High-accuracy camera calibration for underwater binocular laser scanner via virtual-imaging.

Accurate 3-D reconstruction of underwater objects is increasingly important for many applications, and a binocular laser scanner is a common way to achieve an underwater point cloud. However, the perspective-based binocular camera calibration method developed for in-air imaging often causes large errors in underwater 3-D reconstruction because the camera rays are refracted at the water-glass-air interfaces. To improve the accuracy of the underwater binocular laser scanner, this study establishes a binocular camera calibration method based on virtual imaging by considering multilayer refraction, which determines the refractive parameters and camera pose efficiently and accurately. The principles of the binocular laser scanning system are extended to the refractive geometry. The virtual point distribution constraint (VPDC) is employed for closed solution in binocular camera calibration, which estimates the axis, distance, and camera pose for accurate step-by-step initialization. The virtual optical center error (VOCE) is adopted for nonlinear optimization, achieving higher efficiency than forward projection without loss of accuracy. The camera calibration experiments demonstrate that the proposed method yields a more accurate closed solution (about five times and twice improvements for two configurations) and more efficient optimization (above ten times improvement) compared with existing methods. The 3-D reconstruction experiments also show improved accuracy with the proposed binocular camera calibration method.