UNDERWATER COMPARISON OF WAND AND 2D PLANE NONLINEAR CAMERA CALIBRATION METHODS

The purpose of this study was to compare two nonlinear camera calibration methods for 3D underwater motion analysis. The DVideo kinematic analysis system was used for underwater online data acquisition. The system consisted of two gen-locked Basler cameras working at 100Hz, with wide angle lenses that were enclosed in housings. The accuracy of both methods was compared in a dynamic rigid bar test. The mean absolute errors were 1.16mm for wand calibration, 1.20mm for 2D plane calibration using 8 control points and 0.73mm for 2D plane calibration using 16 control points. The results of both nonlinear camera calibration methods provided better underwater accuracy than all previous papers reported in literature. Both methods provided similar and highly accurate results, providing promising alternatives for underwater 3D motion analysis

Pre- and Self-calibration of underwater cameras for photogrammetric documentation of archaeological sites

Underwater photogrammetry has become one of the most affordable and adopted methods for the documentation and the 3D reconstruction of submerged archaeological assets. In digital photogrammetry, images are captured to exploit (using computer vision-based procedures) their intrinsic metric contents. To preserve the metric consistency and to obtain reliable 3D metric products, this process must be followed according to photogrammetric principles that are even more important in underwater photogrammetry. The wide diffusion of low-cost and non-metric sensors requires that some attention be given to proper geometric calibration of the employed cameras. Via calibration, it is possible to opportunely describe geometric distortions that are observable on final images due to lens shapes and construction characteristics of the cameras and the optics used in the survey operations. This research addresses the importance of pre-calibration in underwater cameras, and for this purpose, three calibration datasets are acquired and compared: the first (A) where the camera is pre-calibrated without any addition (flat or dome ports); the second (B) in which the camera is used in combination with a dome port; and the third (C) where the camera setup has been employed in an underwater environment. For both scenarios (dry and wet), self-calibration and pre-calibration procedures are compared. Moreover, is possible to notice how the use of the right camera and lens combinations, specifically designed for underwater survey purposes, are functional to lower the distortion of the images and consequently improve the accuracy of the final 3D products. Different tests have been performed, and preliminary results are presented and discussed in this work-in-progress paper

UNDERWATER NON-LINEAR CAMERA CALIBRATION: AN ACCURACY ANALYSIS

One of the most challenging problems associated with underwater 3D movement analysis is the accurate calibration of the cameras. Additional sources of errors are present in underwater acquisitions such as the nonlinear distortion caused by water interface, camera lenses (ex. wide angle) and housing’s glasses. Despite this, in the literature, systems based on a linear calibration model (DLT) were proposed (Yanai et al., 1996; Machtsiras & Sanders, 2009; Gourgoulis, et al. 2008). However, the results of underwater accuracy were not similar to those obtained out of the water. In Kwon, et al. 1999, the use of a modified DLT algorithm to model the distortion was proposed but the results of accuracy were not substantially improved, with Root Mean Square (RMS) values ranging from 5.6 to 7.2mm. Recently, alternative approaches were proposed to non-linear camera calibration and submillimeter accuracy was reached (Cerveri et al., 1998; Zhang, 2000; Pribanić, Sturm & Cifrek, 2008). However, these approaches were not applied underwater. In previous work, a new non-linear calibration method using a straight line plane object was proposed and tested out of the water (Silvatti et al., 2009 available in http://calib.googlecode.com). In this work, this novel method was tested in underwater conditions and its accuracy evaluated

Integration of optical and acoustic sensors for D underwater scene reconstruction.

Combination of optical and acoustic sensors to overcome the shortcomings presented by optical systems in underwater 3D acquisition is an emerging field of research. In this work, an opti-acoustic system composed by a single camera and a multibeam sonar is proposed, providing a simulation environment to validate its potential use in 3D reconstruction. Since extrinsic calibration is a prerequisite for this kind of feature-level sensor fusion, an effective approach to address the calibration problem between a multibeam and a camera system is presented.Peer Reviewe

Measurement of Micro-bathymetry with a GOPRO Underwater Stereo Camera Pair

A GO-PRO underwater stereo camera kit has been used to measure the 3D topography (bathymetry) of a patch of seafloor producing a point cloud with a spatial data density of 15 measurements per 3 mm grid square and an standard deviation of less than 1 cm A GO-PRO camera is a fixed focus, 11 megapixel, still-frame (or 1080p high-definition video) camera, whose small form-factor and water-proof housing has made it popular with sports enthusiasts. A stereo camera kit is available providing a waterproof housing (to 61 m / 200 ft) for a pair of cameras. Measures of seafloor micro-bathymetrycapable of resolving seafloor features less than 1 cm in amplitude were possible from the stereoreconstruction. Bathymetric measurements of this scale provide important ground-truth data and boundary condition information for modeling of larger scale processes whose details depend on small-scale variations. Examples include modeling of turbulent water layers, seafloor sediment transfer and acoustic backscatter from bathymetric echo sounders

Use of stereo camera systems for assessment of rockfish abundance in untrawlable areas and for recording pollock behavior during midwater trawls

We describe the application of two types of stereo camera systems in fisheries research, including the design, calibration, analysis techniques, and precision of the data obtained with these systems. The first is a stereo video system deployed by using a quick-responding winch with a live feed to provide species- and size- composition data adequate to produce acoustically based biomass estimates of rockfish. This system was tested on the eastern Bering Sea slope where rockfish were measured. Rockfish sizes were similar to those sampled with a bottom trawl and the relative error in multiple measurements of the same rockfish in multiple still-frame images was small. Measurement errors of up to 5.5% were found on a calibration target of known size. The second system consisted of a pair of still-image digital cameras mounted inside a midwater trawl. Processing of the stereo images allowed fish length, fish orientation in relation to the camera platform, and relative distance of the fish to the trawl netting to be determined. The video system was useful for surveying fish in Alaska, but it could also be used broadly in other situations where it is difficult to obtain species-composition or size-composition information. Likewise, the still-image system could be used for fisheries research to obtain data on size, position, and orientation of fish

Action Sport Cameras As An Instrument To Perform A 3d Underwater Motion Analysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Action sport cameras (ASC) are currently adopted mainly for entertainment purposes but their uninterrupted technical improvements, in correspondence of cost decreases, are going to disclose them for three-dimensional (3D) motion analysis in sport gesture study and athletic performance evaluation quantitatively. Extending this technology to sport analysis however still requires a methodologic step-forward to making ASC a metric system, encompassing ad-hoc camera setup, image processing, feature tracking, calibration and 3D reconstruction. Despite traditional laboratory analysis, such requirements become an issue when coping with both indoor and outdoor motion acquisitions of athletes. In swimming analysis for example, the camera setup and the calibration protocol are particularly demanding since land and underwater cameras are mandatory. In particular, the underwater camera calibration can be an issue affecting the reconstruction accuracy. In this paper, the aim is to evaluate the feasibility of ASC for 3D underwater analysis by focusing on camera setup and data acquisition protocols. Two GoPro Hero3+ Black (frequency: 60Hz; image resolutions: 1280x720/1920x1080 pixels) were located underwater into a swimming pool, surveying a working volume of about 6m(3). A two-step custom calibration procedure, consisting in the acquisition of one static triad and one moving wand, carrying nine and one spherical passive markers, respectively, was implemented. After assessing camera parameters, a rigid bar, carrying two markers at known distance, was acquired in several positions within the working volume. The average error upon the reconstructed inter-marker distances was less than 2.5mm (1280x720) and 1.5mm (1920x1080). The results of this study demonstrate that the calibration of underwater ASC is feasible enabling quantitative kinematic measurements with accuracy comparable to traditional motion capture systems.118Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (Sao Paulo Research Foundation) [00/1293-1, 2006/02403-1, 2009/09359-6]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (National Counsel of Technological and Scientific Development) [473729/2008-3, 304975/2009-5, 478120/2011-7, 234088/2014-1, 481391/2013-4]Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (Brazilian Federal Agency for Support and Evaluation of Graduation Education) [2011/10-7, 08/2014]Fundacao de Amparo a Pesquisa de Minas Gerais (Minas Gerais Research Foundation) [PEE-00596-14]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES