2. D
e
p Objectives
t
h
p deals at first with some general theory about visual
e
r depth cues (oculomotor, monocular and binocular
c cues) from the psychological point of view.
e
p details about some possibilities to convey depth
t
i and shape surface in computer based visualization
o
n
Gauri S. Shrestha, M.Optom 2
3. D
e
p Introduction
t
h
p we have developed perceptual systems capable of
e representing three dimensional environment with
r
c sufficient accuracy to guide our behavior.
e
p The human visual system is able to reconstruct
t
i depth information from flat images through
o
n combination of a set of many different impulses,
cues and patterns
Distance estimations, hints about an object's
surface and other visual cues are essential for all
of us
Gauri S. Shrestha, M.Optom 3
4. D
e
p General theory of depth cue
t
h
p Oculomotor cues are, as the name suggests, possibilities
e for depth estimation by controlling the eye axes and the
r lens focus using the eye muscles.
c Convergence and accommodation are counted among that
e
p kind of cues which interact with each other in depth
t perception.
i The angle of convergence of the two eyes and their
o accommodative states are one source of scaling
n information
They must be supplemented by other cues even at close
ranges.
Thus they are considered to be minor cues in depth
perception.
Gauri S. Shrestha, M.Optom 4
5. D
e
p Monocular vs. Binocular Cues
t
h
p Monocular Depth Cues
e
r Cues perceived as strong with one eye as with two eyes
c Binocular Depth Cues
e
p Cues that must be viewed with both eyes
t Greatly enhance depth perception
i
o
n
Gauri S. Shrestha, M.Optom 5
6. D
e
p Monocular Depth Cues
t
h
p Pictorial Depth Cues
e Size
r
c Linear Perspective
e Texture
p
t Interposition
i Clarity
o Lighting and Shadow
n
Motion Parallax
Accommodation
Aerial perspective
Gauri S. Shrestha, M.Optom 6
7. D
e
p Pictorial Depth Cues -Size
t
h
p Larger objects are interpreted
e as being closer
r
c Relative Size- If objects are
e thought to be the same size, the
p
t one that appears smaller is
i perceived as being farther
o away
n
Familiar Size-Used viewing
objects of a known size.
Object distance determined
based on previous knowledge
of its size
Gauri S. Shrestha, M.Optom 7
8. D
e
p Relative size
t
h
p
e
r
c
e
p
t
i
o
n
Gauri S. Shrestha, M.Optom 8
9. D
e
p Relative height
t
h
p Points that are situated
e
r closer to the horizon
c seem to be more
e
p distant than points that
t
i are farther away from
o the horizon.
n
Gauri S. Shrestha, M.Optom 9
10. D
e
p Pictorial Depth Cues-
t
h Linear Perspective
p Related to relative size
e
r Sense of depth is experienced due to our
c
e conception that objects have a constant
p shape and size
t
i
o
n
Gauri S. Shrestha, M.Optom 10
11. D
e
p Pictorial Depth Cues -Texture
t
h
p Smaller, densely packed objects appear far away
e
r Larger, loosely packed objects appear closer
c Observer assumes all objects are the same size
e
p Smaller images are perceived as being farther away
t
i
o
n
Gauri S. Shrestha, M.Optom 11
12. D
e
p Pictorial Depth Cues -texture
t
h
p
e
r
c
e
p
t
i
o
n
Gauri S. Shrestha, M.Optom 12
13. D
e
p Pictorial Depth Cues -Interposition
t
h
p One object blocks view of another
e
r Blocked object is perceived as being farther
c
e away
p
t
i
o
n
Gauri S. Shrestha, M.Optom 13
14. D
e
p Pictorial Depth Cues -Clarity
t
h
p Objects that appear clear are
e interpreted as being closer than
r
c those objects that appear hazy
e Fog, smoke, rain and smog act as
p
t interposing elements that make
i obscured objects seem farther
o
n away
Gauri S. Shrestha, M.Optom 14
15. D
e
p Pictorial Depth Cues -Lighting
t
h and Shadow
p With Light falling on an object, it casts a shadow
e
r The shadow is interpreted as falling behind the
c
e object-creating a sense of depth
p
t
i
o
n
Gauri S. Shrestha, M.Optom 15
16. D
e
p Monocular Depth Cue:
t
h Motion Parallax
p Kinetic Depth Cue
e
r Produced by relative
c motion of two or more
e objects
p
t Near Object Fixation
i Far objects move with
o
n head movement
Distant Object Fixation
Near objects move
opposite of head
movement
Gauri S. Shrestha, M.Optom 16
17. D
e Monocular Depth Cue
p
t
h Motion Parallax
p Whenever we move, the images projected by
e
r objects located at different distances move across
c our retina with different speed whereby nearby
e
p objects tend to cross the retinal image plane faster
t
i than the distant objects.
o
n
Gauri S. Shrestha, M.Optom 17
18. D
e
p Monocular Depth Cue
t
h Accommodation
p Accommodation is necessary to clearly view near
e
r objects
c
e The signal to accommodate contains information
p regarding the distance of viewed objects
t
i
o
n
Gauri S. Shrestha, M.Optom 18
19. D
e
p Aerial perspective
t
h
p Over very long distances in the natural
e
r environment the color of surfaces tends to become
c bluer. (aerial perspective)
e
p The outlines of far away objects become blurred
t
i and the contrast gets reduced
o
n
Gauri S. Shrestha, M.Optom 19
20. D
e
p Binocular Depth Cues
t
h
p Binocular cues are the most important category of cues
e that potentiates humans to experience a three dimensional
r
c world.
e The term binocular cues refers to the fact that both eyes
p
t are required to gain depth information from the projected
i image of the real world.
o
n They are
Stereopsis
Corresponding retinal points
Retinal Disparity
Convergence
Gauri S. Shrestha, M.Optom 20
21. D
e
p Stereopsis
t
h
p Retinal locations of images
e signal the distance of the
r
c object being seen
e Corresponding points-two
p
t retinal points that
i correspond to the same
o
n direction in space (foveas)
Retinal Disparity-distance
between corresponding
points and the image
location
Gauri S. Shrestha, M.Optom 21
22. D
e
p Stereopsis
t
h
p Retinal Disparity
e
r Disparity provides information about spatial depth
c when the images of an object fall upon different retinal
e areas in the left and right eyes
p
t Allows visual system to determine distance of object
i
o It is the function of IPD as well
n Depth is perceived when non corresponding point is
stimulated
Gauri S. Shrestha, M.Optom 22
23. D
e
p Retinal Disparity
t
h
p Uncrossed Retinal Disparity
e Images fall nasal to
r
c fixated object
e Object is farther than
p
t fixated object
i Crossed Retinal Disparity
o
n Images fall temporal to
fixated object
Object is closer than
fixated object
Stereopsis is the perception
of depth produced by
binocular retinal disparity Shrestha, M.Optom
Gauri S. 23
24. D
e
p Stereopsis
t
h
p More important while viewing near objects
e
r Within arm’s length
c Binocular disparity provides depth perception only
e
p if it is not too great
t
i Panum’s Fusional Area-area on the retina that
o corresponds to binocular fusion
n
Physiological diplopia-occurs when images fall on
retinal positions that signal grossly different
directions
Gauri S. Shrestha, M.Optom 24
25. D
e
p Demonstration
t
h
p Hold one index finger 15 cm from nose
e
r Hold other index finger at arms length
c
e Focus on near finger
p
t Notice diplopia
i
o Bring far finger closer to you
n
Notice diplopia disappears
THIS IS PANUM’S AREA
Gauri S. Shrestha, M.Optom 25
26. D
e
p Another way to look at it
t
h
p Every retinal point has a
e corresponding point in the other
r eye
c
e Horopter-a plot of
p corresponding points for a given
t
i distance
o curved plane due to the
n curvature of the retina
Objects relatively close to
the horopter can be fused
Objects are within Panum’s
fusional area
Provides stereopsis for
these objects Gauri S. Shrestha, M.Optom 26
27. D
e
p Disparity Detectors
t
h
p Binocular Cortical Neurons are maximally
e
r responsive to stimuli at a specific distance
c
e The same stimulus placed at different distances,
p elicits a less vigorous response
t
i
o
n
Gauri S. Shrestha, M.Optom 27
28. D
e
p Convergence
t
h
p We constantly converge and diverge our eyes
e
r The amount of convergence required to view a
c
e specific object provides information concerning
p the distance of that object
t
i It is not understood how this information is
o
n incorporated into conscious depth perception
Gauri S. Shrestha, M.Optom 28
29. D
e
p What Are Visual Illusions?
t
h
p Erroneous Perceptions
e
r Commonly result from pictorial depth cues to
c
e judge the size of unfamiliar objects
p
t
i
o
n
Gauri S. Shrestha, M.Optom 29
30. D
e
p Size Constancy
t
h
p The apparent size of
e
r an object does not
c normally change with
e
p viewing distance
t
i Our visual system
o
n compensates for
differences in retinal
image size by taking
into account the
relative distance
Gauri S. Shrestha, M.Optom 30
31. D
e
p Size Illusion
t
h
p Occurs when judgments of distance are erroneous
e
r Size Constancy fails
c
e Examples of Visual Illusions:
p
t Corridor Illusion
i
o Moon Illusion
n
Müller-Lyer Illusion
Gauri S. Shrestha, M.Optom 31
32. D
e
p Corridor Illusion
t
h
p Illustration
e
r Size Constancy fails due to monocular depth cues:
c
e Top line looks farther away
p
t Top line seems larger
i
o Two lines are actually the same size
n
Gauri S. Shrestha, M.Optom 32
33. D
e
p Corridor Illusion
t
h
p
e
r
c
e
p
t
i
o
n
Gauri S. Shrestha, M.Optom 33
34. D
e
p Moon Illusion
t
h
p Moon appears larger if
e viewed on the horizon
r
c Moon appears smaller
e
p when viewed directly
t overhead in a clear sky
i
o Why?
n Moon does not appear to
be at the same distance
On the horizon, moon
appears farther away
Due to background
(trees, houses, fields,
etc.) Gauri S. Shrestha, M.Optom 34
35. D
e
p Müller-Lyer Illusion
t
h
p Lines mimic the corner of a room
e
r The line that appears to be an outgoing corner is
c
e judged as being farther away
p
t Perceived as being a longer line
i
o The line that appears to be an ingoing corner is
n judged as being closer
Perceived as being a shorter line
Gauri S. Shrestha, M.Optom 35
36. D
e
p Müller-Lyer Illusion
t
h
p
e
r
c
e
p
t
i
o
n
Gauri S. Shrestha, M.Optom 36
37. D
e
p Clinically Relevant Topics
t
h
p Stereopsis and Vision Development
e
r Monovision Contact Lenses
c
e
p
t
i
o
n
Gauri S. Shrestha, M.Optom 37
38. D
e
p
t
h Stereopsis Clinically
p Stereopsis Tested Clinically
e
r Patient views flat surface with two identical figures
c
e separated by a small distance
p Each eye views only one of the objects
t
i Polaroid or Red-Green glasses
o
n Minimum amount of disparity that allows patient to
appreciate depth is determined
Gauri S. Shrestha, M.Optom 38
39. D
e
p Stereo-acuity
t
h
p Can be as small as 3 seconds of arc
e
r A high degree of stereo-acuity depends on a
c normal complement of binocular neurons in the
e
p visual cortex
t Anisometropia and Strabismus are two disorders
i
o of binocular vision that retard cortical neuron
n
development
Gauri S. Shrestha, M.Optom 39
40. D
e
p Monovision Contact Lenses
t
h
p One eye for near, One eye for distance
e
r Patient alternates between two eyes to view
c
e objects at various distances clearly
p
t Blurred image is suppressed
i
o Stereopsis is reduced because patient is not using
n both eyes simultaneously
Monocular depth cues are still present
Gauri S. Shrestha, M.Optom 40
41. D
e
p When Will It Matter?
t
h
p
e Stereopsis is more important for near work
r
c Driving can be difficult for these patients
e
p Patients must be advised of ↓ depth perception
t
i Alternatives while driving:
o
n Glasses to correct near eye while driving
Distance only contact lenses for driving only
Gauri S. Shrestha, M.Optom 41
42. D
e
p Thank you
t
h
p
e
r
c
e
p
t
i
o
n
Gauri S. Shrestha, M.Optom 42