Transcript The Human Visual System Part 2: Perception Imaging Science Fundamentals

The Human Visual System
Part 2: Perception
Imaging Science Fundamentals
Chester F. Carlson Center for Imaging Science
Visual Perception
 How
one visually interprets a scene
 4 forms of perception to be studied:
 Depth
 Color
 Temporal
 Motion
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Chester F. Carlson Center for Imaging Science
Depth Perception
 How
does one determine how far away
an object is located?
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Chester F. Carlson Center for Imaging Science
Depth Perception
 Monocular
Cues
 Require
only 1 eye to perceive depth;
Cyclops.
http://www.shag-art.com/cyclops.html
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Chester F. Carlson Center for Imaging Science
Depth Perception
 Binocular
 Require
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Cues
2 eyes to perceive depth.
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Monocular Cue #1
 Interposition
(Overlap)
 An
object that is
partially covered by
another object is
farther away.
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Chester F. Carlson Center for Imaging Science
Monocular Cue #2
 Familiar
Size
 Previous
knowledge of
object sizes aid in judging
distance.
Which object appears closer?
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Chester F. Carlson Center for Imaging Science
Monocular Cue #3
 Linear
Perspective
 The
farther away an
object is the smaller
it appears to be.
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Chester F. Carlson Center for Imaging Science
Monocular Cue #4
 Atmospheric
Perspective
 Objects
farther off in
the distance appear less
saturated and less
sharp (fuzzier) than
those nearby.
 The more atmospheric
particles between the
viewer and a distant
object the more light
that is scattered.
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Chester F. Carlson Center for Imaging Science
Monocular Cue #5
 Motion
Parallax
 Stationary
objects that
are physically closer
to a moving viewer
appear to shift faster
than those farther
away.
 Example

1
Driving by in a car
looking at objects near
and far (animation).
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Chester F. Carlson Center for Imaging Science
Monocular Cue #5

Example 2





Example 3
Close one eye.
Hold your left thumb
upward at arm’s length.
Hold your right thumb
upward at half arm’s length.

Position the thumbs so the
right thumb blocks the left
and move your head to one
side.

Observe that the background
did not move, but the thumbs
appeared to move relative to each
other and the background.
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

Close one eye.
Hold both index fingers
pointing toward each other.
Circle the fingers in a bike
pedaling motion.
Stop them at eye level and
move them inward to make
them meet forming a straight
line.
Try again, but this time move
your head side-to-side to tell the
distance between your index
fingers.
Chester F. Carlson Center for Imaging Science
Monocular Cue #6
 Shading
 Perception
of light falling on an object from a
certain angle gives form and depth to an object.
 Shadows cast by an object aid in locating it.
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Chester F. Carlson Center for Imaging Science
Monocular Cue #7
 Patterns
 Use
contour lines to infer depth.
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Chester F. Carlson Center for Imaging Science
Monocular Cue #8
 Accommodation
 The
change of shape
performed by the eye
lens to focus on an
object aids the brain in
determining the object’s
distance.
Thick lens - object is near
Thin lens - object is far
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Chester F. Carlson Center for Imaging Science
Binocular Cue #1
 Convergence
 The
angle between the line of sight of each eye
is larger as an object moves closer.

This works for nearby objects (with
accommodation)
45°
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20°
Chester F. Carlson Center for Imaging Science
Binocular Cue #2
 Retinal
Disparity
 Each
eye receives a slightly
different view of a scene.

The two views are used to
determine the ratio of
distances between nearby
objects.
Threading a needle utilizes
retinal disparity.
 Example
Close one eye and position
your thumbs so that one
blocks the other with ~1 cm
distance between them.
 Switch your viewing eye.
 Open both eyes.

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Chester F. Carlson Center for Imaging Science
Color Perception
 Trichromats
 Humans
have three cones that correspond to three
ranges of the visible light in the areas of red, green, and
blue light.
Relative response
S
400
I
460 490 500
L
530
600
650
700
Wavelength (nm)
Blue
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Cyan
Green
Red
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Human Color Vision Deficiencies
 Normal
Vision
 Viewer
uses 3 basic
colors (one for each
cone) to match all
colors in the spectrum.
 91% Males
 ~ 99% Females
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 Anomalous
Trichromacy
 Also
uses 3 basic colors
to match all colors, but
the ratios of those three
basic colors differ from
a person with normal
vision.
 ~ 6% Males
Chester F. Carlson Center for Imaging Science
Human Color Vision Deficiencies
 Monochromacy
 Sensitivity
to only
one color (or no
color sensitivity at
all)
 Two types:
Cone
monochromats:
Only one type of
cone (very small %
of population)
 Rod monochromats:
Only the rods
respond (.003%

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 Dichromacy
 Sensitivity
to only two
colors; 8-10% of Caucasion
males
 Four types:
Protanopes: No L cones (1%
males)
 Deuteranopes: No I cones
(1% males)
 Tritanopes: No S cones (very
small % pop.)

Chester F. Carlson Center for Imaging Science
Human Color Vision Deficiencies
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Chester F. Carlson Center for Imaging Science
Temporal Perception
 Negative
Afterimages
A
viewer stares at an image for a period of time.
 The cones become desensitized.
 Upon looking at a plain white surface, the viewer
perceives the previous image with complementary
colors.
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Chester F. Carlson Center for Imaging Science
Temporal Perception
 Positive
Afterimages - Persistence of Vision
 An
intense flash of light allows a viewer to see a
scene.
 When the light goes out the signal from the cones
persist.
 The viewer still perceives the image.
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Chester F. Carlson Center for Imaging Science
Motion Perception
 Real
Movement
 An
object physically
moves.
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 Induced
Movement
 The
background
moves behind an
object causing the
perception that the
object moved.
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Motion Perception
 Stroboscopic
Movement
 Quick,
sequential
flashes of light /
images that imply
motion.
 e.g. television, film,
monitor
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 Autokinetic
Movement
A
still spot of light
appears to move in an
unlit background.
 e.g. star in the night
sky
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