Depth Perception
1
Depth Perception
A Definition…
Registering how far away an object is
Or
How far apart two or more objects are
A Description of the Problem Faced
By the Brain…
The crucial importance of depth
perception
• Recognition of location in space is essential
for almost all activities
– navigating/avoiding objects
– jumping
– catching/throwing
– reaching/grasping
– size judgements and recognition
• As a result, many redundant systems
available for assessing depth and distance
Some Factors Involved in Depth Perception:
Distance, Absolute & Relative
Cues Which Provide Depth Information
Oculomotor: Accommodation
Depth Information
Oculomotor
Accomodation
Visual
Convergence
Binocular
Monocular
Motion
Parallax
Static Cues
Perspective
Size
Interposition
Shading
1
Depth Perception
Oculomotor Cues to Depth Perception:
Accommodation
2
Depth Cues: Oculomotor Convergence
Depth Information
Accomodation
Strain Guage
Oculomotor
Lens
Visual
Lens
Accomodation
Convergence
Binocular
Monocular
Convergence
Strain Guage
Motion
Parallax
Static Cues
Extraocular Muscles
Fattening of the intraocular lens to increase its optical
power and focus images of near objects on the retina
Changing Accommodative Power
Perspective
Size
Interposition
Shading
chameleon
spectacles
accommodation
Convergence
varying lens power on “spectacles” leads
to predicted over- and under-shoot of tongue
Rene Descartes
Traite d’homme
c. 1636
The amount of convergence
required to bi-foveate an
object is related to the
distance of the object.
Descartes’ Hypothesis:
depth via the measurement
of convergence angle
2
Depth Perception
Convergence Depth Cues
3
Visual Cues to Depth: Binocular & Monocular
Depth Information
Accomodation
Strain Guage
Oculomotor
Lens
Visual
Lens
Accomodation
Convergence
Binocular
Monocular
Convergence
Strain Guage
Motion
Parallax
Static Cues
Extraocular Muscles
Monitor muscle strain (afference)?
Monitor neural commands to muscle (efference)?
Experimentally, can monitor with a strain gauge
The Distinction Between Monocular
& Binocular Cues
Perspective
Size
Interposition
Shading
Visual/Monocular/Static/Interposition
Depth Information
Oculomotor
Accomodation
Visual
Convergence
Binocular
Monocular
Motion
Parallax
Static Cues
Perspective
Interposition
(also called overlap)
Size
Interposition
Shading
Interposition Is a More Powerful Cue than Size
One object blocks the view of another
3
Depth Perception
4
Depth Information
Oculomotor
Accomodation
Visual
Convergence
Binocular
Monocular
Motion
Parallax
Static Cues
Perspective
Size
Interposition
Shading
Relative size as a cue
Size alone can influence perceived depth
Which one looks closer?
Size is an ambiguous
cue unless the
physical size of the
object is known.
Smaller squares seem to recede in depth…
Visual Monocular Static Cue: Perspective
Depth Information
Oculomotor
Accomodation
Visual
Convergence
Binocular
Monocular
Motion
Parallax
Static Cues
Perspective
Size
Interposition
Shading
4
Depth Perception
5
Perspective - most important or at least
most studied static, monocular depth cue.
- depends on changes in visual angle with
distance
Albrecht Durer
given a viewing point, locate
real world points on the picture plane
1471-1528
Linear perspective
Linear Perspective convergence of parallel lines in depth
Linear perspective
2 point
3 point
5
Depth Perception
Linear Perspective Can Overwhelm
Our Certainty About Size Cues…
And create illusory perceptions of relative size…
6
Example of a drawing employing
Linear Perspective (ca 1500)
Squares in corner not lined up-gives impression of a rise in the
street
Linear Perspective: A powerful cue…
Does perspective distort our
sense of visual angle in the
picture plane?
Which pair of white dots has the greatest
vertical separation ?
Ames Room
linear perspective dominates our perception
of distance (and size)
(Identical twins!)
6
Depth Perception
7
Ames Room explained
Perceived
room
(relation to
Emmert’s law?)
Real enclosure
one twin is twice as far as the other
Escher Staircase
Pre-Renaissance Paintings
Poor Perspective & Size Cues-Not Solved Until 15th Century
Textural gradients:
An Additional Monocular Cue
Textural gradients
Textural gradient
7
Depth Perception
Texture discontinuities:
8
An Example of Texture Gradients Creating Depth Percepts…
• Similar to interposition; texture changes
abruptly at depth boundaries
An Example of Texture Discontinuity
Relative height in the visual field:
Atmospheric perspective
Atmospheric perspective
• More distant objects are higher in the
field of view
Distant objects have lower contrast and a
blue tinge due to light scatter in the
atmosphere
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Depth Perception
9
Atmospheric (Aerial) Perspective
Shading
The Effect of Inverting an Image on Perceived Depth
An old observation: different distances and angles
From a light source produce different shadows
Same Picture as Last Slide, Only Inverted…
9
Depth Perception
Shading:
• The shading on an object can provide a
sense of 3-dimensionality
10
Shading or Lighting as Cues to Depth
Shading can also mislead…
Shading as a Cue to Depth
An explanation of depth from shading…
Light
Dark
Dark
Light
10
Depth Perception
11
Differential fMRI activation depending
on shading
In viewing a natural scene many pictorial
cues to depth are usually present
The Influence of Shading on Depth
Relative size
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Interposition
Light and Shade
Textural gradient
The Final Monocular Cue: Motion Parallax
Motion Parallax
Depth Information
Oculomotor
Accomodation
Visual
Convergence
Binocular
Monocular
Motion
Parallax
Static Cues
Perspective
Size
Interposition
Shading
11
Depth Perception
Motion Parallax
12
Motion Parallax: The Movie!
QuickTime™
QuickTime™ and
and aa
Sorenson Video decompressor
are
are needed
needed to
to see
see this
this picture.
picture.
The relative apparent motion of objects
Within your field of view whenever you move
Motion Parallax is Dependent on Where You Are Looking
What Happens on the Retina Depends on Fixation Distance
C
Object at B is fixated
B
A
A2
A1
C2
C1
B1
B2
Time 1
Time 2
Direction of Movement
Fovea
Image of object at A moves right to left on the retina, while the
image of object at C moves left to right on the retina.
Image Movement on the Retina is Opposite the Physical World
Of course right to left movement in the world causes the
image on the retina to move from left to right. So, left to right
'image' movement means right to left 'object' movement.
Direction of object movement
Time 3
Time 2
T1
T2
Optic Flow:
• Characteristic changes in the optic flow
field
Time 1
T3
Direction of image movement
• Provides information
about heading rather
than absolute depth
information
12
Depth Perception
13
Limitations of the monocular cues:
• Most of the cues provide only qualitative
information; i.e. indicate that an object is
closer
• Question of whether these cues must
be learned
• Some only apply to distant objects
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