Robust centerline extraction method of laser stripe based on an underwater line scanning lidar

The underwater line scanning lidar (ULSL), which can reconstruct the three-dimensional (3D) profile information of the underwater target from laser stripe images, plays an important role in seabed topography mapping, marine biological research, navigation and military targets detection, etc. The performance of the laser centerline extraction algorithm is crucial to the detection range and accuracy of ULSL. However, the poor optical environment conditions in seawater—the large attenuation and strong backscattering—will seriously hinder the extracting of laser centerlines with traditional algorithms for in-air images. For the ULSL, a special laser centerline extraction algorithm is urgently needed. For this reason, in this paper, an underwater line laser propagation model is established, which theoretically analyzes the energy distribution of the line laser along the propagation direction. On this basis, a distance gradient region energy (DGRE) method for underwater laser centerline extraction is proposed. The influence of backscattering is eliminated by distance gradient map, and the effect of the residual background noise is suppressed by distance gradient region energy map. As a result, the problem of inability to extract the laser centerline can be solved meanwhile mitigating the effects of backscatter and other background noise. Compared with other methods, our method can achieve a detection distance of 25.0 m with a detection accuracy of 50.0 mm under the water attenuation coefficient of $0.28/{\text{m}}^{-1}@532\text{nm}$, while other methods can only reach a maximum of 13.4 m. Finally, an underwater 3D reconstruction experiment was carried out, and a good 3D imaging capacity is realized with this method.