Please join us for the following AOS Seminar Thursday, November 14 at 4:00pm in Spiess Hall 330:
Speaker: Tali Treibitz, Postdoctoral Researcher, Scripps Institution of Oceanography
Title: "Underwater Photometric Stereo"
Photometric stereo is widely used for 3D reconstruction of objects. The method is based on estimating surface normals from intensity changes under varying illumination. The use of photometric stereo in scattering media such as haze, water, and biological tissue has been very limited as the underlying physical light propagation model differs significantly from that of air. In scattering media light is attenuated in an amount that is distance dependent, i.e., depends on the surface shape to be estimated. Forward scattering blurs both the light source and the object signal. Light scattered back from the medium toward the camera is an additive component in the image, with intensity that depends on the light source geometry and object distance. In addition, nearby light sources, which are rarely used for \ps reconstruction, are essential for obtaining proper illumination intensity. In this work our contributions are: we use the insight that by imaging fluorescence emission, the backscatter can be eliminated from the image, and we derive a compact representation for the remaining problem including water attenuation, nearby light sources, and a perspective camera. We analyze the effect of forward scattering on shape reconstruction for uniform light sources and Lambertian surfaces. Our formulation requires four light source locations and can be solved by a simple optimization scheme. We demonstrate the method through experiments in turbid water.