## 5 Conclusions  
Based on digital image processing technology, this paper presents a novel method for structural overall deformation monitoring. Using the proposed method, a series of deformation measurement experiments are carried out on a steel-concrete composite beam, and the main conclusions are as the following:  
1. Due to the limitation on camera sites, orthogonal projection images are usually difficult to be accessed for large engineering structures such as bridges. In dealing with the issue, the perspective transformation method is applied to acquire the orthogonal projection of structures from the originally inclined images. The experimental results show that the orthogonal projection image obtained by the proposed method can correctly reflect the overall deformation of the structure. 2. In order to characterize the key feature of structural deformation, the edge detection operator is utilized to obtain the edge contour of the structure from the processed orthogonal images. Using the operator, the overall deflection curve of the structure can be obtained by locating and calibrating the edge pixels. 3. The edge line of the structure acquired from the position of the pixel shows a notable zigzag effect. Further investigations have been carried out, and the result suggests that the illumination environment can be mainly attributed to the zigzag effect. Since the image edge of the structure has a certain bandwidth, the final position of edge pixels in the bandwidth range will be affected by the illumination environment, which eventually results in the fluctuation around the actual deflection curve. On this end, the fitting method is used to minimize the fluctuation and obtain the linear approximation of the actual deflection curve. After comparison with the data measured by the dial meters, it shows the error of the proposed method is less than 5%. 4. Since the proposed method is based on digital images, the accuracy is dependent on the quality of available images even if some advanced image processing methods are utilized. For instance, a major limitation of the method is that the overall deformation cannot be directly obtained when \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Loading Condition/kN**} & \multirow{2}{*}{**Dialgauge Position**} & \multicolumn{2}{c}{**Dialgauge Measured Value R2/mm**} & \multirow{2}{*}{**Photogrammetric Value R2/mm**} & \multirow{2}{*}{**\(\mathbf{R2-R1}\) = S/mm**} & \multirow{2}{*}{**Error S/R1/\%**} \\  & & & & & \\  & & & & & \\ \hline \multirow{4}{*}{200} & Left support & 7.50 & 7.25 & 0.25 & 3.33 \\  & L/4 & 15.95 & 15.50 & 0.45 & 2.82 \\  & 2L/4 & 19.09 & 18.75 & 0.34 & 1.78 \\  & 3L/4 & 16.90 & 16.19 & 0.71 & 4.20 \\ \hline \multirow{4}{*}{300} & Left support & 7.74 & 7.45 & 0.29 & 3.75 \\  & L/4 & 22.13 & 22.69 & 0.56 & 2.53 \\  & 2L/4 & 27.37 & 27.80 & 0.43 & 1.57 \\  & 3L/4 & 23.49 & 23.78 & 0.29 & 1.23 \\ \hline \multirow{4}{*}{400} & Left support & 8.82 & 8.47 & 0.35 & 3.97 \\  & L/4 & 28.42 & 27.56 & 0.86 & 3.03 \\  & 2L/4 & 35.45 & 34.15 & 1.30 & 3.67 \\  & 3L/4 & 29.89 & 28.94 & 0.95 & 3.18 \\ \hline \multirow{4}{*}{500} & Left support & 8.72 & 8.36 & 0.36 & 4.13 \\  & L/4 & 34.34 & 33.14 & 1.20 & 3.49 \\  & 2L/4 & 43.46 & 42.18 & 1.28 & 2.95 \\  & 3L/4 & 36.08 & 36.91 & 0.83 & 2.30 \\ \hline \multirow{4}{*}{600} & Left support & 10.33 & 9.84 & 0.49 & 4.74 \\  & L/4 & 41.76 & 42.81 & 1.05 & 2.51 \\ \cline{1-1}  & 2L/4 & 52.75 & 51.26 & 1.49 & 2.82 \\ \cline{1-1}  & 3L/4 & 43.28 & 42.19 & 1.09 & 2.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of displacement measurement error. some parts of the measuring structures are obscured. Under such a situation, the postprocessing method, such as the fitting, can be applied to obtain the approximation data of the blocked parts. [PERSON] conceived this study, designed the computational algorithms, wrote the program code, and wrote the manuscript. [PERSON] proposed some valuable suggestions and guided the experiments.