Multichannel shape from shading techniques for moving specular surfaces

Abstract

This paper describes a shape from shading technique for the reconstruction of transparent moving specular surfaces such as the winddriven wavy water surface. In contrast to classical shape from shading techniques that are based on reflection, the new technique is based on refraction. Specular surfaces require area-extended light sources in order to apply the shape from shading principle. With three or more properly arranged light sources, the surface gradient can be coded almost linearly in image irradiance ratios in order to achieve a maximum accuracy for the surface normals. This retrieval technique is also in first-order independent of the transmittance of the refracting surface. Two realizations of this system are discussed. The first system uses a color illumination scheme where the red, green, and blue channels of the light source radiance are changing linearly in different directions and a 3-CCD color camera. The second system uses a monochromatic light source with more than 16 000 LEDs and a four-way control electronic that generates four pulsed intensity wedges shortly after each other. Both systems have been used to retrieve the small-scale shape of wave-undulated water surfaces in wind/wave facilities and the ocean. This paper thus demonstrates a successful example how computer vision techniques have helped to solve a longstanding experimental problem in environmental sciences and now give an unprecedented insight into complex spatiotemporal phenomena.