3D rendering of shipwrecks from synthetic aperture sonar

Synthetic aperture sonar (SAS) bathymetric mapping provides centimeter-scale lateral and vertical resolution of the seafloor. Typically, SAS data has been visualized as 2D images or gridded 2.5D surfaces, but it can also be plotted as 3D point clouds. Even if the imaging system is not fully volumetric, the imaging geometry leaves the possibility of having data points that reside in the same lateral positions in a ground coordinate system. Visualizing these without geometric distortion or loss of data is challenging. In this paper we have investigated the use of full-fledged 3D rendering with artificial lighting and shading techniques using the open-source software Blender in order to convey more of the available information to a human observer. The motivation for this study is to produce better 3D visualization of shipwrecks from a chemical munitions dump site in the Skagerrak sea south of Norway. We provide an overview of which data products are available from state-of-the-art SAS processing, and explore existing visualization methods. We discuss the use of 2.5D surfaces, 3D surfaces and point clouds for 3D rendering, as well as various shading and rendering techniques. These include lighting, surface shading, volumetric shading, emission shading and camera techniques. Our goal is to display SAS bathymetry in a way that is both intuitive and appealing to a general audience, and can convey more supplemental information to the observer than other visualization techniques. Finally, we show image and video examples of rendering large shipwreck scenes using data collected by a HISAS1032 SAS carried by a HUGIN autonomous underwater vehicle.