Lecture 10 Perception: The problem of depth perception

Question 1

The problem of depth perception

Answer 1

the world is 3D but the retinal image is 2D issue of the visual system extracting 3D depth from the 2D retinal image

Question 2

Two-dimensional image in each eye

Answer 2

straight lines project onto curved retina
euclidean input
non-euclidean image

Question 3

What changes the visual angle

Answer 3

both size and distance

Question 4

Relation between the degree of visual angle on the retina and size of the object

Answer 4

directly proportionate
the bigger the object the larger the angle on the retina

Question 5

Relation between visual angle and how far away the object is from the person

Answer 5

inversely proportional - the same size object at twice the distance from a person is going to generate a smaller angle on the retina.

Question 6

Monocular depth cue: based on projective geometry: occlusion

Answer 6

occluded objects are farther in depth, unoccluded objects are nearer

Question 7

Monocular depth cue: based on projective geometry: relative size

Answer 7

All else being equal:

• Retinal image size is smaller for objects farther in depth
• Retinal image size is larger for objects nearer in depth

Question 8

Monocular depth cue: based on projective geometry: relative height

Answer 8

The height of the image from the base of the image/image plane.

If the object is lower in the image plane then it will be closer to you, if the object is higher it will be further away.
Objects farther away from us in the visual field lower in the retinal image

Question 9

Monocular depth cue: based on projective geometry: texture gradients

Answer 9

Gradual change in the appearance of an object from coarse to fine - some objects appear closer because they are coarse and more distinct, but gradually become less and less distinct and and more fine, which makes the objects appear to get further and further away.

Question 10

Monocular depth cue: based on projective geometry: familiar size

Answer 10

depth cue based on knowledge of the typical size of objects

Question 11

Monocular depth cue: aerial perspective

Answer 11

light scatter by the atmosphere causes farther objects to appear hazier, less distinct

Question 12

Monocular depth cue: linear perspective

Answer 12

parallel lines in a 3D world converge in the 2D image
Vanishing point: point at which lines converge

Question 13

Anamorphic art

Answer 13

use of linear perspective to create depth in a 2D image from a single view point
only works if the person viewing the scene is standing at a particular point

Question 14

motion parallax

Answer 14

to do with moving through a scene
depth cue bases on head movement, or retinal image at two different points in time
Objects in depth are displaced more in the image than objects further away

Question 15

Extraretinal depth cues: accomodation

Answer 15

When we focus on an object close to us our lens is bulging - more convex

When we focus on an object far away from us our lens is becoming flatter’

Change in accommodative state - a source of info as to how far away the object is from us

Question 16

When does accomodation become an ineffective cue to depth

Answer 16

objects further than 2-3 metres as there is a limit as to how flat the lens can get

Question 17

Extrarential depth cues: convergence

Answer 17

Eyes rotate inward or outward focus on objects: Vergence angle is a depth cue.

Looking at something far away the vergence angle is smaller, looking at something close the vergence is smaller

The convergence state of the eye tells us how far an object is away from us.

Near: converge, Far: diverge

Question 18

convergence

Answer 18

the way your eyes move together and point inward when you look at nearby objects

Question 19

diverge

Answer 19

an object farther away, they rotate away from each other

Question 20

human field of vision compared to rabbits

Answer 20

humans: 190 deg - predator eyes close in front of the head
rabbits: 360 deg - prey need to see all round

Question 21

binocular disparity

Answer 21

110 deg seen by both eyes
but eyes separated in the head by 6 cm so each eye sees a slightly different image
disparity between retinal image is a depth cue

Question 22

stereopsis

Answer 22

the ability to use disparity as a depth cue

Question 23

where is disparity

Answer 23

centre of gaze

Question 24

where is non-zero disparity

Answer 24

outside the centre of gaze

Question 25

zero disparity meaning

Answer 25

means that the images received by both eyes are identical

Question 26

non-zero disparity meaning

Answer 26

a difference in the image seen by each eye

Question 27

corresponding retinal points

Answer 27

fovea and pairs of points equidistant from fovea

Question 28

what is the Vieth-Müller circle:

Answer 28

points in space whose images fall on corresponding retinal points theoretical geometric prediction in reality isn't exactly a circle

Question 29

what happens to objects that arent in Vieth-Müller circle

Answer 29

will have some non-zero disparity

Question 30

What is a horopter

Answer 30

true zero-disparity plane, flatter than a circle deviated systematically from V-M circle measured behaviourally using special apparatus always included the objects of fixation (fovea)

Question 31

Zero-disparity objects and horopter

Answer 31

lie on the horopter and are seen as fused

Question 32

Non-zero disparity objects and horopter

Answer 32

lie off the horopter and produce diplopia (double vision)

Question 33

Panum's fusional area

Answer 33

Zone of non-zero disparity around the horopter where objects are perceived as fused.

Question 34

Crossed disparity

Answer 34

objects closer than fixation (nearer to you than the horopter)Fixating at something far away and the object closer to you will appear double.

Question 35

uncrossed disparity

Answer 35

objects farther from fixation (farther from you than the horopter) Fixating at something near the object far away will appear double

Question 36

how the disparity information is resolved

Answer 36

by disparity sensitive neurons in the brain

Question 37

Stereoacuity

Answer 37

The smallest difference in depth can be detected by from disparity How much objects can be displaced before realising they arent lined up 0.1 arc minute (i.e., 1/60 x 1/10 = 0.0016 degrees)

Question 38

Stereoblindness

Answer 38

the inability to use binocular disparity as a depth cue 3-5% of population

Question 39

Strabismus:

Answer 39

misalignment of the ocular axes a condition where the eyes are not aligned properly, causing them to point in different directions, potentially leading to double vision or other vision problems 2-5% of the population

Question 40

Strabismus: esotropia

Answer 40

eye deviates inward

Question 41

Strabismus: exotropia

Answer 41

eye deviates outwards

Question 42

How do disparity-sensitive neurons in the brain work

Answer 42

Binocular neurons tuned to sign and magnitude of disparity * some neurons respond more for zero or low disparity * others for higher disparity * Similar orientation and spatial frequency preference

Question 43

3D illusions and applications from stereopsis: Wheatstone stereoscope (Charles Wheatstone, 1830s)

Answer 43

an optical instrument that uses two slightly different images, one for each eye, to create the illusion of a three-dimensional image

Question 44

3D illusions and applications from stereopsis: Holmes stereoscope, 1850s Inventors: David Brewster, Oliver Wendell Holmes

Answer 44

Creates a more realistic impression of what it would actually be like if you were at the scene

Question 45

3D illusions and applications from stereopsis: Stereoscopic camera, 1950’s

Answer 45

- Sputnik: took 3D photos that could be viewed with a stereoscope - Screens in the camera separated by same distance as eyes in the head are if it was much more cant fuse if it is too less doesn't match out disparity perception - Causes fusion to happen - producing a 3D image

Question 46

3D illusions and applications from stereopsis: Free fusion (poor man’s stereoscope)

Answer 46

refers to the ability to view stereoscopic images in 3D without the aid of special glasses or a stereoscope, by simply converging or diverging your eyes to fuse the two images.

Question 47

Random dot stereograms

Answer 47

refers to the ability to view stereoscopic images (like those in a stereoscope) in 3D without the aid of special glasses or a stereoscope, by simply converging or diverging your eyes to fuse the two images.

Question 48

Dichoptic viewing

Answer 48

the input has been split to the two eyes using the filters, the key difference in the image is the disparity as if projected to each eye in the head.

Question 49

Polarized glasses (modern dichoptic method)

Answer 49

- Light waves vibrate in different directions, but can be filtered to vibrate in a single direction - Two cameras, spaced 6-7 cm apart, capture the scene with different polarizations (e.g., horizontal vs. vertical) - The two images are projected on the screen, each with its own polarization - Viewers wear glasses with polarizing filters that allow only the corresponding image to reach each eye, creating 3D effect

Question 50

Other applications of stereopsis:

Answer 50

- 3D medical imaging and surgical planning - 3D terrain mapping - 3D ground scanning, surveillance and other defence applications - Robotics, machine vision, self-driving cars

Question 51

Size constancy

Answer 51

Perceiving objects as constant in size changes in viewing distance Closer objects cast larger retinal images, further objects cast smaller retinal images Yet perceived size doesn’t change that much Perceived size is constant becuase we discount distance.

Question 52

Ames room:

Answer 52

A person appears to grow in size when walking across the room room appears regular but is actually distorted (trapezoidal) * corners of the room are at different distances from the observer * retinal image size differs (is smaller for farther corner) * perceived distance to the two corners is the same (while actual distance differs) * therefore, perceived size differs Unable to use the distance cues correctly to judge their size correctly.

Question 53

Ponzo illusion

Answer 53

Same physical size, upper line appears longer * could be due to depth induced by linear perspective (upper line appears farther away) * could be an artifact of how the visual system processes tilted lines when they are shown with horizontal lines (i.e., not related to depth)