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05 Color Vision

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COLOR
VISION
I The Spectrum
II Trichromatic Vision
–
1.
2.
Cones
Additive Mixing
Subtractive Mixing
III Color Opponency
–
–
–
Complimentary Colors
Color Blindness
Color Constancy
IV Color Vision
–
–
Memory & Imagery
Form & Motion
Anthony J Greene
1
Light and Frequency
The Rays, to speak properly, are not coloured.
In them there is nothing else than a certain
Power and Disposition to stir up a Sensation of
this or that Colour. . .
So Colours in the Object are nothing but a
Disposition to reflect this or that sort of Rays
more copiously than the rest.
-Isaac Newton
Anthony J Greene
2
Light and Frequency
• Color is how we discriminate frequencies
• Light does not have color, light has
frequency
• Objects do not have color, objects have
pigments which absorb all frequencies
except those which we see
Anthony J Greene
3
The
Spectrum
Color Properties
• Brightness amount of light
• Saturation richness of color:
desaturated colors
are are grayish or whitish
• Hue - frequency :
position on the spectrum
Violet - Indigo - Blue - Green - Yellow - Orange - Red
Short -------------------Medium------------------------ Long
400 nm ----------------550 nm-----------------------700 nm
Color Solid
Hue
Brightness
Saturation
Anthony J Greene
5
Trichromatic ColorVision
• Each of the three primary
colors can be considered as
a dimension on a 3-D graph
- Combinations of these
three colors, or positions on
the graph specify all visible
colors
• Combinations of cone
activity likewise specify all
percievable colors Anthony J Greene
Red
Green
Blue
6
Anthony J Greene
7
Color Mixing
• Additive mixing =>
mixing light - when
two colored lights are
mixed wavelengths
which were present in
either original source
are now present in the
mixture
Anthony J Greene
8
Color Mixing
• Subtractive mixing =>
mixing pigments - because
pigments absorb all colors
except for that which you see
reflected, when two colored
pigments are mixed the
result is that only
wavelengths which were
reflected in both original
pigments are reflected in the
mixture
Anthony J Greene
9
Subtractive Color Mixing
Anthony J Greene
10
Color Opponency
• Three types of cones connect to two types
of ganglion cells
Anthony J Greene
11
Color Opponency
• Starting with an Y-B pathway & the evolution of an R-G pathway
• Implications for human R-G color blindness and animal vision
Anthony J Greene
12
Color Opponency
• Red-Green cells will increase their firing rate in
response to green and decrease their firing rate in
response to red (or vice-versa)
• At-home-experiment: Take a half circle of yellow
paper, and a half circle of blue paper, glue them to
a cardboard disk, push a pencil through the center
of the disk, then spin the disk rapidly: What is the
hypothesis?
Anthony J Greene
13
Color Opponency
• Color opponency determines
complementary colors
• When complementary colors are additively
combined (i.e., simultaneous activation of
red and green or of blue and yellow)
opponent processes cancel and a shade of
gray is percieved
Anthony J Greene
14
Color
Opponency
Color opponency
determines
complementary colors
Anthony J Greene
15
Color Opponency
• Color After-images
Anthony J Greene
16
Anthony J Greene
17
Color Opponency
• Color
Afterimages
Anthony J Greene
18
Anthony J Greene
19
Color Opponency
• Color
Afterimages
Anthony J Greene
20
Color Opponency
• Color Blindness
Anthony J Greene
21
Color Constancy
• Discounting the illuminant - adaptation
• The color of the ambient lighting quickly
fatigues photorecptors to that color -- There
is no eye position that allows the
photoreceptors to recover
• Once fatigued to the ambient color, that color
is subtracted, or discounted from the visual
scene and colors appear close to the way they
would in white light
Anthony J Greene
22
Color Constancy
There is a cognitive
component as well.
Anthony J Greene
23
Color Constancy
There is a cognitive
component as well.
Anthony J Greene
24
Color constancy
Anthony J Greene
25
Color Vision
1. Memory & Imagery
–
Achromatopsia
2. Form & Motion
–
Interactions of color system with other visual
components
Anthony J Greene
26
Case of Achromatopsia
• Damage to V4 can cause the complete loss
of color vision (as opposed to red-green
color blindness): V4 is more sensitive to
oxygen deprivation
• In addition, color imagery and color
memory are also lost
• What are the implications for perception,
imagery and memory?
Anthony J Greene
27
Anthony J Greene
28
Anthony J Greene
29
Anthony J Greene
30
Color, Form & Motion
• Although V4 interacts with other areas (V3
& V5 are monochromatic), its interactions
are limited
• Equiluminant color conditions makes form
and motion perception difficult -- but not
impossible
Anthony J Greene
31
Equiluminant Colors
Anthony J Greene
32
Equiluminant Colors
Anthony J Greene
33
Equiluminant Colors
Anthony J Greene
34
This is
Difficult
to Read
This is
Easier
to Read
This is
Easiest
to Read
Anthony J Greene
35
Dad: Wow Honey, your missing a beautiful
sunset out here.
Mom: I’ll count to 10 and then… POW
Calvin: Dad, how come old photographs are
always black and white. Didn’t they have color
film back then
Dad: Sure they did. In fact those old
photographs are in color. Its just the world was
black and white then.
Calvin: Really
Dad: Yep. The world didn’t turn color until
sometime in the 1930s, and it was pretty grainy
color for a while too.
Calvin: That’s really weird.
Dad: Well, truth is stranger than fiction
Calvin: But then why are old paintings in color?
If the world was black and white, wouldn’t artists
have painted it that way?
Dad: Not necessarily, a lot of great artists were
insane.
Calvin: But… but how could they have painted in
color anyway? Wouldn’t their paints have been
shades of gray back then?
Dad: Of course, but they turned colors like
everything else did in the 30s.
Calvin: so why didn’t old black and white photos
turn color too?
Dad: Because they were color pictures of black
and white, remember?
Calvin: The world is a complicated place
Hobbes.
Hobbes: Whenever it seems that way, I take a
nap in a tree and wait for dinner.
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