Download Human Eye - Image Processing

Visual Perception and Illusions
Optical Illusions
Which of the above gray rectangles is darker?
Optical Illusions
Do you see gray spots at the intersections?
Structure of the Retina
Rods
Direct (black) and
indirect (blue)
connections
What your eye
tends to see
(edges are
important)
Neurons
Intensity (as “seen” by brain)
Pattern of
incoming
light
Lateral Inhibition
Indirect (lateral) rod signals tend to cancel out
(inhibit) the direct connection signals
Result: Changes in intensity (amount of
incident light) are most important for
the visual system.
Mach Bands
Direct connections 0
+0.2
+0.4
+0.8
Indirect connections
-0.05
-0.1
-0.2
+1.6
Mach Bands
Intensity we perceive
0.4
0.2
Lateral Inhibition
Parts of retina responding to gray rectangle
on the right are receiving strong inhibitory
signals, fatiguing the retina and making it
less sensitive (so the gray part seems darker).
Lateral Inhibition
b
a
Lateral Inhibition
The point “a” is surrounded on four sides by white area.
Inhibitory signals from receptors responding to “white”
tell receptors responding to the point “a” to become less
sensitive.
Point “b” is surrounded by white on two sides and black
on two sides; there are not as many signals from receptors
trying to “desensitize” the rods responding to the area at “b.”
100,000,000 rods and cones (sensors) lead to 1,000,000
neurons - the brain is throwing away 99% of the input!!
Stratton Article
1. What was the point of Stratton's experiments?
2. Can the eye + brain be fooled into making a major
new adaptation as described by Stratton?
3. What is the optical instrument Stratton uses to
alter his vision?
Negative Afterimages
"Successive lightness contrast"
Negative Afterimages
Stare at this for 30 sec., then look at a white surface
Positive Afterimages
Time response, or “persistency”
You see a white image if there was a
white stimulus  camera flash
Persistency
Movies – frames flash by faster than your
eye can change its response
Strobe lights – a flashing light can make a
moving object appear stationary
Binocular Vision
Eyes in front or eyes on the side?
How do we judge distances?
Overlapping “field-of-view gives
us the possibility of depth
perception
Binocular Vision
Lions need binocular vision to help them
in catching dinner.
Rabbits need a large field of view to
avoid being dinner.
Depth Perception
Accomodation
What if all parts of a scene are at a distance
for which the eye is relaxed?
Rene Magritte,
Depth Perception
Convergence
L
Big angle
R
L
R
Small angle
Depth Perception
Parallax
Different views of a scene from
different positions.
Driving a car - nearby objects
“move past” quickly; distant
objects seem to follow.
Depth Perception
Impossible figures and ambiguous
depth cues
Ambiguous Depth Cues
Depth Perception
Why might one wish to “play games”
with depth perception and visual cues?
Look at drawings and woodcuts
by M.C. Esher such as
“Ascending and Descending”
or “Relativity.”
Poggendorf Illusion