Shape from underwater image shading

In the field of machine vision, most research in recovering shape from image shading assumes a non-attenuating non-scattering medium, such as air. Generally, the developed methods cannot be directly used for undersea applications where the absorption and scattering of the medium may not be negligible. The authors discuss several issues associated with recovering shape from underwater image shading. The authors have developed an underwater image formation model, which takes into account the effect of the attenuation phenomenon on the irradiance. Based on this, the reflectance map for a Lambertian surface illuminated by a point source is given. This model has been used to recover the orientation of uniform planar patches. The experimental data show that the simplified model also provides a quite accurate approximation of the image brightness of curved surfaces. The authors also investigate the uniqueness of shape recovery from underwater images. The authors prove that the shape of a curve-generated surface can be uniquely determined from its image shading, provided that the depth at the object boundary is given. These findings are useful for underwater image understanding, object recognition, and other undersea applications