Fig. 1. System block diagram of the detection system

Fig. 2. Photo of the detection system

Fig. 3. Schematic diagram of polar coordinates

Fig. 4. A picture of a simulated tunnel

Fig. 5. Three-dimensional point cloud image of a tunnel

Fig. 6. Raw point cloud data

Fig. 7. Point cloud data after segmentation

Fig. 8. Schematic for defining concept of DBSCAN

Fig. 9. Flowchart of segmentation algorithm based on density clustering

Fig. 10. Segmentation results based on density clustering algorithm

Fig. 11. Scene 1

Fig. 12. Scene 2

Fig. 13. 3D point cloud of scene 1

Fig. 14. 3D point cloud of scene 2

Fig. 15. Result of scene 1. (a) Result of region growing segmentation; (b) result based on density clustering segmentation method

Fig. 16. Result of scene 2. (a) Result of region growing segmentation; (b) result based on density clustering segmentation method

Fig. 17. Deformation of the tunnel is simulated by object compression

Fig. 18. A deformed simulated tunnel

Fig. 19. Segmentation results of object point cloud in the deformed tunnel. (a) Result of region growing segmentation; (b) result based on density clustering segmentation method