Week 5 : Depth Perception Flashcards
1
Q
The problems for the visual system…
A
- interpreting a 3D world on a 2D retina
- we have an organized representation of visual space that is maintained in the visual cortex
- but… the representation is 2D which our world is 3D
- regardless of the distance an object is from the eye, light is imaged by the same receptors on the flat retina… so there is no depth perception at the retina
2
Q
Monocular depth cues
A
- cues that can be perceives with one status eye
- 2 categories… pictorial cues & motion cues
3
Q
Pictorial cues (monocular)
A
- cues from which we can judge depth from static non-moving pictures
- top-down understanding from experience
4
Q
Occlusion - pictorial cue
A
- occurs when one object partially hides/occludes the view of a second object
- we infer the hidden object is farther away from us than the object that obstructs it
5
Q
Relative height - pictorial cue
A
- objects closer to the horizon are seen are farther away (horizon = earth meets sky)
- objects at the lower bound of an image are closed to us than the horizon & objects above horizon are closer (e.g. cloud, tree branch)
- used a lot in art for depth
6
Q
Familiar size - pictorial cue
A
- when we judge distance based on our existing knowledge of the sizes of objects
- if we know something is smaller than another object but it is taking up more space on the retina, we assume it is closer to us
- this allows us to predict the relative distance each object is located from the eye
- this is why you arent likely to be fooled by a pic of someone leaning on the tower of Pisa
7
Q
Linear perspectives - pictorial cues
A
- percept that parallel lines appear to get closer as they get farther away
- relies on experience & we know parallel lines dont meet to must be cause we are far away
- perceptually parallel lines meet at the horizon (relative height)
- 1st cue to appear in kids art
8
Q
Texture gradients - pictorial cue
A
- many environments are comprised of an arrangement of objects that we know are approx. same size & same distance from each other
- so, an area of the image where objects appear larger & less tightly packed it closer to us than smaller tightly packed objects
- texture finer/smoother when far away
9
Q
Atmospheric perspective - pictorial cue
A
- objects in distance looks blurred and bluish
- our atmosphere scatters more blue light than other wavelengths, so more distant objects should appear blueish
- because far away objects appear fuzzy cuz not all the light is travelling in a straight line to us
10
Q
Shadows - pictorial cue
A
- shadows provide a depth cue cuz the object is in front of the shadow and the angle of the shadow gives cues
- objects in the shadows must be farther from the light
- depend on our life experiences w/ light sources like the sun
11
Q
Motion cues (monocular)
A
- arise from the motion of an object across the retina, or our own body movements to judge depth & distance
12
Q
Deletion & accretion - motion cue
A
- simplist motion cue
- essentially dynamic occlusion cues generated by a moving object
- an object moves across the visual field, if it disappears behind another object (deletion) it must be rather than the occluding object
- e.g. walking behind bookshelves
13
Q
Motion parallax - motion cue
A
- different speeds at which objects at different depths sweep across the retina
- objects closer to us, sweep across a larger portion of the retina when we move the same distance as objects farther away
- looking out car window, far objects move slow & close objects fly by
- eye can be stationary & still observe it
- objects moving sideways through the visual field
14
Q
Optic flow - motion cue
A
- motion of objects as we move forward/backward through environment
- point of focus = focus of expansion
- far away objects appear to move more slowly than close objects that rush toward us quicker (periphery so fast)
- helps navigate movement -> flow & movement cycle (e.g. gymnasts)
- crucial to balance & staying upright esp in early development
15
Q
Oculomotor cues
A
- involve the movement of the eyeballs themselves
- accommodation & vergence
16
Q
Accommodation - oculomotor cue
A
- to focus an image produced by incoming lights at different distances, the lens changes shape
- process can be felt & provide cues to depth, but its minimal use cause of the imprecision observed across different lighting conditions
17
Q
Vergence - oculomotor cue
A
- vergence occurs when the eyes rotate inward to see a near object & outward (diverges) when looking at a more distant object
- automatic process but brain senses movements & this feedback provides info about distance
- can provide info to about 2m in distance
- challenge for developing VR
18
Q
Binocular vision & disparity
A
- a large portion of what is visible to humans exists in binocular vision
- visible to both eyes
- representations on each retina aren’t identical
- our brain stitches these images together to extract depth cues & perceive a continuous single scene (stereopsis)
- this is binocular disparity
- how 3D glasses work
19
Q
Corresponding retinal points
A
- the 2 retinae can be thought of as having corresponding points/locations that would overlap perfectly if the 2 eyes were superimposed
- non corresponding points would not overlap perfectly = double image (think of finger and lamp)
20
Q
Horopter
A
- corresponding points fall on the horopter
- horopter is an imaginary arc through the visual field that contains the fixation object & all other objects located the same distance from viewer as fixation object
21
Q
Horopter & double vision
A
- non-corresponding points not in the horopter give rise to seeing doubles (diplopia)
22
Q
Horopter & visual disparity
A
- horopter related to point of fixation so it changes
- location of horopter also influences the type of disparity generated by other objects in the visual field
- crossed vs uncrossed disparity
23
Q
Crossed disparity
A
- direction of disparity for objects that are closed to the viewer than the horopter (cross eyes)
- image in the left eye is to the right of the image of the object in the right eye
24
Q
uncrossed disparity
A
- farther than the horporter (uncrossed eyes)
- image in the left eye is to the left of the image in the right eye
25
Q
Disparity tuning
A
- use magnitude of disparity to provide depth cues
- in cortex, after 2 images have been combined, we have binocular cells that have similar tuning properties & discern object depth
- many of these cells receive their inputs from corresponding points on the retina
- however, there is a subset of binocular cells that receive their inputs from non-corresponding retinal areas & thus tuned to visual disparities (V4-V1)
- we do not have binocular depth info till we are 4 months old