read article by land for thursday article by anne treisman coming up in about two weeks

• Read article by Land for Thursday• Article by Anne Treisman coming up in

about two weeks

Color Vision

• Primary colors

Perceiving Color

What are the primary colors?

Color Vision

• Primary colors

Perceiving Color

Red Green Blue

Color Vision

• Primary colors

Perceiving Color

What makes them primary?

Color Vision

• Primary colors

• Every color (hue) can be created by blending light of the three primary colors in differing proportions

Perceiving Color

Color Vision• Primary colors

• Every color (hue) can be created by blending light of the three primary colors in differing proportions

• Led to prediction that there must be three (and only three) distinct color receptor types

Perceiving Color

Color VisionPerceiving Color• Four absorption peaks in retina: 3 cone types plus

rods

Abs

orpt

ion/

Con

e re

spon

se

Color Vision

“Blue”

“Green”

“Red”

Blue

Wavelength Input Cone Signal to Brain

Theories of Color Vision: Trichromatic Theory

Color Vision

“Blue”

“Green”

“Red”

Green

Wavelength Input Cone Signal to Brain

Theories of Color Vision: Trichromatic Theory

Color Vision

“Blue”

“Green”

“Red” Red

Wavelength Input Cone Signal to Brain

Theories of Color Vision: Trichromatic Theory

Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Theories of Color Vision: Trichromatic Theory

Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Theories of Color Vision: Trichromatic Theory

Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Theories of Color Vision: Trichromatic Theory

Color Vision• Trichromatic theory of color vision:

– brain interprets the relative amounts of signaling from each of these cone types

Theories of Color Vision: Trichromatic Theory

Color Vision• Trichromatic theory of color vision:

– brain interprets the relative amounts of signaling from each of these cone types

• This means that some colors can be matched by a pair of wavelengths– metamers: colors that have no definite

single wavelength (e.g. yellow)

Theories of Color Vision: Trichromatic Theory

Color Vision• Trichromatic theory of color vision:

– brain interprets the relative amounts of signaling from each of these cone types

• This means that some colors can be matched by a pair of wavelengths– metamers: colors that have no definite single

wavelength (e.g. yellow)• This also means that any color can be matched by

mixing (not more than) three different wavelengths

Theories of Color Vision: Trichromatic Theory

• use color sliders to adjust R,G,B values

• What color can only exist as a metamer (an additive mixture of wavelengths)? In other words, what color cannot be made with a single wavelength?

• What color can only exist as a metamer (an additive mixture of wavelengths)? In other words, what color cannot be made with a single wavelength?

MagentaThink about why!

Color VisionTheories of Color Vision• Trichromatic Theory can explain some

aspects of colorblindness:– most of us are trichromats– someone missing one of the three cone

types is a dichromat– someone missing two is a monochromat– someone missing all cone types is called a

rod monochromat (very poor vision!)

Color VisionTheories of Color Vision• Trichromatic Theory can explain some aspects of

colorblindness:

– dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths to which they are sensitive

Color Vision• Trichromatic Theory can explain some aspects of

colorblindness:

– dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths to which they are sensitive

– most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones

Theories of Color Vision: Trichromatic Theory

Color Vision• Trichromatic Theory can explain some aspects of

colorblindness:

– dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths to which they are sensitive

– most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones

– cannot see color difference between reds and greens - but they can see luminance difference

Theories of Color Vision: Trichromatic Theory

Color Vision

Ishihara Color Plates can indicate color blindness

Theories of Color Vision: Trichromatic Theory

Color Vision

DON’T DO THIS !

…~3% of male readers will have trouble seeing it!

Theories of Color Vision: Trichromatic Theory

Color Vision

But this is OK.

Theories of Color Vision: Trichromatic Theory

Color Vision

So is this.

Theories of Color Vision: Trichromatic Theory

Color Vision

Even this is good.

Theories of Color Vision: Trichromatic Theory

• Problem with Trichromatic Theory:

Theories of Color Vision: Trichromatic Theory

• Problem with Trichromatic Theory:

YELLOW

Theories of Color Vision: Trichromatic Theory

• Problem with Trichromatic Theory:– most people categorize colors into four primaries:

red, yellow, green, and blue

– some colors simply cannot be perceived as gradations of each other

• redish green !?• blueish yellow !?

– It is as if these colors are opposites

Theories of Color Vision: Trichromatic Theory

• Opponent-Process Theory– color is determined by outputs of two

different continuously variable channels:• red - green opponent channel• blue - yellow opponent channel

Theories of Color Vision: Opponent-Process Theory

• Opponent-Process Theory– Red opposes Green– (Red + Green) opposes Blue

• Opponent-Process Theory explains color afterimages

Theories of Color Vision: Opponent-Process Theory

Color Vision

• White light is a mixture of wavelengths– prisms decompose white light into assorted

wavelengths– OR recompose a spectrum into white light

Wavelength and Color

• Additive vs. Subtractive

There are two different ways to mix colors.

• Additive vs. Subtractive

What do you get if you use a prism to combine all wavelengths of light?

• Additive vs. Subtractive

What do you get if you use a prism to combine all wavelengths of light?

• Additive vs. Subtractive

What do you get if you mix a bunch of paint?

• Additive vs. Subtractive

What do you get if you mix a bunch of paint?

• Additive vs. Subtractive

• Additive mixing is most intuitive:

ADD wavelengths:

red+green = yellowred+blue = magentablue+green = cyanred+green+blue=white

• Subtractive mixing is much less intuitive (but much more common)

• Subtractive mixing happens when we mix pigments (paint) together

• Different pigments subtract different wavelengths:– red subtracts all but red, blue all but blue,

green subtracts blue and red, etc…

• Example: blue + yellow = green

Technically it’s called “cyan”

• The result of a mixture depends on what wavelengths don’t get absorbed by the two pigments

wavelength

Am

ount

of r

efle

ctio

n

blue green yellow red

• Both yellow and blue pigments reflect a bit of green

wavelength

Am

ount

of r

efle

ctio

n

blue green yellow red

• Subtractive mixing is commonly used in color printers

• Everything you’ve learned so far is wrong.

• Everything you’ve learned so far is wrong.

• Well, not really wrong, just far from complete.

• What color is this box?

• What color is this box?