Fig. 1.

The geometry of stereopsis. The eyes are fixating the white sphere, which therefore falls on the fovea in both retinae. The black sphere is closer to the observer; its images therefore fall at non-corresponding points in the two retinae. Its distance z from the fixation point can be deduced from its retinal disparity δ = r−l and the interocular distance I. To deduce its absolute position in space, we also need to know the positions of the eyes. In principle, these can be deduced from purely retinal information, provided the locations of the retinal images of enough objects are known (Hartley and Zisserman, 2000).