CS5600 Computer Graphics by Rich Riesenfeld Spring 2006

CS5600 Computer Graphics

by

Rich Riesenfeld

Spring 2006

Lect

ure

Set

11

Spring 2006 Utah School of Computing 2

• Physical nature of color

• Eye mechanism of color

–Rods, cones, tri-stimulus model

• Brain mechanism of color

• Color spaces

• Aesthetic and physiological


• Color is complicated!–Highly nonlinear–No single model to explain all

• Many simplistic models, explanations

• Many myths• Much new knowledge


• Many phenomena to explain

–High light / low light

–Illusions

–Color blindness

–Metamers


Additive Primaries: (r,g,b)Additive Primaries: (r,g,b)

(1,0,1)(1,0,1)

(0,0,1 )(0,0,1 )

(0,1,0)(0,1,0)

(0,1,1)(0,1,1)

(1,0,0)(1,0,0)

(1,1,0)(1,1,0)

magentamagentamagentamagenta

blueblueblueblue

cyancyancyancyan

greengreengreengreenredredredred

yellowyellowyellowyellow

whitewhitewhitewhite(1,1,1)(1,1,1)



…………...<www.jgiesen.de/ColorTheory/

RGBColorApplet/rgbcolorapplet.html>


Traditional, Artistic:• rgb• cmy• cmyk• hsv• hsl

Perceptually Based:• XYZ (Tristimulus)• Xyz• Hunter-Lab• CIE-L*ab• CIE-L*CH° • CIE-L*CH°• CIE-L*ab• CIE-L*uv



(0,1,0)(0,1,0)(1,0,0)(1,0,0)

(1,1,0)(1,1,0)

greengreengreengreenredredredred




(1,0,1)(1,0,1)

(0,0,1 )(0,0,1 )

(1,0,0)(1,0,0)

magentamagentamagentamagenta

blueblueblueblue

redredredred


Subtractive Primaries: (c,m,y)Subtractive Primaries: (c,m,y)

(1,1,0)(1,1,0)yellowyellowyellowyellow

(0,1,0)(0,1,0)greengreengreengreen

(0,1,1)(0,1,1)cyancyancyancyan

(0,0,1 )(0,0,1 )blueblueblueblue

(1,0,1)(1,0,1)magentamagentamagentamagenta

(1,0,0)(1,0,0)redredredred

blackblackblackblack(0,0,0)(0,0,0)

blackblackblackblack


Additive Primaries: (c,m,y)Additive Primaries: (c,m,y)





(1,1,0)(1,1,0)(1,1,0)(1,1,0)yellowyellowyellowyellow

(1,0,1)(1,0,1)(1,0,1)(1,0,1)magentamagentamagentamagenta

(1,0,0)(1,0,0)(1,0,0)(1,0,0)redredredredblackblackblackblack









(0,0,1 )(0,0,1 )bbllueuebbllueue




Wavelength Spectrum

infrared light

ultraviolet light

Wavelength (nm)700 600 500 400

• Seen in physics, physical phenomena (rainbows, prisms, etc)

• 1 Dimensional color space

http://elchem.kaist.ac.kr/vt/chem-ed/light/graphics/em-visib.jpg


Wavelength Spectrum

Note that the rainbow does not contain any magenta. It is nonspectral.


• “Navigating,” moving around in a

color space, is tricky

• Many color representations (spaces)

• Can you get to a nearby color?

• Can you predictably adjust a color?


Color Cube: (r,g,b) is RHSColor Cube: (r,g,b) is RHS (0,0,1 )(0,0,1 )

blueblueblueblue





(1,0,0)(1,0,0)redredredred

whitewhitewhitewhite(1,1,1)(1,1,1)

(0,0,0)(0,0,0)blackblackblackblack

graygraygraygray






(1,1,0)(1,1,0)yellowyellowyellowyellow(1,0,0)(1,0,0)

redredredred

(1,1,1)(1,1,1)whitewhitewhitewhite


Complementary Colors Add to GrayComplementary Colors Add to Gray

(0,0,1 )(0,0,1 ) (0,0,1 )(0,0,1 )blueblueblueblue

magentamagentamagentamagenta(0,1,1)(0,1,1)

cyancyancyancyan

(1,1,1)(1,1,1)whitewhitewhitewhite(1,0,1)(1,0,1)

(0,1,0)(0,1,0)(0,1,0)(0,1,0)greengreengreengreen

((1,1,01,1,0))yellowyellowyellowyellow(1,0,0)(1,0,0)

redredredred


Complementary ColorsComplementary Colors

Looking at color cube along major diagonal


James Clerk Maxwell’s ColorJames Clerk Maxwell’s Color

magentamagentamagentamagentablueblueblueblue

cyancyancyancyan

greengreengreengreen

redredredred


unsaturated cyan unsaturated cyan unsaturated cyan unsaturated cyan

whitewhitewhitewhite


Newton’s Color Wheel Newton’s Color Wheel

Replaced Aristotle’s color model based on light and darkness.


Color AppletsColor Applets

www.cs.brown.edu/exploratories/freeSoftware/catalogs/repositoryApplets.html


( H, S, V ) Color Space( H, S, V ) Color Space

• Introduced by Albet Munsell, late 1800s

– He was an artist and scientist

• Hue: Color

• Saturation/Chroma: Strength of a color

– Neutral gray has 0 saturation

• Brightness/Value: Intensity of light emanating from image


The hue of an object may be blue,

but the terms light and dark

distinguish the brightness of one

object from another.

(Hue, Saturation, Value/Intensity)(Hue, Saturation, Value/Intensity)

( H, S, V ) Color Space


SaturationSaturation


Other HSX Color Spaces (Cones)Other HSX Color Spaces (Cones)

120˚

0˚

V

SH

1.0

0.0

240˚

yellowyellowyellowyellowgreengreengreengreen

cyancyancyancyanredredredred

magentamagentamagentamagentablueblueblueblue


Another HSX Color Space(double cone)

Another HSX Color Space(double cone)

1.0

S

L

29

0˚

0.0

H



redredredred


Tristimulus Color TheoryTristimulus Color Theory

• Any color can be matched by a mixture of three fixed base colors (primaries)

• Eye has three kinds of color receptors called cones

• Eye also has rods (low light receptors)


Color Receptors in EyeColor Receptors in Eye

• ((RedRed, GreenGreen, BlueBlue))

• ((LongLong, MediumMedium, ShortShort))

• ((RedRed, GreenGreen, BlueBlue))

• ((LongLong, MediumMedium, ShortShort))

Fra

ctio

n of

ligh

t ab

sorb

ed

by e

ach

type

of

cone

Wavelength λ (nm)

0.20

0 .18

0 .16

0 .14

0 .12

0 .10

0 .08

0 .06

0 .04

0 .02

0 .00

600560440 520 640400 680480


Color Receptors in EyeColor Receptors in Eye

600550450 500 650400 700Wavelength λ (nm)

Rel

ativ

e se

nsiti

vity

0.0

1.0


• Why are runway lights blue?

• Why are console lights green?

• What color is the Kodak box?

• Why are green lasers directed toward pilots for destructive purposes?

• Why do soldiers read maps in the dark using dim red light?

Color Response Color Response


Color Matching ExperimentsColor Matching Experiments

• Given a reference color, try to match it identically

• What does “negative red,” or “negative color” mean??


CIE* Color SpaceCIE* Color Space

( X, Y, Z ) represents an imaginary basis that does not correspond to what we see

Define the normalized coordinates:x = X / ( X + Y + Z )

y = Y / ( X + Y + Z )

z = Z / ( X + Y + Z )

* Commission Internationale de l'Êclairage


CIE Color Space of Visible ColorsCIE Color Space of Visible Colors

x + y + z = 1

x = X / ( X + Y + Z )

y = Y / ( X + Y + Z )

z = Z / ( X + Y + Z )

z

y

x

The projection of the plane of the triangle onto the (X,Y) plane forms the chromaticity diagram that follows.


Color Gamuts: CIE Color ChartColor Gamuts: CIE Color Charty

x

1.0

0 .9

0 .8

0 .7

0 .6

0 .5

0 .4

0 .3

0 .2

0 .1

1.0 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1.0

cyancyan

magentamagentaredredblueblue

green green

yellowyellow

400

490

500

510

520

600

700

580

560

540



Color Gamuts: CIE Color ChartColor Gamuts: CIE Color Chart

ideal redideal redideal redideal redideal blueideal blueideal blueideal blue

ideal greenideal green ideal greenideal green

redredredredblueblueblueblue

green green green green

cyancyanyellowyellow


400 nm

490 nm

500 nm

510 nm540 nm

560 nm

580 nm

700 nm

600 nm

520 nm



www.cs.rit.edu/~ncs/color/a_chroma.html

400

490

500

510

520

600

700

580

560

540




700 nm

510 nm

400 nm

500 nm

490 nm

600 nm

520 nm540 nm

560 nm

580 nm




400

490

500

510

520

600

700

580

560

540

C2

C3

C1

The additive colors C1and

C2 combine to

form C3 on the

line connecting C1 and C2.




400

490

500

510

520

600

700

580

560

540

R

B

G

The Color Gamuts of different displays and printers are not likely to match. Printers usually have smaller gamuts.


www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/cs/exploratories/applets/

combinedColorMixing/combined_color_mixing_java_browser.html


CIE L*a*b* Color SpaceCIE L*a*b* Color Space

greengreengreengreen

--bb**

L*=0

L*=1

light

ness

redredredred

blueblue

blueblue

++aa**++aa**

-a*-a*-a*-a*

yello

wye

llow

yello

wye

llow

+b*+b*+b*+b*

Equal distances represent

approximately equal color difference.




Important ConceptsImportant Concepts

• Adaptation–Slow process

• Constancy–Immediate process


Output to the Brain from Lateral Geniculate BodyOutput to the Brain from Lateral Geniculate Body

RRRR

GGGG

BBBB

+

+

-

+

-

B B - B B -

RR- GGRR- GG

BBBB

RRRR

GGGG

YYYY

YYYY

+

Color processing unit: lateral geniculate body


Eye’s MechanismEye’s Mechanism

+++ +++

+++

- --

+++

- --

+++

- --

+++

- --

- --

- --

- --

- --

- --

- --

- --

- --- -

-

---

- --

---

- --

---

- --

---

- --

---

- --

---

48

Lect

ure

Set

11

Documents

CS5600 Computer Graphics by Rich Riesenfeld Spring 2006