While the study of geometry has mainly concentrated on single viewpoint (SVP) cameras, there is growing attention to more general non-SVP systems. Here, we study an important class of systems that inherently have a non-SVP: a perspective camera imaging through an interface into a medium. Such systems are ubiquitous: They are common when looking into water-based environments. The paper analyzes the common flat-interface class of systems. It characterizes the locus of the viewpoints (caustic) of this class and proves that the SVP model is invalid in it. This may explain geometrical errors encountered in prior studies. Our physics-based model is parameterized by the distance of the lens from the medium interface, besides the focal length. The physical parameters are calibrated by a simple approach that can be based on a single frame. This directly determines the system geometry. The calibration is then used to compensate for modeled system distortion. Based on this model, geometrical measurements of objects are significantly more accurate than if based on an SVP model. This is demonstrated in real-world experiments. In addition, we examine by simulation the errors expected by using the SVP model. We show that when working at a constant range, the SVP model can be a good approximation.

Flat Refractive Geometry

A giant pockmark colonised by dense cold-seep assemblages near 3160 m depth along the CongoAngola margin has been surveyed by the ROV Victor 6000. The quantitative distribution of chemosynthetic communities was mapped along the dive tracks from a video study using GIS and image mosaicking. Several types of faunal assemblages, either dominated by bivalves of the families Mytilidae (Bathymodiolus sp.) or Vesicomyidae (Calyptogena sp., ‘Vesicomya’ aff. chuni), or by Siboglinidae polychaetes (Escarpia southwardae) were mapped over the 800-m diameter pockmark area and sampled for fauna, water and sediment. The isotopic analyses (δ 13 C) of tissues from symbiont-bearing species were within the range typical of nutrition via symbiosis using methane for mussels and sulphide for vesicomyids and siboglinids. The living chemosynthetic communities were distributed on a SW-NE axis, corresponding to the expression at the sediment surface of a main buried channel providing fluids to the pockmark. The site was characterised by a more active central part in a depression with abundant carbonate concretions where high-density clusters of siboglinids and mytilids dominate. Large fields of dead and live vesicomyids with a lower mean density were observed in the external areas. The mean coverage of each of the three symbiotic taxa in these two contrasted areas was estimated from mosaic analysis and was up to 30% in the central area dominated by E. southwardae bushes (23%). Symbiont-bearing species distribution was consistent with methane concentrations in seawater that were generally higher in mytilid beds than in the vicinity of siboglinids and vesicomyids. A Principal Component Analysis performed on environmental factors at the ten sampling sites revealed that 37% of the observed variance in the distribution of symbiontbearing species may be explained by variation in both methane and oxygen concentrations, while a Canonical Redundancy Analysis selected methane concentration as the only variable which explains symbiont-bearing species densities. This spatial distribution of chemosynthetic species at the pockmark scale may reflect temporal patterns of succession of both substrate and fauna, and may be related to different individual pockmarks visible on the microbathymetry mapped using ROV data.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1439-0485.2006.00145.x

Cold-seep assemblages on a giant pockmark off West Africa: spatial patterns and environmental control

Ship hulls, as well as bridges, port dock pilings, dams, and various underwater structures need to be inspected for periodic maintenance. Additionally, there is a critical need to provide protection against sabotage activities, and to establish effective countermeasures against illegal smuggling activities. Unmanned underwater vehicles are suitable platforms for the development of automated inspection systems, but require integration with appropriate sensor technologies. This paper describes a vision system for automated ship-hull inspection, based on computing the necessary information for positioning, navigation, and mapping of the hull from stereo images. Binocular cues are critical in resolving a number of complex visual artifacts that hamper monocular vision in shallow-water conditions. Furthermore, they simplify the estimation of vehicle pose and motion, which is fundamental for successful automatic operation. The system has been implemented on a commercial remotely operated vehicle (ROV), and tested in pool and dock tests. Results from various trials are presented to demonstrate the system capabilities

An ROV Stereovision System for Ship-Hull Inspection

Underwater photogrammetry provides an efficient nondestructive means for measurement in environments with limited accessibility. With the growing use of consumer cameras, its application is becoming easier, thus benefiting a wide variety of disciplines. However, utilizing cameras for underwater photogrammetry poses some nontrivial modeling problems due to refraction effect and the extension of the imaging system into a unit of both the camera and the protecting housing device. This paper studies the effect that the underwater environment has on the photogrammetric process, and proposes a model for describing the geometric distortions and for estimating the additional parameters involved. The proposed model accounts not only for the multimedia effect, but also for inaccuracies related to the setting of the camera and housing device. The paper shows that only a small number of additional parameters is needed to model both elements and to preserve the collinearity relation. The results show that no unique setup is needed for estimating the additional parameters and that the estimation is insensitive to noise or first approximations. Experiments show that high levels of accuracy can be achieved.

Photogrammetric modeling of underwater environments

While the study of geometry has mainly concentrated on single-viewpoint (SVP) cameras, there is growing attention to more general non-SVP systems. Here we study an important class of systems that inherently have a non-SVP: a perspective camera imaging through an interface into a medium. Such systems are ubiquitous: they are common when looking into water-based environments. The paper analyzes the common flat-interface class of systems. It characterizes the locus of the viewpoints (caustic) of this class, and proves that the SVP model is invalid in it. This may explain geometrical errors encountered in prior studies. Our physics-based model is parameterized by the distance of the lens from the medium interface, beside the focal length. The physical parameters are calibrated by a simple approach that can be based on a single-frame. This directly determines the system geometry. The calibration is then used to compensate for modeled system distortion. Based on this model, geometrical measurements of objects are significantly more accurate, than if based on an SVP model. This is demonstrated in real-world experiments.

/pdf/flat-refractive-geometry-1633selbxb.pdf

Flat refractive geometry

This paper presents a self-calibration technique for a camera mounted on an underwater vehicle designed to perform the 3D reconstruction of underwater scenes. Our aim is to identify the intrinsic parameters of the camera with methods that are adapted to the operational constraints on Ifremer’s underwater vehicles. The optical system is composed by a single vertical camera located below the underwater vehicle and looking downwards. The motion of the vehicle can be measured through navigation sensors and the observed 3D scene is always unknown. The use of a moving camera is not an obstacle for the application of stereoscopic methods. Nevertheless, the camera motion enables the use of robust algorithms for points matching, but impoverishes perspective effects between several images. Therefore, we are interested in the analysis of the conditions in which the procedure of self-calibration is valid and reliable, i.e.: the 3D characteristics of the scene and the camera motion. This paper presents the steps necessary for the camera self-calibration in an underwater environment: the extraction and the tracking of features in several successive images, the fundamental matrix estimation and the intrinsic parameters identification. Several tests and results are presented.

/pdf/camera-self-calibration-in-underwater-environment-1nwxgsb02s.pdf

Camera self-calibration in underwater environment

This paper presents the MATISSE system: Mosaicking Advanced Technologies Integrated in a Single Software Environment. This system aims at producing in-line and off-line geo-referenced video mosaics of seabed given a video input and navigation data. It is based upon several techniques of image and signal processing which have been developed at Ifremer these last years in the fields of image mosaicking, camera self-calibration or correction and estimation of navigation data.

Real-time geo-referenced video mosaicking with the MATISSE system

This paper presents a robust self-calibration method as well as results from experiments and simulations. The algorithm tracks points in an image sequence and exploits successive views in order to estimate the fundamental matrix and finally the intrinsic parameters of the camera. In the considered application, the camera is mounted on an underwater vehicle in a vertical position. Taking into account the constraints and the controllability of the underwater vehicle's movements, we study the influence and exploitation of these movements for the estimation of the intrinsic parameters. This paper presents the major steps of the robust self calibration algorithm. Starting from a set of matched points that may comprise bad matches, we describe the estimation of the fundamental matrices using a RANSAC algorithm scheme and the computation of the intrinsic parameters. Experiments are explained and a sensible motion for self-calibration is proposed.

Nathalie Pessel

Papers

Camera self-calibration in underwater environment

Real-time geo-referenced video mosaicking with the MATISSE system

An experimental study of a robust self-calibration method for a single camera

Seabed Video Mosaicking With MATISSE: A Technical Overview And Cruise Results