Color and Perception - Purdue University · Color and Perception CS635 Spring 2010 Daniel G. Aliaga ... HSV/HSL Color Space. HSV/HSL Color Space • Intensity/Value – total amount

Color and PerceptionColor and Perception

CS635 Spring 2010

Daniel G AliagaDaniel G. AliagaDepartment of Computer Science

Purdue University

Elements of Color PerceptionElements of Color PerceptionElements of Color PerceptionElements of Color Perception

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Elements of ColorElements of ColorElements of ColorElements of Color

• Physics: y– Illumination

• Electromagnetic spectra; approx. 350 – 720 nm

R fl ti– Reflection• Material properties (i.e., reflectance, transparency) • Surface geometry and micro geometry (i.e., polished versus matte

b h d)versus brushed)

• Perception– Physiology and neurophysiologyPhysiology and neurophysiology– Perceptual psychology

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Physiology of the EyePhysiology of the Eye

• The eye:

• The retinaThe retina– 100 M Rods

• B&WB&W

– 5 M Cones• ColorColor

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Physiology of the RetinaPhysiology of the Retina

• The center of the retina is a densely packed region called the fovearegion called the fovea. – Cones much denser here than the periphery

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Types of ConesTypes of Cones

• Three types of cones:L or R most sensitive to red light (610 nm)– L or R, most sensitive to red light (610 nm)

– M or G, most sensitive to green light (560 nm)

– S or B, most sensitive to blue light (430 nm), g ( )

– Color blindness results from missing cone type(s)

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Color Color BlindnessBlindnessNormal

Protan (L-cone) “red insensitivity”

Deutan (M-cone) “green insensitivity”

Tritan (S-cone) “B=G and Y=violet”

Opponent Color TheoryOpponent Color TheoryOpponent Color TheoryOpponent Color Theory• Humans encode colors by differencesHumans encode colors by differences

• E.g R‐G, and B‐Y Differences

Perception: MetamersPerception: Metamers• A given perceptual sensation of color derives from the stimulus of all three cone types

• Identical perceptions of color can thus be caused by very different spectra

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Perception: Other GotchasPerception: Other Gotchas• Color perception is also difficult because:

– It varies from person to person (thus need p p (“standard observers”)

– It is affected by adaptationy p

– It is affected by surrounding color

– There is Mach‐bandingg

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Summary of Human Color PerceptionSummary of Human Color PerceptionSummary of Human Color PerceptionSummary of Human Color Perception

• Subjectively, the human eye seems to perceive colorSubjectively, the human eye seems to perceive color by three conceptual dimensions: – hue,

– brightness, and

– saturation.

• This suggests a 3D color space.

• Hardware reproduction of color cannot match human perception perfectly.

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Color SpacesColor Spaces• Three types of cones suggests color is a 3D quantity. How

to define 3D color space?p

• Idea: shine given wavelength (λ) on a screen, and mix three other wavelengths (R,G,B) on same screen. Have

dj t i t it f RGB til l id ti luser adjust intensity of RGB until colors are identical:

• How closely does this correspond to a color CRT?

• Problem:• Problem: • sometimes need to “subtract” R

to match λ

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CIE Color SpaceCIE Color Space

• The CIE (Commission Internationale d’Eclairage) came up with three hypothetical lights X, Y, and Z with these spectra:

A i t l• Approximately:X ~ RY GY ~ GZ ~ B

• Idea: any wavelength λ can be matched perceptually by positive combinations of X,Y,Z

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p p y y p , ,

CIE Color SpaceCIE Color Space1931


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CIE Color SpaceCIE Color Space1931


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• The gamut of all colors perceivable is thus a three dimensional shape in X YZ:three‐dimensional shape in X,Y,Z:

For simplicity, we often project to the 2D plane X+Y+Z=1, e.g.:

X = X / (X+Y+Z)Y = Y / (X+Y+Z)Z 1 X Y

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Z = 1 - X - Y

Device Color GamutsDevice Color GamutsDevice Color GamutsDevice Color Gamuts

• X Y and Z are hypothetical light sources; noX, Y, and Z are hypothetical light sources; no real device can produce the entire gamut of perceivable colorperceivable color

• Example: CRT monitor

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• The RGB color cube sits within CIE color spaceThe RGB color cube sits within CIE color space something like:

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• We can use the CIE chromaticity diagram toWe can use the CIE chromaticity diagram to compare the gamuts of various devices:

• Note for example• Note, for example, that a color printercannot reproducecannot reproduceall shades availableon a color monitoron a color monitor

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Converting Within Color SpaceConverting Within Color SpaceConverting Within Color SpaceConverting Within Color Space

• Simple matrix operation:Simple matrix operation:

⎥⎤

⎢⎡⎥⎤

⎢⎡

⎥⎤

⎢⎡

GR

YYYXXX

GR BGR

''

⎥⎥⎥

⎦⎢⎢⎢

⎣⎥⎥⎥

⎦⎢⎢⎢

⎣

=⎥⎥⎥

⎦⎢⎢⎢

⎣ BG

ZZZYYY

BG

BGR

BGR

''

• The transformation C2 = M‐12 M1 C1 yields RGB

it 2 th t i i l t t i RGBon monitor 2 that is equivalent to a given RGB on monitor 1A l t h f di t t

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• Analogous to change of coordinate system.

RGB Color SpaceRGB Color SpaceRGB Color SpaceRGB Color Space


• Convenient colors (screen phosphors)Convenient colors (screen phosphors)

• Decent coverage of the human color

C il i d i h 0 2• Customarily quantized in the range 0…255

• Full color = 3 bytes/pixel

• Not a particularly good basis for human interaction– Non‐intuitive

– Non‐orthogonal (perceptually)Non orthogonal (perceptually)


• The RBG colors can be arranged in a cube, in a spaceThe RBG colors can be arranged in a cube, in a space with the dimensions R, G, and B. The colors at the vertices of the RGB cube are then:

Color R G Bblack 0 0 0white 255 255 255red 255 0 0green 0 255 0green 0 255 0blue 0 0 255cyan 0 255 255

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magenta 255 0 255yellow 255 255 0

RGB Cube PropertiesRGB Cube PropertiesRGB Cube PropertiesRGB Cube Properties

• The main diagonal from black to white contains the ggray scale.

• If a specific color is given as (R,G,B) and k is a number ll h h (k k k ) h lsmaller than 1, then (kR, kG, kB) has approximately

the same hue and is dimmer. So, we can model color intensity byintensity by– (kR, kG, kB), k < 1– Note that the brightness of (R,G,B) is not g ( , , )exceeded

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HSV/HSL Color SpaceHSV/HSL Color SpaceHSV/HSL Color SpaceHSV/HSL Color Space

HSV/HSL Color SpaceHSV/HSL Color SpaceHSV/HSL Color SpaceHSV/HSL Color Space

• Intensity/ValueIntensity/Value– total amount of energy

• Saturation• Saturation– degree to which color is one wavelength

• Hue– dominant wavelength

HSVHSVHSVHSV

• Max = max(R, G, B)Max max(R, G, B)• Min = min(R, G, B)• S = (max – min)/max• S = (max – min)/max• If R==Max → h = (G‐B)/(max‐min)If G M → h 2+(B R)/( i )• If G==Max → h = 2+(B‐R)/(max‐min)

• If B==Max → h = 4 + (R‐G)/(max‐min)• If h<0 → H = h/6 + 1• If h>0 → H = h/6

HSV User InteractionHSV User InteractionHSV User InteractionHSV User Interaction

HSLHSLHSLHSL

if G>B if G<Bif G>B, if G<B

if G=B

YIQ Color SpaceYIQ Color SpaceQ pQ p

• YIQ is the color model used for color TV in the USYIQ is the color model used for color TV in the US– Y is luminance; I & Q are color

– Note: Y is the same as CIE’s Y – Result: backwards compatibility with B/W TV!

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Converting Between Color SpacesConverting Between Color Spacesg pg p

• Converting between color models can also beConverting between color models can also be expressed as such a matrix transform, e.g.:

⎤⎡⎤⎡⎤⎡ RY 110590300

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡−−=

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

BGR

QIY

31052021032.028.060.0

11.059.030.0

⎥⎦⎢⎣⎥⎦⎢⎣ −⎥⎦⎢⎣ BQ 31.052.021.0

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Gamma CorrectionGamma CorrectionGamma CorrectionGamma Correction

• We generally assume color brightness is linearWe generally assume color brightness is linear

• But most display devices are inherently nonlinear– brightness(voltage) ≠ 2×brightness(voltage/2): γVI =brightness(voltage) 2 brightness(voltage/2):

• Common solution: gamma correction– Post‐transformation on RGB values to map them to linear

sVI =

prange on display device:

γ1

sc VV =– Can have separate γ for R, G, B– γ is usually in range 1.8 to 2.2

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• Camera• (gamma encoding)

Display

• Overall

• Display• (gamma expansion)

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• Camera• (gamma encoding)

Display

• Overall

• Display• (gamma expansion)

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Color and Perception - Purdue University · Color and Perception CS635 Spring 2010 Daniel G. Aliaga ... HSV/HSL Color Space. HSV/HSL Color Space • Intensity/Value – total amount