Depth Perception and Stereopsis

Stereopsis

the brain’s magical trick for turning two flat pictures into one vivid, 3D view of the world. Each of your eyes sees the same scene from a slightly different angle because they’re spaced a few centimeters apart. (Depth)

Binocular summation

When your two eyes team up to give your brain a stronger, clearer visual signal than either eye could provide alone. (Clarity)

Monocular depth cues

The clever visual hints your brain uses to judge depth and distance using just one eye. Even without stereopsis (the two-eye trick), these cues help you see the world in 3D.

Occlusion

When one object blocks part of another, the blocker is clearly closer to you.
If a cat walks in front of a chair, you know the cat is nearer—not because of any fancy math, but because your brain reads the overlap as a depth signal.

Size/position

Objects that are smaller or higher up in your field of view usually appear farther away, while larger or lower ones look closer.
Your brain assumes the world is built on familiar proportions, so when two similar objects differ in size, it uses that difference as a distance clue.

Perspective

When parallel lines seem to meet in the distance—like railroad tracks narrowing toward the horizon—your brain interprets that convergence as depth.
The sharper the angle, the farther away things look. Artists love this trick because it instantly makes a 2D scene feel 3D.

Triangulation cues

There are depth cues that depend on comparing the slightly different views each of your two eyes sees. They’re part of binocular vision—the brain’s way of measuring depth by geometry.

Motion

depth cue that gives life and dimension to what you see—especially when you or the objects around you are moving.

Focus

also called the accommodation cue—is a subtle but important monocular depth cue that comes straight from your own eyes’ mechanics.

• When you look at something close, your eye’s lens thickens (muscles contract).

• When you look at something far away, the lens flattens (muscles relax).So if your eyes have to work hard to focus, your brain knows the object is near; if the lenses stay relaxed, it’s far.

Convergence

It is a binocular depth cue—it depends on both eyes working together to judge the distance of an object. When you focus on a near object, your eyes naturally turn inward, or “converge,” so both are pointing at the same target. When the object is far away, your eyes stay more parallel.

Binocular disparity

One of the brain’s most powerful binocular depth cues—the magic behind 3D vision.

Each of your eyes sits a few centimeters apart, so they see the world from slightly different angles. This creates a small difference between the two retinal images, called disparity.

Your brain compares these two images, measures how much they differ, and uses that information to figure out how far away things are.

• Large disparity → the object is close (the views from each eye are very different).

• Small disparity → the object is far away (the two views look almost the same)

Absolute disparity

The measure of how much each eye’s image of an object falls in a different place on the retinas—a key ingredient in how your brain judges depth.

Here’s the simple breakdown:

• Each eye focuses on a single fixation point (the thing you’re looking directly at).

• Anything that isn’t at that exact distance projects onto slightly different retinal positions in each eye.

• If an object is in front of where you’re focusing, it has crossed disparity (images shift outward).

• If it’s behind your point of focus, it has uncrossed disparity (images shift inward).

Relative disparity

The brain’s way of figuring out how far two objects are from each other in depth—not just how far either one is from you.

Here’s how it works:

• Each object in your view produces its own absolute disparity (a small difference between the left and right eye images).

• Relative disparity is the difference between those absolute disparities.

That difference tells your brain which object is closer or farther away—even if your eyes are focused somewhere else.

Corresponding Retinal Points

Locations on each retina that, when stimulated by the same object, make that object appear in the same place in space.

Noncorresponding Retinal Points

areas that don’t match up—stimulation there makes an object appear off in depth, either closer or farther than the fixation point.

Vieth–Müller Circle (Horopter)

This is an imaginary circle that passes through:

• The point you’re fixating on,

• The centers of both eyes, and

• Other points in space that project onto corresponding retinal points.

Everything on that circle appears single and in focus—it’s your binocular “comfort zone.”

Panum’s Fusional Area

Just outside the Vieth–Müller circle lies a small zone where the brain can still fuse slightly different images from each eye into one.
Within this area, small disparities create depth rather than double vision.
If the disparity gets too large—outside Panum’s area—you see double.

Diplopia (Double Vision)

When an object’s image falls on noncorresponding points that are too far apart, your brain can’t fuse them—so you see two of the same object.
Objects closer than your focus cause crossed diplopia; objects farther cause uncrossed diplopia.

Free Fusion

This is when you voluntarily fuse two slightly different images (like in a stereogram) without using a special device.
By crossing or relaxing your eyes, you get both images to land on corresponding points—suddenly a 3D image pops out.
It’s your binocular muscles showing off.

The Correspondence Problem

Your brain must decide which parts of the image in the left eye match which parts in the right eye.
Solving this problem is how your brain figures out depth and keeps your world aligned and single rather than scrambled.

Size Constancy

Even when an object moves closer or farther away (so its image size on your retina changes), your brain perceives its actual size as constant.
A person walking away doesn’t shrink—they just look farther.
Your brain combines depth cues and context to maintain that stable sense of size.

Binocular Rivalry

When each eye sees completely different images that can’t be fused (say, stripes in one eye and dots in the other), your brain can’t combine them—so it alternates between the two.
You’ll see stripes for a moment, then dots, switching back and forth.
It’s a fascinating glimpse into how your visual system negotiates conflicts.