Theories Of Vision Flashcards
what are the three theories of vision
marr - computational theory
gibson - ecological theory
Bayesian Inference
define Marr’s computational theory
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
give an example for 1. computational theory
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
give an example for 2. algorithm
stereoalgorithms - light/dark boundaries, course/fine detail as input to compute depth
ie. very blurred likely to be CLOSE
give an example for 3. implementation
‘disparity sensitive neurons’ - used in stereopsis to implement depth
what is marrs heirachy of representations
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
define raw primal sketch
first in hierachy of representations
detects changes in intensity, geometric structures and illuminations
ie. gausian blurring
what does the raw primal sketch map
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
define blobs (RPS)
small areas bounded by changes in intensity
define edge segments (RPS)
sudden changes in intensity
define bars (RPS)
two parallel edge segments
define termination (RPS)
sudden discontinuity
define full primal sketch
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
define 2 1/2 D sketch
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
what does 2 1/2 d sketch use to determine depth
motion cues
texture cues
disparity between two eyes
how do we determine orientation in 2 1/2 D sketch
primitives contain vectors
description viewer centred not object centred as contains no external info about the object
define 3d model
simple primatives are hierachied to produce a more abstract image ie shapes
object centred and doesnt matter about perception/movement
define gibson’s ecological approach
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
what kind of heuristics are used in the ecological approach
finding where youre headed avoiding obstacles passing through a gap judging contact time catching a ball
what provides directional cues in ecological approach
optic flow
movement of the world around in specific directions ie image in centre gets bigger and close moves faster - moving towards
what invariants are used to depict depth
texture gradient horizon ratio relation flow and ambient optical array motion parallax occlusion
define texture gradient
the density of elements increases with distance - becomes more compact the further away they are
define horizon ratio relation
distance - distance of person from below horizon to the horizon (bottom up)
height - distance of person from above horizon to the horizon (top down)
define flow and ambient optical array
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
define outflow of ambient optical array
signifies approach
centre coming closer
peripheral objects move outwards
define inflow of ambient optical array
signifies retreat
centre becoming smaller and further away
peripheral objects move inwards, more detail becomes more compact (texture)
define change in pole direction
centre becomes shifted as observer changes directin
define motion parallax
acts as a depth cue
objects closer to us move slower than those further away
define occlusion
percieve depth by obects in front of one another
perceive objects continuing behind another despite occlusion
describe lee and reddish 1981 ganets and visual triggers
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
compare marr and gibson
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
define bayesian inference
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
