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Color Image Processing

Selim Aksoy

Department of Computer Engineering

Bilkent University

saksoy@cs.bilkent.eu.tr

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Color

Use (eavily in (uman vision. )isible spectrum for (umans is !%% nm

*blue+ to ,%% nm *re+.

-ac(ines can see/ muc( more0 e.g.# 12rays# infrare# raio 3aves.

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8uman visual system

Color perception 9ig(t (its t(e retina# 3(ic( contains p(otosensitivecells.

 :(ese cells convert t(e spectrum into a fe3 iscretevalues.

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8uman visual system

 :(ere are t3o types of p(otosensitive cells; Cones

Sensitive to colore lig(t# but not very sensitive to im lig(t.

!> nm *4"% nm *

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actors t(at a=ect perception

9ig(t; t(e spectrum of energy t(atilluminates t(e obect surface.

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Color moels

 :(ey provie a stanar 3ay of specifying aparticular color using a 7D coorinatesystem.

8ar3are oriente

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Aitive an subtractive colors

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C- moel

Cyan2-agenta2ello3 is a subtractive moel3(ic( is goo to moel absorption of colors.

Appropriate for paper printing.

−

=

 B

G

 R

Y 

 M 

C 

1

1

1

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CIE c(romaticity moel

 :(e Commission Internationale elHEclairage ene 7 stanar primaries; 1#

 # J t(at can be ae to form all visiblecolors.

  3as c(osen so t(at its color matc(ingfunction matc(es t(e sum of t(e 7 (umancone responses.

−

−−

−−

=

 Z 

Y 

 X 

 B

G

 R

8986.01185.00583.0

0283.09984.19843.0

2883.05326.09107.1

=

 B

G

 R

 Z 

Y 

 X 

1149.10661.00000.0

1143.05868.02988.0

2001.01736.06067.0

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CIE c(romaticity moel

K# y# 5 normali5e 1# # Jsuc( t(at

K L y L 5 M &.

Actually only K an y

are neee because5 M & 2 K 2 y.

Pure colors are at t(ecurve bounary.

6(ite is *&N7# &N7# &N7+.

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CIE 9ab *9OaOb+ moel

ne luminance c(annel *9+an t3o color c(annels *a anb+.

In t(is moel# t(e colori=erences 3(ic( you

perceive correspon toEucliian istances in CIE 9ab.

 :(e a aKis eKtens from green*2a+ to re *La+ an t(e b aKis

from blue *2b+ to yello3 *Lb+. :(e brig(tness *9+ increasesfrom t(e bottom to t(e top oft(e 7D moel.

(ttp;NN333.f(o2emen.eN(o=mannNcielab%7%$$%%7.pf Aapte from 9ina S(apiro# U of

6as(ington

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 IF moel

8ave better compression properties. 9uminance is encoe using more bits

t(an c(rominance values I an F *(umansare more sensitive to t(an I an F+.

9uminance use by blackN3(ite :)s.

All 7 values use by color :)s.

−

−−=

 B

G R

Q

 I Y 

311.0532.0212.0

321.0275.0596.0114.0587.0299.0

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8S) moel

8S); 8ue# saturation# value are non2linearfunctions of

−−+−

−+−−=

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8S) moel

Uniform; eQual *small+ steps give t(e sameperceive color c(anges.

8ue is encoe as an angle *% to $π+. Saturation is t(e istance to t(e vertical aKis *% to

&+. Intensity is t(e (eig(t along t(e vertical aKis *% to

&+.

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8S) moel

*9eft+ Image of foo originating from a igital camera.

*Center+ Saturation value of eac( piKel ecrease $%R.

*

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Color moels

C- 

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EKamples; pseuocolor

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EKamples; pseuocolor

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EKamples; pseuocolor

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EKamples; segmentation

Can cluster on color values an piKellocations.

Can use connecte components an anapproKimate color criteria to n regions.

Can train an algorit(m to look for certaincolore regions T for eKample# skin color.

riginal

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EKamples; segmentation

Skin color in

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EKamples; segmentation

*9eft+ Input vieo frame.

*Center+ PiKels classie accoring to

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EKamples; (istogram

8istogram is fast an easy to compute. Si5e can easily be normali5e so t(at

i=erent image (istograms can becompare.

Can matc( color (istograms for atabaseQuery or classication.

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EKamples; (istogram

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EKamples; image retrieval

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Summary

 :o print *