New generation of underwater 2-D forward-look sonar video cameras operating at near and over 1-MHz frequency offer images with enhanced target details. Within their limited range of only tens of meters, they are the most suitable imaging systems for conducting visually guided missions in turbid waters. These include, but are not limited to inspecting underwater structures for routine maintenance, search, and surveillance, the detection, localization, and identification of small sought after objects, as well as target-based positioning and navigation. To this end, the automatic 3-D reconstruction of 3-D target shape and establishing the 3-D spatial location of interest points are highly desired capabilities in many such operations. As achieved with 2-D optical images, multiple images from nearby positions may be utilized. This paper investigates the estimation of 3-D point locations from two overlapping images collected with two forward-scan (FS) sonar systems in stereo configuration, or with one FS sonar at known relative poses, established from visual motion cues. The first contribution includes the analysis of a sonar stereo epipolar geometry. Beyond reducing the correspondence problem to a 1-D search along epipolar curves, this reveals unique properties associated with sonar measurements of range and azimuth angle. Next, certain linear closed-form reconstruction solutions are presented, their degeneracies are established, and adjustments under degenerate conditions are proposed. Two preferred method for the degenerate and nondegenerate stereo configurations are identified: a regularization-based method and one employing a range constraint approximation, respectively. These primarily provide the initial estimate for an iterative nonlinear optimization scheme based on gradient descent. Finally, the results of experiments with synthetic data are presented to assess various estimation methods, and with real data sets to demonstrate performance under two subsea operational scenarios.

中文翻译：

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前向扫描声纳立体声在3D场景重建中的应用

以接近和超过 1 MHz 频率运行的新一代水下二维前视声纳摄像机提供具有增强目标细节的图像。在只有几十米的有限范围内，它们是最适合在浑水中执行视觉引导任务的成像系统。这些包括但不限于检查水下结构以进行日常维护、搜索和监视、探测、定位和识别小型目标，以及基于目标的定位和导航。为此，3-D 目标形状的自动 3-D 重建和建立兴趣点的 3-D 空间位置是许多此类操作中非常需要的功能。正如使用 2-D 光学图像实现的那样，可以利用来自附近位置的多个图像。本文研究了从使用两个立体配置的前向扫描 (FS) 声纳系统或一个 FS 声纳在已知相对姿势下收集的两个重叠图像中估计 3-D 点位置，根据视觉运动线索建立。第一个贡献包括对声纳立体对极几何的分析。除了将对应问题简化为沿极线的一维搜索之外，这还揭示了与声纳距离和方位角测量相关的独特属性。接下来，提出了某些线性封闭形式的重建解决方案，建立了它们的退化，并提出了退化条件下的调整。确定了退化和非退化立体配置的两种优选方法：基于正则化的方法和采用范围约束近似的方法，分别。这些主要为基于梯度下降的迭代非线性优化方案提供初始估计。最后，展示了使用合成数据的实验结果以评估各种估算方法，并使用真实数据集展示两种海底操作场景下的性能。