Theories Of Vision

Question 1

what are the three theories of vision

Answer 1

marr - computational theory
gibson - ecological theory
Bayesian Inference

Question 2

define Marr’s computational theory

Answer 2

visual system works by info processing
3 tiered system
1. theory - define input and output, and establish constraints
2. algorithm - use a specified logical/mathematical algorithm to yield an output from input
3. implementation - nerve cells carry out algorithm

Question 3

give an example for 1. computational theory

Answer 3

binocular stereopsis (3d vision)
input = image from eyes, output = depth due to view as midpoint from both eyes 
contraint - images correspond to only one scene point

Question 4

give an example for 2. algorithm

Answer 4

stereoalgorithms - light/dark boundaries, course/fine detail as input to compute depth
ie. very blurred likely to be CLOSE

Question 5

give an example for 3. implementation

Answer 5

‘disparity sensitive neurons’ - used in stereopsis to implement depth

Question 6

what is marrs heirachy of representations

Answer 6

image - primal sketch - 2 1/2 D sketch - 3D model - action

primal sketch and 2 1/2 D sketch seperates imageinto its seperate components ie geometry, reflectance, illumination and viewpoint

Question 7

define raw primal sketch

Answer 7

first in hierachy of representations
detects changes in intensity, geometric structures and illuminations
ie. gausian blurring

Question 8

what does the raw primal sketch map

Answer 8

important primatives within an image
ie blobs - small areas bounded by changes in intensity
edge segments - sudden changes in intensity
bars - two parallel edge segments
termination - sudden discontinuity

Question 9

define blobs (RPS)

Answer 9

small areas bounded by changes in intensity

Question 10

define edge segments (RPS)

Answer 10

sudden changes in intensity

Question 11

define bars (RPS)

Answer 11

two parallel edge segments

Question 12

define termination (RPS)

Answer 12

sudden discontinuity

Question 13

define full primal sketch

Answer 13

after raw primal sketch, primatives are grouped and sumarised to construct a larger and more abstract geometic illusion
produces a hierachy of primatives ie hairs > stripes > texture
presents location, shape, texture and details

Question 14

define 2 1/2 D sketch

Answer 14

occurs during full primal
imput of info about slants and distance - look at depth relation, distance between obecs and observer to work out how far an object extends

Question 15

what does 2 1/2 d sketch use to determine depth

Answer 15

motion cues
texture cues
disparity between two eyes

Question 16

how do we determine orientation in 2 1/2 D sketch

Answer 16

primitives contain vectors

description viewer centred not object centred as contains no external info about the object

Question 17

define 3d model

Answer 17

simple primatives are hierachied to produce a more abstract image ie shapes
object centred and doesnt matter about perception/movement

Question 18

define gibson’s ecological approach

Answer 18

emphasise importnce of natural scenes and perceptual guidance of movement
visual info controls action
form judgement via direct percepion without representations/computations
ns resonates scene and picks up info
image and heuristics cause action

Question 19

what kind of heuristics are used in the ecological approach

Answer 19

finding where youre headed
avoiding obstacles
passing through a gap
judging contact time
catching a ball

Question 20

what provides directional cues in ecological approach

Answer 20

optic flow
movement of the world around in specific directions ie image in centre gets bigger and close moves faster - moving towards

Question 21

what invariants are used to depict depth

Answer 21

texture gradient
horizon ratio relation
flow and ambient optical array
motion parallax
occlusion

Question 22

define texture gradient

Answer 22

the density of elements increases with distance - becomes more compact the further away they are

Question 23

define horizon ratio relation

Answer 23

distance - distance of person from below horizon to the horizon (bottom up)
height - distance of person from above horizon to the horizon (top down)

Question 24

define flow and ambient optical array

Answer 24

ambient optical array - light reflect off surface of objects and converge at observer, this changes as the observer moves
the movement of the observer determines flow of ambient optical array - outflow and inflow and change in pole direction

Question 25

define outflow of ambient optical array

Answer 25

signifies approach
centre coming closer
peripheral objects move outwards

Question 26

define inflow of ambient optical array

Answer 26

signifies retreat
centre becoming smaller and further away
peripheral objects move inwards, more detail becomes more compact (texture)

Question 27

define change in pole direction

Answer 27

centre becomes shifted as observer changes directin

Question 28

define motion parallax

Answer 28

acts as a depth cue

objects closer to us move slower than those further away

Question 29

define occlusion

Answer 29

percieve depth by obects in front of one another

perceive objects continuing behind another despite occlusion

Question 30

describe lee and reddish 1981 ganets and visual triggers

Answer 30

use ecological theory t odetermine action time
see fish in water and estimates time of contact
time = initial retinal size / rate of expansion
uses to determine wing closure for maximum impact

Question 31

compare marr and gibson

Answer 31

marr use ventral “what” stream - necessary for fine detail and pre existing knowledge

gibson use dorsal “where” stream - necessary for action, and rapid decisions - not require memory

both may be correct as explain different phenomena

Question 32

define bayesian inference

Answer 32

emphasises the importance of prior knowledge when faced with poor sensory info
converges what our senses tell us about the world ie what we observe but cannot recognise, and what our knowledge tells us about the world ie previous experience about what things look like
this can determine what we see ie see cow in blurred image
compare prior knowledge and the image to give the likelihood of different interpretations