Specular Stereo

A glossy highlight, viewed stereoscopically, can provide information about surface shape. For example, highlights appear to lie behind convex surfaces but in front of concave ones. A highlight is a distorted, reflected image of a light source. A ray equation is developed to predict the stereo disparities generated when a point source of light is reflected in a smooth, curved surface. This equation can be inverted to infer surface curvature properties from observed stereo disparities of the highlight. To obtain full information about surface curvature in the neighbourhood of the highlight, stereo with two different baselines or stereo with motion parallax is required The same ray equation can also be used to predict the monocular appearance of a distributed source. A circular source, for instance, may produce an elliptical specular patch in an image, and the dimensions of the ellipse help to determine surface shape. 1 INTRODUCTION When the reflectance of a surface has a specular as well as a diffuse component, the viewer may see highlights. Highlights can give extra information about surface shape Ikeuchi [8] uses photometric stereo with specular surfaces to determine surface orientation Beck [l] notes that stereo vision might be able to perceive highlights on a convex surface as lying beneath the surface Grirnson [7] incorporated lambertian and specular components of reflectance into stereo But he found that the computation of surface orientation could be numerically unstable. Here a computation is proposed that is less ambitious than Crimson's, in that it. attempts to determine only local surface geometry, at specular points But it avoids relying on precise assumptions a b o u t surface reflectance Instead, the only assumption is that a specularity can be detected in an image, and its position measured For instance a method like that of Ulltnan [ 15] could be used Thereafter specularities are matched in the same way as features in conventional stereo [6,9.10]. The disparity of a stereo-matched specular point is then compared with the disparity of any nearby surface features The basic principle of the surface shape estimation relies on the properties of curved mirrors (fig 1). To interpret, specular stereo, both horizontal and vertical disparities are used. Ideally, three non-collinear eyes are needed to obtain full information about local curvature. Alternatively, parallax from a known vertical motion of a viewer, combined with conventional stereo geometry, is just as good If only a static, stereo view is …