As a biologically inspired guess, we consider two stereo information channels. One is the traditional channel that works on the basis of the horizontal disparity between the left and right projections of single points in the 3D scene; this channel carries information regarding the absolute depth of the point. The second channel works on the basis of the projections of pairs of points in the 3D scene and carries information regarding the relative depth of the points; equivalently, for a given azimuth disparity of the points, the channel carries information of the ratio of the orientations of the left and right projections of the line segment between the pair of points.