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PPA > 4/5 - DEPTH AND SIZE CUES/ILLUSIONS > Flashcards

4/5 - DEPTH AND SIZE CUES/ILLUSIONS Flashcards

1
Q

depth cues

A

‘sources of information about depth and distance in retinal images’

  • provide information about the third dimension (depth) from two dimensional retinal images

depth perception = ‘visual perception of the third dimension’

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2
Q

binocular depth cues

A
  • occur only when both eyes are open due to the differences between the retinal images occurring due to the eyes being in different locations on the head
  • both images are presented simultaneously
  • there is only 1 binocular cue to depth
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3
Q

monocular depth cues

A
  • only present in a singular image

- but if you close one eye then you can still see depth so there must be cues in the image

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4
Q

pictorial depth cues

A
  • cues present in the picture
  • when you move your head or eyes the image is the same (on the whole) so it must be cues in the picture that provides information about depth
  • only for monocular!!
  • photographs, paintings, drawings etc
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5
Q

non-pictorial depth cues

A
  • not present in static images

- present in moving images and are present when the head is in motion

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6
Q

occlusion

A

‘when one object obscures an object that is behind it from an observer’

‘occluded object must be further away’

‘only know depth order, don’t know distance’

ORDINAL CUE - depth order

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7
Q

metric cue

A

‘cue that gives information about absolute or relative distance’

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8
Q

how does size provide a basis for depth cues?

A

1 - image size can NOT provide information about distance

2 - image size depends on the objects distance from the eye, the larger an object is, the closer it is

knowing an objects size can be used to estimate its distance by comparing it to the size of the image

image size is a depth cue to very familiar objects (that you know the size of)

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9
Q

assumed focal length of eye is 25mm

A

-

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10
Q

familiar size cue

A

‘the use of prior knowledge about an objects size’

METRIC CUE

  • involves assumptions that the object viewed does have the same size
  • ‘get information about the distance of an object from the size of its image if you know the actual size of the object’
  • but very limited evidence of this
  • measured in how many eye diameters away it is
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11
Q

what if actual size is unknown?

RELATIVE SIZE CUE

A
  • size can still provide depth information
  • only if there are objects of the same type at different distances to one another
  • the smaller ones must be further away as the objects are meant to be the same size
  • metric cue
  • only provides relative information about depth
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12
Q

retinal image variation

A

the size of the retinal image changes with the distance between the eye and the object

a greater distance will results in a smaller image

a retinal image which covers the entire retina will only be 2.5cm across

foveal retinal images (what we’re looking at) = 1.5mm across

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13
Q

perceptions of size

A
  • don’t see the size of the image, you see a ‘perceptual estimate of its physical size’
  • ‘this perceived size does not change when the distance between you and the objects gets smaller or larger’
  • IMAGE SIZE CHANGES

SIZE CONSTANCY - as physical size doesn’t change so it must be the perceived physical size you’re estimating
- only works up to 100m away

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14
Q

how far does size constancy work til?

A

100m

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15
Q

size constancy

A

SIZE CONSTANCY - as physical size doesn’t change so it must be the perceived physical size you’re estimating
- only works up to 100m away

  • ‘not seeing properties of the retinal images (the proximal stimulation, which changes if the viewing conditions change), you’re seeing properties of the world (distal stimuli) that do not vary with the viewing conditions’
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16
Q

the perceptual problem of getting size information from a retinal image

A
  • ‘the size of the image depends upon how far away the object is’
  • very far = near zero
  • mm from the eye = whole retina
  • ‘shows size and distance are connected through image size’
  • need something else other than just the size of the image
  • a small image could be a large thing far away or a small thing close
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17
Q

EQUATION

familiar size cue

A

object size (S) / distance from eye (D)

=

image size (s) / eye diameter (E)

  • objective quantities
  • basic geometry
  • page 3 week 5
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18
Q

REARRANGED EQUATION - for object distance

A

object distance (D)

= (S x E) / s

S = object size 
E = eye diameter (2.5cm usually)
s = image size
19
Q

what are the units of size and distance perception?

A
  • not really known

- maybe eye diameter, leg length, hand width, interpupillary distance (distance between both pupils of the eyes)

20
Q

emmert’s law information

A
  • discovered by Emil Emmert (swiss opthalmologist)
  • ‘described the relationship between visually perceived size and visually perceived distance based on his studies of after images’
  • visually perceived (properties of experience) not actual physical size and distance (properties of the world)
  • eg looking at a bright bulb and then looking away and seeing a ‘bright blob in the retinal position of where the bulbs image was located’
  • ‘appear to change in colour’
  • the after image can ‘vary in size depending upon where you look’
  • look nearby = after image appears relatively small
  • look far away = after image appears much larger
  • ‘perceived size of an after image increases as the perceived distance of the background against which it is viewed increases’
  • ‘the region of the retina from which the after image originates remains the same’
  • ‘the after image always has the same retinal size regardless of the distance of the background against which it’s viewed’
21
Q

emmert’s law definition

A

‘objects with images that are the same size will be perceived as having a physical size that depends on how far away they appear to be, such that the perceived size increases roughly in proportion to increases in perceived distance’

  • but this law ‘doesn’t tell us how big something will appear to be if we know how big it’s image is and we know how far away it is perceived to be’
22
Q

size-distance invariance hypothesis (SDIH)

A

perceived size / perceived distance

  • ‘the ratio of perceived size to perceived distance is equal to ratio of measures image size to eye diameter (E)’

measured image size / eye diameter

  • perceived size depends on perceived distance
  • these are subjective quantities as they’re psychologically known
  • this is the ‘relationship between perceived and measured quantities, not the actual physical ones’
  • ‘perceived quantities are related in exactly the same way as the physical ones’
23
Q

SDIH perceived size in terms of perceived distance

A

perceived size = perceived distance x (measured image size / E)

  • ‘the size of something is seen to be depends on how far away it is perceived to be (perceived distance) and how big its image is’
24
Q

difference between emmert’s law and SDIH?

A
  • emmert’s law is a ‘restricted and imprecise version of the SDIH’
25
Q

SDIH and size constancy

A
  • ‘if distance perception is accurate, then the SDIH implies size constancy’
  • so ‘changes in image size are compensated for by corresponding changes in perceived distance and perceived size will remain approximately the same’
  • PERCEIVED SIZE IS CONSISTENT
26
Q

size constancy

A

= ‘the perceived size of an object remains the same despite changes in image size that occur due to changes in its distance’

only when objects are less than 10-20m away

  • this is because we are ‘familiar with very distant things appearing to be very small - thats size constancy not working’
  • doesn’t mean SDIH is wrong !!
  • maybe means distance perception isn’t accurate at large distances
27
Q

ponzo size illusions

A

having two objects of the same size look like different sizes due to the contour lines (depth cues - linear perspective) of the background

  • works as an illusion as if the image was real life, then the one that looks larger would be larger
  • the monsters in the tunnel
  • they’re identical so their retinal images would be identical
  • emmert’s law = ‘something with the same sized image will look larger if it is perceived as further away’
  • monster higher in tunnel = seems further away so it perceived as larger
  • depth cues in the picture mislead the visual system into thinking the higher monster is larger
  • ‘the depth cues are being used to estimate the distance of the two monsters and their sizes’ follow emmert’s law and SDIH
  • the tunnel is also illusionary as it’s not a tunnel, it’s just the way the lines are drawn that gives the effect of a tunnel
  • but most people wouldn’t say the tunnel is an illusion because it’s not surprising as they know it’s a drawing
  • the monsters are usually considered an illusion as it’s surprising and the visual percept differs from reality
28
Q

most depth cues are relative

  • ‘give information that one object is further than another, but not in absolute units of length/distance’
  • distance information = is in terms of the object themselves and the relative depth
  • exact distance cannot be determined by relative depth cues
A

-

29
Q

which two cues give absolute distance information?

A

familiar size cue and vergence cue

30
Q

relative height cue

A

‘closer objects on the ground plane are lower in the visual field mm’

  • height in visual cue
31
Q

texture gradient

A

‘multiple similar elements distributed over the image that are larger lower down in the image’

‘get progressively smaller and higher’ > ‘produces a pattern that gives a strong impression of depth’

‘ground plane receding in the distance - don’t need discrete elements for the effect’

  • ‘if elements are laid out in an orderly fashion = texture gradient cue and a perspective cue (later)’
    ^ eg a brick pavement
32
Q

foreshortening cues

A
  • increased foreshortening = type of depth cue
    ^ eg further away lily-pads appearing more oval shaped and smaller
  • can create 3D effects
33
Q

most depth cues are relative

  • ‘give information that one object is further than another, but not in absolute units of length/distance’
  • distance information = is in terms of the object themselves and the relative depth
  • exact distance cannot be determined by relative depth cues
A

-

34
Q

which two cues give absolute distance information?

A

familiar size cue and vergence cue

35
Q

aerial perspective (haziness)

A

‘a cue based on the implicit understanding (an assumption) that light is scattered by the atmosphere’

  • this can give an impression of depth
  • ordinal cue (depth order)
  • no information about relative or absolute distance
  • kinda shows shadows etc
  • provides ordinal depth information - what’s in front or behind
36
Q

linear perspective

A
  • pictorial depth cue
  • ‘based on the fact that lines are parallel in the three dimensional world will appear to converge in a two dimensional image
  • vanishing point = point of convergence
  • need to be aligned correctly with the radial lines
  • remove all texture, aerial perspective, shadowing and occlusion cues
37
Q

list of cues

A

occlusion

  • ordinal
  • near to&raquo_space;30m

familiar size

  • metric (absolute)
  • near to >30m

relative size

  • metric (relative)
  • near to >30m

relative height

  • metric (relative)
  • near to >30m

texture gradient
- combination is relative size and height

aerial perspective

  • ordinal
  • > > 30m

shadows

  • metric?
  • <10m?

linear perspective

  • metric (relative)
  • 2-»30m
38
Q

metric monocular cues

A
  • linear perspective
  • texture gradient
  • motion parallex
39
Q

non-visual cues to depth

A
  • accommodation of the eyes

- vergence

40
Q

over 100m+

A

no vergence cue and binocular disparity

41
Q

phenomenologically a visual perception

A
  • multimodal percept
  • ‘closing one eye’ = ‘no massive effect in judgement’
  • removing a cue = negligible effect
42
Q

absolute distance

A
  • vergence
  • familiar/known size
  • vertical disparity
  • provides a scale factor
43
Q

relative distance

A
  • occlusion (ordinal)
  • ‘known distance in a proportional manner’ (objects and yourself)
  • ‘don’t know the scale of the relative distance
  • ‘need an absolute distance for at least one visible object in the scene’

PPA (19 decks)

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