Color Vision 1

Color

Color Vision 2

Review of last week

Color Vision 3

Review of color• Spectrum• Cone sensitivity function• Metamers

– same color, different spectrum• Opponent

– black-white, blue-yellow, red-green• Color spaces

– Linear algebra– Problem with negative values– Standard: CIE XYZ

• Perceptually uniform color space– CIE Lab (non-linear wrt XYZ)

Color Vision 4

CIE-Lab (a.k.a. CIE L*a*b*)

Image adapted from Wyszecki, G., and W. S. Stiles,Color science: Concepts and methods, quantitative data and formulae.Wiley-Interscience; 2nd Edition. July 2000. ISBN: 0471399183

Color Vision 5

Perceptually Uniform Space: MacAdam• In color space CIE-XYZ, the perceived

distance between colors is not equal everywhere

• In perceptually uniform color space, Euclidean distances reflect perceived differences between colors

• MacAdam ellipses (areas of unperceivable differences) become circles

560550540530520510

490

480

470460 450

.05

x

y

.10 .15.20

.45.50.55.60.65

.25

.25

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.35 .40 .45

Spectrum locus

.50 .55 .60.65 .70.30

400 nm

700 nm650

620610600

590580570

Purple line

Wyszecki, G. and W. S. Stiles. Color science: Concepts and Methods, Quantitative Data and Formulae. Wiley-Interscience, 2nd ed. July 2000. ISBN: 0471399183.

Image adapted from:

Color Vision 6

Hue Saturation Value• Value: from black to white• Hue: dominant color (red, orange, etc)• Saturation: from gray to vivid color• HSV double cone

value

saturation

saturation

hue

Color Vision 7

Hue Saturation Value• One interpretation

in spectrum space• Not the only one

because of metamerism

• Dominant wavelength (hue)

• Intensity• Purity (saturation)

Color Vision 8

CIE color space• Match color at somepoint A

• A is mix of whiteC, spectral B!

• What is dominant wavelength of A?

• What is excitationpurity (%) of A?

– Move along AC/BC

C

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ECF

G

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Hunt, R. W. G. The Reproduction of Colour. John Wiley & Sons Incorporated. September2004. ISBN: 0-470-02425-9.

Image adapted from:

Color Vision 9

Plan• Color Vision• Color spaces• Color effects

– Definitions– Spatial sensitivity– Color illusion and color appearance

• Producing color

Color Vision 10

Color terms (Fairchild 1998• Color• Hue• Brightness vs. lightness• Colorfulness and Chroma• Saturation• Unrelated and related colors

Color Vision 11

Color– chromatic and achromatic content. This attribute can

be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names.

– Note: Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure, and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observations.

Color Vision 12

Related and Unrelated Colors• Unrelated Color

– Color perceived to belong to an area or object seen in isolation from other colors.

• Related Color – Color perceived to belong to an area or object seen in

relation to other colors.

Color Vision 13

Hue• Hue

– Attribute of a visual sensation according to which an area appears be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them.

• Achromatic Color– Perceived color devoid of hue.

• Chromatic Color– Perceived color possessing a hue.

Color Vision 14

Brightness vs. Lightness• Brightness

– Attribute of a visual sensation according to which an area appears to emit more or less light.

• Lightness: – The brightness of an area judged relative to the

brightness of a similarly illuminated area that appears to be white or highly transmitting.

Color Vision 15

Colorfulness & Chroma• Colorfulness

– Attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic.

• Chroma: – Colorfulness of an area judged as a proportion of the

brightness of a similarly illuminated area that appears white or highly transmitting.

Color Vision 16

Saturation– Colorfulness of an area judged in proportion to its

brightness.

Color Vision 17

Plan• Color Vision• Color spaces• Color effects

– Definitions– Spatial sensitivity– Color illusion and color appearance

• Producing color

Color Vision 18

Cornsweet illusion• Two opposite gradients• We judge only the contrast at the edge

Color Vision 19

Contrast processing• Receptors are wired to

other neurons• Center-surround

organization• Sensitive mostly to local

contrast

+ ++ + +

+++

+++

++

+++

++

- - ----

------

--

- ---

Color Vision 20

Land Retinex and local contrast

Color Vision 21

Mach Bands• Contrast is enhanced at

region boundaries

a

light

Perc

eive

d B

right

ness

(e.g

. sca

le)

Act

ual I

nten

sity

(e.g

. lum

ina)

dark

light

dark

b c d e

a b c d e

e d c b a

Color Vision 22

Hermann Grid

----

- --+ +++

++++ +

+---

- ----------- - -

----- - ------

---

- --

--------- - -

- -

--

-----

-- -

- ---------

-----

- ----

- -

Color Vision 23

Brightness vs. lightness• Brightness: subjective amount of light• Lightness: how “white”

The white cells in shadow are as darkas the black illuminated cells

(Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.)

Color Vision 24

Brightness vs. lightness• Brightness: subjective amount of light• Lightness: how “white”

The white cells in shadow are as darkas the black illuminated cells

Color Vision 25

Brightness vs. lightness• Brightness: subjective amount of light• Lightness: how “white”

The white cells in shadow are as darkas the black illuminated cells

Color Vision 26

Lightness and transparency

(Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.)

Color Vision 27

Opponents and image compression• JPG, MPG• Color

opponents instead of RGB

• Compress color more than luminance

Color Vision 28

Contrast Sensitivity• Sine Wave grating• What contrast is necessary to make the grating

visible?

(Image removed due to copyright considerations.)

Color Vision 29

Contrast Sensitivity Function (CSF)

(Image removed due to copyright considerations.)

Color Vision 30

Contrast Sensitivity Function (CSF)

-10.01

0.1

1

0

Luminance

Red-Green

Blue-Yellow

Log Spatial Frequency (cpd)

Con

trast

Sen

sitiv

ity

1 2

10

100

Spatial contrast sensitivity functions for luminance and chromatic contrast.

Image adapted from:Mark D. Fairchild. Color Appearance Models. Prentice Hall PTR, November 1997. ASIN: 0201634643

Color Vision 31

JPEG Compression• Perform DCT to work in frequency space

– Local DCT, 8x8 blocks• Use CSF for quantization (more bits for

sensitivity with more contrast)• Other usual coding tricks

Color Vision 32

Plan• Color Vision• Color spaces• Color effects

– Definitions– Spatial sensitivity– Color illusion and color appearance

• Producing color

Color Vision 33

Color constancy• Chromaticity of light sources vary• Chromatic adaptation

– Similar to white balance on camera– Different films, filters

Color Vision 34

Chromatic adaptation• Von Kries adaptation• Different gain control on L, M, S• Similar to white balance on camera

0.75, 1, 1

Gain control:*1.33, *1, *1

0.75, 1, 1

0.15, 1, 0.2 0.2, 1, 0.20.2, 1, 0.2

Color Vision 35

Crispening• Increased sensitivity

Color Vision 36

Stevens effect• Stevens effect

– Perceived contrastincreases with luminance

0.1

1.0

1.0

L (97 dB)

L (79 dB)L (63 dB)L (Dark)

Image adapted from:Fairchild, Mark D. Color Appearance Models. Prentice Hall PTR. November 1997.ASIN: 0201634643.

Rel

ativ

e B

right

ness

Relative Luminance

Color Vision 37

Hunt and Stevens effect• Stevens effect

– Perceived contrast increases with luminance• Bartleson-Breneman effect

– Image contrast changes with surround– A dark surround decreases contrast

(make the black of the image look less deep)– Important for movies

• Hunt effect– Colorfulness increases with luminance

• Hence the need for gamma correction (see later)

Color Vision 38

Color appearance models• Predict the appearance

of a color depending on– Objective stimulus– Surrounding, context

Color Vision 39

Plan• Color Vision• Color spaces• Color effects• Producing color

Color Vision 40

Color synthesisAdditive Subtractivered, green, blue cyan, magenta, yellow

Add light (CRT, video projector)

Remove light (e.g. filter) printer, photos

Color Vision 41

Color synthesis: a wrong exampleAdditive Subtractivered, green, blue cyan, magenta, yellow

RIGHTWRONG

Color Vision 42

Device Color Models(Subtractive)Start with white, remove energy (e.g.,w/ink, filters)

Example: CMY color printer

Cyan ink absorbs red light

Magenta ink absorbs green light

Yellow ink absorbs blue light

C+M+Y absorbs all light: Black!RGB => CMY conversion

Some digital cameras use CMY

Color Vision 43

CMYK

CMY model plus black ink (K)

Saves ink

Higher-quality black

Increases gamut

Conversion not completely trivial

Color Vision 44

The infamous gamma curveA gamma curve x->xγ is used for many reasons:• CRT response

– The relation between voltage and intensity is non-linear• Color quantization

– We do not want a linear color resolution– More resolution for darker tones– Because we are sensitive to intensity ratios

• Perceptual effect– We perceive colors in darker environment less vivid– Hunt and Stevens effect

• Contrast reduction– Keep some contrast in the highlights

Color Vision 45

Gamut• Every device with three primaries can produce only colors inside some (approx.) triangle

– Convexity!

• This set is called a color gamut

– (Why can’t RGB can’t give all visible colors?)

• Usually, nonlinearities warp the triangle

– Also, gamut varies with luminance

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R. W. G. Hunt. The Reproduction of Colour. John Wiley & Sons,Incorporated, September 2004. ISBN: 0-470-02425-9.

Image adapted from:

Color Vision 46

Gamut MappingGamut Mapping• Color gamut of

different processes may be different (e.g. CRT display and 4-color printing process)

• Need to map one 3D color space into another

• Color gamut of different processes may be different (e.g. CRT display and 4-color printing process)

• Need to map one 3D color space into another

CIE-Lab

Perceptually-uniform Color space

Typical CRT gamut

4-color printing gamut

Color Vision 47

Gamut MappingGamut MappingTypical CRT

gamut4-color CMYK printing

gamut

a*

b*

Gamut mapping is a morphing of 3D color space according to adopted schemeGamut mapping is a morphing of 3D color space according to adopted scheme

Color Vision 48

ICC standard• Every device has a

different color response (gamut, spectrum)

• Picture file should store this color profile

• ICC standard• Unfortunately not spread

enough

http://www.color.org/

Color Vision 49

Selected Bibliography

Vision Scienceby Stephen E. PalmerMIT Press; ISBN: 0262161834760 pages (May 7, 1999)

Billmeyer and Saltzman's Principles of Color Technology, 3rd Editionby Roy S. Berns, Fred W. Billmeyer, Max SaltzmanWiley-Interscience; ISBN: 047119459X304 pages 3 edition (March 31, 2000)

Vision and Art : The Biology of Seeingby Margaret Livingstone, David H. HubelHarry N Abrams; ISBN: 0810904063 208 pages (May 2002)

Color Vision 50

Selected Bibliography

The Reproduction of Colorby R. W. G. HuntFountain Press, 1995

Color Appearance Modelsby Mark FairchildAddison Wesley, 1998