Underwater 3D Surface Capture Using Multi-view Projectors and Cameras with Flat Housings

This paper is aimed at realizing a practical image-based 3D surface capture system of underwater objects. Image-based 3D shape acquisition of objects in water has a wide variety of academic and industrial applications because of its non-contact and non-invasive sensing properties. For example, 3D shape capture of fertilized eggs and young fish can provide a quantitative evaluation method for life-science and aquaculture. On realizing such a system, we utilize fully-calibrated multiview projectors and cameras in water (Fig. 1). Underwater projectors serve as reverse cameras while providing additional textures on poorly-textured targets. To this end, this paper focuses on the refraction caused by flat housings, while underwater photography involves other complex light events such as scattering [3,16,17], specularity [4], and transparency [13]. This is because one of the main difficulties in image-based 3D surface estimation in water is to account for refractions caused by flat housings, since flat housings cause epipolar lines to be curved and hence the local support window for texture matching to be inconstant. To cope with this issue, we can project 3D candidate points in water to 2D image planes taking the refraction into account explicitly. However, projecting a 3D point in water to a camera via a flat housing is known to be a time-consuming process which requires solving a 12th degree equation for each projection [1]. This fact indicates that 3D shape estimation in water cannot be practical as long as it is done by using the analytical projection computation. To solve this problem, we model both the projectors and cameras with flat housings based on the pixel-wise varifocal model [9]. Since this virtual camera model provides an efficient forward (3D-to-2D) projection, we can make the 3D shape estimation process feasible. The key contribution of this paper is twofold. Firstly we propose a practical method to calibrate underwater projectors with flat housings based on the pixel-wise varifocal model. Secondly we show a system for underwater 3D surface capture based on space carving principle [12] using multiple projectors and cameras in water.