Depth, Size And Illusions Flashcards
We perceive…
A 3D world but the image on our retina is 2D
Cues
Use cues to extract depth info
Cues are info in the retinal image that’s correlated with depth
Across our lives we learn to associate cues with depth
Associations become automatic through repeated exposure
What happens to the image on the retina
The image on the retina is upside down. Perception flips it
3 broad cues
Bits of info that are:
1: oculomotor = minds get from muscles around eye (sensing position of eyes)
2: monocluar= extracted from images by either eye independently
3: binocular = extracted from images by both eyes working together
Ocular convergence
Oculomotor Cue
Inward movement of eyes and the corresponding muscle tension when focusing on near objects
Lens accomodation
Oculomotor cue where tension is detected in the ciliary muscles
Accomodation is the change in lens shape when focusing on near or far objects
Far = relaxes
Close = contacts
Monocular cues are mostly
Pictorial cues - information extracted from 2D images
Also non-pictorial cues - info cannot be extracted from static 2D image
Occlusion
When an object partially covers another
Object that is occluded is usually deeper in the visual field
Relative height
Looking below horizon = objects higher are more distant
Looking above horizon = objects lower are more distant
Relative size
When objects are equal size, the closer ones will take up more of your visual field
Perspective convergence
Parallel lines appear to approach each other at greater distances
Perspective convergence in art
European art- effect loss in middle ages and reemerged in Renaissance
Familiar size
Knowledge of familiar objects can help us infer which objects are closer
Atmospheric perspective
Distant objects are blurrier and can have blue/grey tint
Texture gradient
Equally spaced elements are more closely packed as distance increases
Shadow
Position of objects shadow can be helpful to perceive depth
Cues combined
Cues are more useful when they give consistent info
Motion parallax
Non-pictorial monocular cue
Near objects glide rapidly past and far objects move slowly
Occlusion via movement
Non-pictorial cue
Objects are covered and uncovered as we move relative to them
Covering = deletion
Uncovering = accretion
Binocular cues involve
Comparing the slightly different views presented in each eye
Binocular disparity
Disparity between different images created by objects falling in different locations on the retina
Corresponding points
The forea is always the corresponding point as whatever is focused on falls on the same spot on both retinas
Horopter
Unfocused things fall on the corresponding point if they are on the an arc called the horopter
Images not on the horopter fall on non-corresponding points
Crossed disparity
Crossed: object inside horopter = appears closer
Uncrossed: object outside horopter= appears further
Stereoscopes
Take advantages of binocular disparity to create depth in a pair of 2d images
Chromostereopsis
See red circle in front of blue
Some see no differences and minority reverse
Effect reverses on white background
From slightly different refractions of light from colours = non-corresponding points on eye
Corresponding problem
How does our visual system combine images
Potentially from specific features of objects but this doesn’t explain objects like random-dot stereograms
Depth perception other species
Animals use same eves as humans
But binocular disparity requires front facing eyes for overlapping fields of view
Predators tend to have front facing
Prey tend to have side facing (lateral)
Environmental cues to size
Size and depth perception are interrelated
Size is a cue for depth and depth is a cue for size
Retinal image size
Visual angle is the amount of the retina that an image occupies
Far away = smaller image = less angle
Close = big image = greater angle
Your thumb at arms length covers 2 degrees of visual angle
When depth cues are absent, perceived size determined by retinal image size eg- moon and sun have almost same visual angle but moons bigger so it covers sun during eclipse
Removing depth cues
With depth cues provided judgements were based on physical size
With depth cues removed judgements were based on the size of the retinal image eg the visual angle
Size constancy
Perception that objects size is relatively constant even if the size of retinal image changes
Expressed in the size-distance scaling equation
Brains have no control over visual angle but try to keep perceived size of object the same
Eg visual angle goes up distance from object must go down
Emmert’s law
Retinal size of an after image stays constant
Perceived size changes with distance
Closer afterimage = smaller it seems (follows size- distance scale)
Nonveridical perception
Occurs during visual illusions
Stimulus dimensions that are only subjectively perceived to be there
Muller-lyer illusion
Arrows with different faces fins - shorter arrows have inward fins
Explanation: size constancy misapplied to 2D
Perceive fins as belonging to objects pointing
If an objects coming towards us its closer = smaller
Miller-lyer problems with size constancy
Dumbbell version (no fins but circles) = same perception Also occurs in 3D images that don't implicate depth (things at the same distance but spread apart)
Conflicting cues muller-lyer
Perception depends on:
1: actual length of lines
2: overall length of figure
Conflicting cues are integrated into a compromised perception of length
Ponzo illusion
Horizontal objects placed over train tracks
Upper object seems longer but same size
Misapplied Size constancy
Ames room
Room constructed to look normal with one eye view
Left corner is actual further away
Explanations:
1 - size-distance scaling: person on left has smaller retinal image + perceived distance is the same = perceived size must be smaller
2- relative size: size perception is relative to other objects (filling whole room)
Moon illusion
Moon appears larger on the horizon
Explanations:
1 - apparent depth theory: horizon is surrounded by other depth cues
2- hastened heavens theory: horizon is perceived as further away than sky = small retinal image size larger on horizon
Forced perspective
When photos remove depth cues to change the size of people
Leaning tower of Pisa where people look like they’re holding it
