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Table of contents

In: IEEE 12th international conference on computer vision, pp 72—79 Bay, H. Bingham, B. Field Robot.

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Botterill, T. Bulow, H. IROS , pp — Caballero, F. In: IEEE international conference on robotics and automation, pp — Choukroun, A. Day, W. Donovan, G.

IEEE J. Elibol, A. Escartin, J. Estrada, C. IEEE Trans. Ferreira, F. Robotics 5 , 55—71 Garcia, R. In: Robotics and automation in the maritime industries, pp — IEEE Retrieved from Gracias, N. Lisbon, Portugal Haralick, R. In: 9. Theoretical Foundations of Computer Vision, pp.

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Hartley, R. Ho, K. Karypis, G. SIAM J. Kekec, T.

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Lirman, D. Marcon, Y. Nemhauser, G. Wiley-Interscience Park, J. SI 64 , — Pizarro, O. Prados, R. For instance, in [ 43 ], Brahim et al. Other exotic systems have been researched, combining IS with conventional cameras to enhance the 3D output and to better correlate the sonar correspondences. In [ 44 ], Negahdaripour uses a stereo system formed by a camera and an imaging sonar. Correspondences between the two images are described in terms of conic sections.

In [ 45 ], a forward looking sonar and a camera are used, and feature correspondences between the IS and the camera image are provided manually to perform reconstructions. Furthermore, in [ 46 ], an SfM approach from a set of images taken from an imaging sonar is used to recover 3D data.

The object shadows in a sonic image can also be used to recover 3D data, as in [ 47 ], where Aykin et al. Its main requirement is that the shadow is distinguishable and that it lays on a known flat surface. Beamforming BF is a technique aimed at estimating signals coming from a fixed steering direction, while attenuating those coming from other directions. When a scene is insonified by a coherent pulse, the signals representing the echoes backscattered from possible objects contain attenuated and degraded replicas of the transmitted pulse.

It is a spatial filter that combines linearly temporal signals spatially sampled by a discrete antenna. This technique is used to build a range image from the backscattered echoes, associated point by point with another type of information representing the reliability or confidence of such an image. One pulse of this sonar system covers a footprint of 3. In [ 50 ], Castellani et al. Kunz et al. Table 1 shows a comparison of the 3D reconstruction techniques using sonar.

Airborne scanning light detection and ranging LiDAR is widely used as a mapping tool for coastal and near shore ocean surveys.

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Similar to LLS, but surveyed from an aircraft, a laser line is scanned throughout the landscape and the ocean. Depending on the laser wavelength, LiDAR is capable of recovering both the ocean surface and the sea bottom. In this particular case, a green nm laser that penetrates the ocean water over 30 m [ 53 ] is used in combination with a red or infrared laser.

Both lasers return the echo from the sea surface, but only one reaches the underwater domain. It is normally surveyed at heights of hundreds of meters Pellen et al. In [ 53 ], a resolution of 0. Moreover, the LiDAR signal can be modulated, enhancing its range capabilities and rejecting underwater backscatter [ 56 , 57 ].

Although this paper focuses on underwater sensors, LiDAR has been briefly mentioned, as it is capable of reconstructing certain coastal regions from the air. No 3D reconstruction. To increase the resolution of the systems exposed above, laser combined with imaging devices can be used.

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Green lasers working at nm are a common solution as a light source because of their good trade-off between price, availability and low absorption and scattering coefficients in seawater. At the reception side, photomultiplier tubes PMT or photon counters can be used, although many approaches also use photodiodes or cameras.

For a larger operational range, preventing the effects of light scattering in the water, some LLS systems send out narrow laser pulses that will be gathered by range gated receivers. Some additional references are listed in Table 3 , as well. This subcategory uses a triangulation method to recover the depth.

A camera-based triangulation device using a laser scan concept can be built using a moving laser pointer made of a mirror galvanometer and a line-scan camera, as shown in [ 58 , 59 ].

Sea Glass Mosaic Frame Preparation

The geometric relationship between the camera, the laser scanner and the illuminated target spot is shown in Figure 3. The depth D of a target can be calculated from Equation 4. This ToF sensor has a simple principle: it illuminates a narrow area with a laser light pulse while keeping the receivers shutter closed.

Then, it waits for the return of the light from the object by estimating its distance from the sensor and then opens the shutter so that only the light returning from the target is captured. For instance, in Figure 4 , the shutter should have been opened from 80 to ns to get rid of the unwanted backscatter.