How do we percive space
- we percive 3D shapes and objects
- our eyes (retina image) is only 2D
- our brain has to reconstruct the 3D image
Types of depth cues
monocular cues
- Pictorial (static) cues
- movement cues
Binocular cues (binocular disparity)
non-retinal cues
Monocular cue meaning
one eye
so all the cues within the Monocular cue category are available for one eye
binocular cue
two eyes
What are the monocular pictorial cues
occlusion
linear perspective
size
texture gradient
atmospheric perspective
shading
height in visual field
What is occlusion cue? (in monocular -> pictorial (static) cue)
something that is closer to you will hide something further away from you
If A blocks B, A is closer.
Tells you order (near vs far) but not exact distance.
What is linear perspective cue? (in monocular -> pictorial (static) cue)
Parallel lines appear to converge
train track example
What is size cue? (in monocular -> pictorial (static) cue)
Even though an object may physically be the same size if one appears smaller, the brain perceives it to be further away
What is texture gradient cue? (in monocular -> pictorial (static) cue)
in near surfaces you see textured gradients with detail
In further away surfaces you see blurrier or denser surfaces
What is atmospheric perspective cue? (in monocular -> pictorial (static) cue)
Far mountains look lighter, hazier, bluer than the moutains right in front of you that look darker or grenner
What is a shading cue? (in monocular -> pictorial (static) cue)
So: shading is a guess about shape based on assumed light direction.
Same circle with different light/shadow can look convex (bulging out) or concave (sunken).
The brain assumes light comes from above (because of the sun).
What is a height in the visual field cue? (in monocular -> pictorial (static) cue)
The things near me appear lower in my view.
The things farther away appear higher up in my view.
That’s because as objects get farther away from me, the angle from my eyes to them gets smaller, so their image ends up higher on my retina (or in a photo).
What are the monocular movement cues
motion parallax
kinetic depth affect
What is motion parallax (monecular movment cue)
The idea that objects that is closer to you moves more compared to objects far away from you.
You move → near objects move a lot across the retina; far objects move a little
But also
When you move, objects in your view seem to move too — but the direction they appear to move depends on how far they are from the point you’re looking at (your fixation point).
Objects closer than your fixation point (like a fence by the road)
→ seem to move in the opposite direction to your movement.
(You move right → the fence seems to move left.)
Objects farther than your fixation point (like mountains)
→ seem to move in the same direction as your movement.
(You move right → the mountains seem to move right too.)
What is the kinetic depth effect?
A 2D pattern can be perceived as providing depth information when moving
When something moves, your brain can “fill in” its 3D shape — that’s the Kinetic Depth Effect. Even without texture or shading, movement alone reveals the object’s structure and depth
What is binocular disparity? (binocular cue)
because our eyes are in different locations - the image we get through our right eye vs our left eye is different
The greater the disparity is between the two eyes - the further away the object must be
What is Stereopsis
the process where depth information is processed from binocular disparity
What are the non retinal cues?
Monocular and binocular cues are cues due to images on the retina, which are the source of the depth information.
Non-retinal cues are not due to the retina:
- accommodation cue
- convergence cue
What is accommodation? (non-retinal cue)
How you change the thickness of lens of the eye
Your eyes’ way of adjusting the lens shape to keep things in focus indicates to your brain how far away something is.
Because the amount of muscle movement changes depending on the distance, the brain sees how much muscle movement there is and utilises it as a depth cue.
What is convergence? (non-retinal cue)
When you focus on something, the two eyes have to rotate inward or outward to converge on the said object.
This rotation changes systematically according to distance - indicating to the brain a depth cue
What happens when cues are insufficent/ambigious?
Because many depth cues are ambiguous, sometimes the brain can find two valid 3D interpretations from the same 2D image. You cant see the two at the same time though
examples
necker cube
shroeder staircase
Called bistable (2 states) or multistable perception.
What is motion perception?
- Objects in the environment are constantly moving
- Even if they are not in motion, you are rarely perfectly still
- Perceiving images out of this busy environment that is constantly moving is difficult
Cues
observer movement
object movement
Illusory (apparent movement)
When detecting observer movement, what is the powerful cue?
Optic flow
What is optic flow (observer movement)
When you move through the world, the whole retinal image flows in a lawful pattern → optic flow.
o Move forward → pattern expands from the point you’re heading to.
o Move backward → pattern contracts.
o Rotate → everything slides across the retina.
We are very sensitive to small changes in optic flow → helps with navigation and posture.
