# Experimental Analysis of the Behavior of Mirror-like Objects in LiDAR-Based Robot Navigation
## 2 Related Works  
Several strategies have been proposed to tackle the difficulties posed by reflective objects in LiDAR-based SLAM. Some researchers have suggested multisensor fusion solutions to provide complementary information. [PERSON] and [PERSON] [6] used LiDAR and sonar sensor fusion to remove specular reflections by detecting all surfaces as solid objects. [PERSON] et al. [7] combined laser and sonar data using a Bayesian filter to estimate the distance to transparent objects more accurately. [PERSON] et al. [8] proposed a similar laser and sonar sensor fusion approach to detect mirrors and windows. Since multisensor fusion approaches need different sensors, this approach has higher computational and financial costs. Therefore, most previous efforts have relied only on LiDAR sensors and have striven to obtain supplementary information from laser data. In [PERSON] et al. [9], windows were detected by extracting facade planes from LiDAR point clouds by combining bottom-up with top-down strategies. Cluster point clouds were clustered into potential facade regions by using principal component analysis (PCA) in the bottom-up approach. Then, the top-down approach was performed to utilize the random sample consensus (RANSAC) to extract the facade from the potential facade region. [PERSON] et al. [10] proposed a window detection method based on building wall extraction from scene point clouds. The building walls were extracted from the scene point clouds according to a collection of characteristics, then the building facade was sliced both horizontally and vertically to detect window regions. [PERSON] et al. [11] also attempted to detect glass by distinguishing facade features such as walls and roofs. They used the obtained knowledge about these features' sizes, positions, orientations, and topology to detect windows from laser point clouds. The drawback of these methods is that auxiliary information, such as glass frames or walls, cannot be extracted in modern buildings containing frameless glass or glass-walled environments. To address this issue, some researchers have utilized the inherent properties of glass and mirror objects. Reflection intensity characteristics were studied by [PERSON] et al. [12] to detect glass. They used the specular reflection phenomenon, which occurs when the irradiation angle is close to the perpendicular angle to the glass surface. In this case, the reflection intensity is maximum. They also studied the transmission phenomenon when there is an object in the transmission destination. In this scenario, the strongest peak occurs at the incident angle, and the reflection intensity drops as laser light passes through the glass. Similarly, [PERSON] et al. [13] recognized glass panels based on the specular reflection of laser beams from glass. Then, the glass detection method was combined with a SLAM algorithm to avoid collisions with glass obstacles. Based on the reflective characteristics of a laser beam, [PERSON] et al. [14] designed a scan-matching algorithm to differentiate different scenarios, such as diffuse reflection, specular reflection, and beam penetration. However, several factors, such as the laser incident angle, affect the received intensity, making intensity-based methods unreliable and error prone. Therefore, some researchers have focused on glass detection alone. [PERSON] et al. [15] considered the variation in neighboring LiDAR point clouds to differentiate the pulses that pass through glass and the pulses that directly hit objects. Then, two filters were applied using intensity and range discrepancy to identify the boundary of the glass. [PERSON] et al. [16] proposed a method to detect mirrors based on the symmetries of real objects and their images in the mirror. To find symmetrical relationships from the point cloud, the robot itself was considered a reference point, and its position was estimated by the Rao-Blackwellised particle filter (RBPF) SLAM algorithm. [PERSON] et al. [17] also used the geometric property of mirror symmetry.