## 3 Image Feature-Based Extraction of Damage Indicators  
Since damage assessment has certain requirements for image quality, drone images of bridge piers and bridge decks can be collected, and drone image collection has the advantages of operational flexibility and equipment selectability, which can ensure the quality of image data to a certain extent. After the acquisition of images, there are many factors that affect image quality due to this process, including weather, environment, shooting viewpoint, and so on. Therefore, in order to suppress the image noise and enhance the damage features, the image must be preprocessed to facilitate the distinction between the background and the bridge while accurately extracting the damage regions. The histogram equalization algorithm can improve the contrast between the bridge target and the background to facilitate the extraction of the target [39], and the image alignment algorithm can attenuate the effect of different shooting angles to facilitate change detection [11]. In bridge damage assessment, in order to ensure the accuracy of the assessment results, there are certain requirements for image acquisition. First, the distance between the camera and the bridge needs to be able to comprehensively cover the entire bridge structure and be able to recognize the crater features, while recognizing small weak features such as tiny cracks is not necessary. Second, the image resolution does not need to be very high. Too high a resolution, although richer in detail, increases the processing time and computational cost significantly, and 720 pixels is sufficient to detect an impact crater. Therefore, in order to obtain accurate damage assessment results, it is recommended that the appropriate distance and resolution be selected during image acquisition. Specifically, the distance between the camera and the bridge should be between 10 and 30 m (to capture the bridge structure in its entirety), and the image resolution should be kept in the 720p to 4K range. When studying the damage assessment of bridges after a munitions strike, the state of the bridge prior to damage has a relatively small impact on the damage assessment. This is because munitions strikes are usually extremely high in energy density and usually result in fracture or collapse of critical parts of the structure (e.g., piers, abutments, etc. ), and this macroscopic damage is far beyond the impact of the initial defects. The degree of damage and the pattern of damage of new and old bridges under the same munitions strikes are basically the same, and the difference may be reflected mainly in the subtle crack expansion rate, but it has little effect on the overall results of the assessment. In summary, although the state of the bridge before damage will have some influence on the structural performance, these effects are relatively small under the action of high-energy munitions strikes. Therefore, when assessing bridge damage after a bombing, more attention can be paid to key factors such as location and intensity of the explosion, without excessive consideration of the initial state of the bridge. After using the UAV to take visible-light images, damage analysis of the bridge structure is carried out to identify the bridge pier and bridge deck, and the strike crater is simplified to a circle for the convenience of the analysis. Based on the above analysis, the abutments play the role of support and force transmission in the bridge structure, so the impact of the abutment damage on the bridge is primarily reflected in the change of bearing capacity, and the bridge deck mainly provides the passageway and transmits the load, so the impact of the bridge deck damage on the bridge is mainly reflected in the change of the path of the passageway and the remaining bearing capacity. At the same time, for the bridge blow, there may be penetration and loss of quality in two types of conditions and different conditions on the components of different impacts.