3D vision 1

Question 1

what does a binocular observer gain?

Answer 1

binocular stereopsis (and convergence of eyes)

simple geometry would suggest that binocular stereopsis apparently provides complete information about the relative distances of P1 and P2 - as long as we know the distance between the two eyes

However BS is often said to only provide relative rather than absolute information

this is because we can converge our eyes such that fixation point falls on the fovea of both eyes

whatever our eyes converge on has zero disparity

non-zero disparities provide information relative to point zero

Question 2

what is the vieth-muller circle

Answer 2

the locus of all positions in space that have zero disparity such that they stimulate anatomically corresponding points on the two retinae

zero disparity does not tell us that two points are the same distance

Question 3

what are horizontal disparities

Answer 3

disparities measured around a horizontal axis on the two retini (which are separated horizontally)

disparities tell us about the position of a point with respect to the Vieth-Muller circle

outside V-M circle: uncrossed/divergent disparities

inside V-M circle: crossed or convergent disparities

Question 4

do disparities tell us about depth

Answer 4

No - The same disparity is created by a small amount of depth at a close viewing distance or a large
amount of depth at a far viewing distance.

Question 5

what is the inverse square law

Answer 5

the disparity of the retina is inversely proportional to the square of the viewing distance

Question 6

do we use disparities?

Answer 6

to test must control stimuli and remove cues about depth and distance

Wheatstone’s stereoscope allows images to be presented to the left and right eyes independently

doesn’t show binocular disparity is sufficient for depth perception

need to remove other depth cues

Question 7

do random-dot stereograms provide evidence we use disparities

Answer 7

pictorial stereograms contain other cues/sources of information

important to use random-dot stereograms

in a key region, dots in the left eye’s image are shifted relative to the dots in the right eyes image

this should produce the impression of a square floating above the background

when this is convergent: the square appears to protrude

when this is divergent: the square appears to recede

Question 8

how can red-blue glasses create the same effect as random-dot stereograms

Answer 8

Essentially the same pattern but one is blue and black while the other is red and black
In the image presented to the left eye some of the dots have been shifted from the centre
The red dots got through the red filter and the blue dots go through the blue filter
When these are viewed as superimposed there is a central square floating in depth
Perceiving the depth distance rather than the shape of the object

Question 9

what do RDS tell us about stereopsis

Answer 9

stereopsis can occur according to a new model where object recognition happens after binocular combination and stereopsis

this differs from the traditional model where it was though that monocular object recognition occured before binocular combination and stereopsis

Not true that random dot kinematograms have don’t have monocular features
There must be some features that are compared between the two eyes: dots, edges, micropatterns
Can’t achieve a process of monocular combination if there is no structure in the eyes able to do so
Arguing about a continuum of extent of monocular processing
What is being compared?

Question 10

what is being matched in RDS

Answer 10

not individual dots as they can be seen over a wide range of dot sizes and densities and look identical

could be clusters as local clusters look very different, may be that the statistical properties are inferred rather than the shape

similarity of contrast is arguably essential

Question 11

how well do we see depth in RDS

Answer 11

Test whether participant correctly reports whether centre square is in front or behind.
Manipulate the stimulus to make the depth discrimination easier or harder, and search for the point
where the participant gets it right 75% of time.
(note that 50% correct is chance performance).

features that are matched must be:
similar size
similar shape
same contrast in two eyes

Question 12

what is the correspondence problem (Julesz)

Answer 12

for every dot in one eye’s view, there are thousands of possible matches with dots of the same brightness

Question 13

what are the solutions to the correspondence problem

Answer 13

Julesz suggests two processes
local matching
global matching - (disambiguates local matches)

problem is ill-posed

problem of matching/ correlating individual dots

if, instead local areas are matched the problem can be solved (best local correlation)

Question 14

what is Julesz’s shift and subtract model

Answer 14

based on his solution to the correspondence problem

the amount of shift needed to bring areas into alignment is equivalent to the disparity

Question 15

what is Julesz’s spring-couples dipole model

Answer 15

local matches COOPERATE to achieve a global solution

Question 16

what constraints is marr & poggio (1976) model of stereopsic matching based on

Answer 16

investigated constraints to exploit to find a solution that corresponds to properties of the world and our visual system:
similarity
only matching features which are similar
Eyes are closely spaced so images to the two eyes are generally similar

uniqueness
only match one feature with one other
we live in a world of opaque surfaces, therefore there can’t be more than one match down a given line of sight

smoothness
select neighbouring matches that are similar in disparity
depth generally varies smoothly because of the world of surfaces

Question 17

what is marr & poggio (1976) model of stereopsic matching

Answer 17

incorportated these constraints into a biologically plausible model containing binocular cells or nodes of all possible disparity matches

similarity is incorporated because only white dots can be matched with white dots and vice versa

uniqueness is incorporated through inhibition down line-of-sight columns

smoothness is incorporated through excitation between cells responding to the same disparity

running all constraints produces black circles as most probable

Good model but limited to a single spatial scale as it only works for dots of a given size

Question 18

what is Marr & Poggio (1979) new radically different model

Answer 18

separate processing in the left and right eyes

monocular spatial frequency channels at different spatial scales (Low and High)

local independent spatial frequency tuned disparity matching - separate matches at different spatial scales

unitary global broad-band stereopsis

stage 1: create multiple representations at different spatial scales (coarse, medium, fine)

stage 2: matching between zero crossings (edges) separately for the different spatial scales

matching is multiscale
order of processing is from coarse to fine scale matching

no correspondence problem as there aren’t multiple possible candidate matches
at the coarse scale there is one match between L and R

this coarse scale match then guides vergence eye movements to bring middle and finer scale representations in the two eyes into approximate alignment, restricts the range of disparity matches so we don’t have to consider all possible matches

disparity matches are limited to the size of the filter (size-disparity correlation)

false matches never arise - avoids rather than solves correspondence problem

Question 19

summary

Answer 19

3D vision is not a special problem, need to consider if there is information available not whether the retina is 2D

binocular disparities proide information about location with respect to the vieth muller circle

RDSs tell us that some monocular processing must take place in stereopsis and similarity is important in the matching process

however the correspondence problem may not be a real problem that has to be solved explicitly depending on what you assume is being matched