Ray Tracing for Underwater Illumination Estimation

Oceanographic research often involves taking high resolution underwater images which can be analyzed in many ways including counting specific species such as scallops, identifying the spread of invasive species like didendum, and determining the effects of environment changes on ecosystems. In order for large scale deployment of imaging systems to be feasible, automatic techniques are required to assist in data analysis. Due to the optical properties of water, light is significantly attenuated due to both absorption and scattering in a manner dissimilar from air. Absorption affects light as a function of wavelength, attenuating the low more than high visible wavelengths. Also, since research vehicles are often operated at depths well below the euphotic depth, which by definition, is the depth at which less than 1% of sunlight penetrates, they must provide their own lights. These strobes cast a nonuniform illumination pattern on the seafloor which varies with height and orientation of the craft. Computer vision algorithm are often sensitive to uneven illumination so correcting the illumination is a very important preprocessing step. This paper approaches illumination correction from a modeling perspective. We attempt to accurately model the camera, seafloor, lights and absorption of water in full spectrum for a specific sled vehicle called the HABCAM. The hope is that the HabCam can be accurately modeled through the use of ray tracing without photon mapping. Our primary global assumptions are (1) scattering effects of the water column are uniform, and can therefore be approximated alongside our absorption parameter, (2) the ocean floor is roughly planar, (3) the reflectance properties of the ocean floor can be modeled by a simplified Phong model.