In the recent paper by Weijer et al. [9], the authors proposed the Grey-Edge hypothesis, which assumes that the average edge difference in the scene is achromatic. The Minkowski norm of color derivatives is used to approximate the light source color. In this paper, we point out that the Minkowski norm of these derivatives can actually be interpreted by the raw moment of the distribution of these derivatives. Furthermore, we discovered that the central moment of the distribution can also be utilized to estimate the light source color, and comparable results may be obtained.

If an object is interreflected between two planar mirrors, we may take an image containing both the object and its multiple reflections, i.e., simultaneously imaging multiple views of an object by a single pinhole camera. This paper emphasizes the problem of recovering both the intrinsic and extrinsic parameters of the camera using multiple silhouettes from one single image. View pairs among views in a single image can be divided into two kinds by the relationship between the two views in the pair: reflected by some mirror (real or virtual) and in a circular motion. Epipoles in the first kind of pairs can be easily determined from intersections of common tangent lines of silhouettes. Based on the projective properties of these epipoles, efficient methods are proposed to recover both the imaged circular points and the included angle between two mirrors. Epipoles in the second kind of pairs can be recovered simultaneously with the projection of intersection line between two mirrors by solving a simple 1D optimization problem using the consistency constraint of epipolar tangent lines. Fundamental matrices among views in a single image are all recovered. Using the estimated intrinsic and extrinsic parameters of the camera, a euclidean reconstruction can be obtained. Experiments validate the proposed approach.