Fig. 1. Binocular disparities used for stereopsis.

(A) Two views of a simple 3D scene. The eyes are fixating point F₁, which lies straight ahead. Point P is positioned above and to the right of the viewer’s face, and is closer in depth than F₁. The upper panel shows a side view and the lower panel a view from behind the eyes. Lines of equal azimuth and elevation in Helmholtz coordinates are drawn on each eye. (B) Retinal projections of P from the viewing geometry in (A). The yellow and orange dots correspond to the projections in the left and right eyes, respectively. The difference between the left and right eye projections is binocular disparity. The difference in azimuth is horizontal disparity, and the difference in elevation is vertical disparity. In this example, point P has crossed horizontal disparity, because it is closer than point F₁ and the image of P is therefore shifted leftward in the left eye and rightward in the right eye. (C) For a given point in the scene, the disparity at the retinas can change substantially depending on where the viewer is fixating. In the left panel, the same point P is observed, but with a different fixation point F₂ that is now closer to the viewer than P (indicated by the arrow). The original fixation point F₁ is overlaid in gray. In the right panel, the retinal disparities projected by P are shown for both fixations [disparities from (B) are semitransparent]. For this viewing geometry, point P now has uncrossed horizontal disparity: that is, the image of P is shifted rightward in the left eye and leftward in the right eye.