack. Moreover, the Wa calculated by the estimated pixel indicated an error of up to 5% and a great agreement with the ground truth. However, the biggest Em was 19.32% at the working distance of 1800 mm, while the rest of them were less than 10%. As the working distance increased, the Wm did not show a clear trend in error change and repeated the absolute increase or decrease, because the Pm was measured somewhat higher or lower than the value of Pa or Pr. As mentioned in Section 4.3, in order to accurately measure crack width, the Pm must be in decimal units. However, the Pm was computed as an integer, and it was found that the difference between Pm and Pr led to an error in the crack measurement. In addition, it can be seen that Pr becomes 3 at the position of 1450 mm where the Pm, Pr, and Pa intersect in Figure 18. Moreover, it can be seen that the measurement error at the same working distance represents positive or negative values, as shown in Table 9. As shown in Table 9 and Figure 18, it was indicated that the error increased as the distance increased, moving away from the reference position.