Binocular geometry · seen from above
Two eyes, one fixation point, two objects. Move things around and see where each object lands in each eye. The difference between the two views is binocular disparity, one of the brain’s cues to depth.
All of the readouts come from one comparison: where an object sits relative to the fixation point in the left eye, compared with the right eye.
Load “Thumb at 20 cm” and compare the left-eye and right-eye rows. Then load “Object across the room”. How far does each object jump between the two eyes? Turn the difference into a rule your brain could use.
Watch the disparity label as A moves from inside the circle to outside it. When A is seen double, which ghost image comes from the left eye? Does that flip when A is farther than fixation? Check with your own thumb by closing one eye.
Load “Floating finger” and look at the Both eyes row. Which two images overlap to make the floating piece? Drag the fixation point closer to the fingers. What happens to the sausage, and why?
Compare the two “10 cm behind” setups. The physical gap is the same in both. How many times larger is the disparity up close? What does this say about where binocular disparity is a useful depth cue?
How it’s modelled. Each eye is a single nodal point, and the eyes sit 6.3 cm apart (a typical adult interocular distance) on a horizontal line. An object’s visual direction in each eye is measured relative to the fixation point. Disparity is the left-eye direction minus the right-eye direction, which equals the angle the two eyes make at the object minus the angle they make at fixation. Zero-disparity points fall on the Vieth–Müller circle through both eyes and the fixation point. The real (empirical) horopter is somewhat flatter than this circle. Objects within 15′ (0.25°) of zero disparity are drawn fused in the Both eyes row; beyond that they are drawn double. Real fusion limits (Panum’s area) grow with eccentricity and vary between people. Only horizontal positions are modelled; height in the eye-view rows is decorative.