13-1

Chapter 13: Color Processing

。 Color: An important descriptor of the

world

。 The world is itself colorless

。 Color is caused by the vision system

responding differently to different

wavelengths of light.

13-2

。 Image color depends on:

(1) The color of the incidence light (2) The color of the scene surface (3) The nature of the visual sensor

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○ The Human Eye

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Two kinds of photoreceptors: rods, cones

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Rods -- sensitive to lightCones -- sensitive to color

Three types of cones:

, , ( , , )S M L R G B

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○ RGB Color Space -- Many colors are made

up of varying amounts of red, green and

blue

R, G, B: primary colors, real 1 2 3( ) ( ) ( ) ( )C w R w G w B

1 2 3, ,w w w : color matching functions may be negative

13-7

○ CIE XYZ Color Space

CIE (Commission Internationale d’Eclairage):

an organization responsible for color standardX,Y,Z: not real primaries, Y: luminanceTheir color matching functions are positive everywhere

。 The volume of visible

colors in CIE XYZ

space is a cone

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。 The relationship between RGB and XYZ

0.431 0.342 0.178

0.222 0.707 0.071

0.02 0.130 0.939

X R

Y G

Z B

3.063 1.393 0.476

0.969 1.876 0.042

0.068 0.229 1.069

R X

G Y

B Z

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○ CIE xy Color Space -- A constant brightness

section intersects the XYZ space with the

plane 1X Y Z

, , X Y Z

x y zX Y Z X Y Z X Y Z

Since x + y + z = 1, a color can be specifiedby x and y alone.

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。 Chromaticity Diagram

(i) Spectral locus: the curved boundary along which colors of single wavelengths are viewed(ii) Neutral point: the color whose weights are equal for all three primaries(iii) Colors that lie farther away from the neutral point are more saturated

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。 RGB Gamut – The colors correspond to

positive matching values

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。 Secondary colors (primaries of pigments):

Magenta (purple) = R + B = W - G

Cyan = G + B = W - R

Yellow = R + G = W - B

。 Pigments remove color from incident light,

which is reflected from paper

e.g., Red ink absorbs green and blue light;

incident red light passes through the

ink and is reflected from the paper

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○ HSV (Hue, Saturation, Value) Color Space

Hue: varies from red greenSaturation: varies from red pinkBrightness: varies from black white

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○ (i) RGB HSV

If R = V, then

If G = V, then

If B = V, then

If H ends up being negative, add 1

If (R,G,B) = (0,0,0), then (H,S,V) = (0,0,0)

max{ , , },

min{ , , }, /

V R G B

V R G B S V

1

6

G BH

1

26

B RH

1

46

R GH

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。 Example: (R, G, B) = (0.2, 0.4, 0.6)

max{ , , } max{0.2,0.4,0.6} 0.6

min{ , , } 0.6 min{0.2,0.4,0.6} 0.4

/ 0.4 / 0.6 0.6667

V R G B

V R G B

S V

Since 0.6,

1 1 0.2 0.44 4 0.5833

6 6 0.4

B V

R GH

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(ii) HSV RGB

6

6

(1 )

(1 )

[1 (1 )]

H H

F H H

P V S

Q V SF

T V S F

0

1

2

3

4

5

H R G B

V T P

Q V P

P V T

P Q V

T P V

V P Q

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。 Example: (H, S, V) = (0.5833, 0.6667, 0.6)

6 6(0.5833) 3

6 6(0.5833) 3 0.5

(1 ) 0.6(1 0.6667) 0.2

(1 ) 0.6(1 0.6667 0.5) 0.4

[1 (1 )] 0.6[1 0.6667

(1 0.5)] 0.4

H H

F H H

P V S

Q V SF

T V S F

Since 3,

( , , ) ( , , ) (0.2, 0.4, 0.6)

H

R G B P Q V

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○ YIQ Color Space – Used for TV and video

Y : luminance information

I, Q : color information

0.299 0.587 0.114

0.596 0.274 0.322

0.211 0.523 0.312

Y R

I G

Q B

1.0 0.956 0.621

1.0 0.272 0.647

1.0 1.106 1.703

R Y

G I

B Q

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○ Uniform Color Space

-- The distance in the space is a guide to

color difference

。 Noticeable difference – the difference when

modifying a color until one can tell it has

changed

。 Macadam ellipse -- the noticeable difference

of a color forms the boundary of the color in

a color space and can be fitted with an ellipse

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The color difference in CIE xy space is poor: (a) the ellipses at the top are larger than those at the bottom (b) the ellipses rotate as they move up

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。 CIE u’v’ Color Space – a more uniform

space than the CIE xy color space 4 9

( , ) ( , )15 3 15 3

X Yu v

X Y Z X Y Z

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○ CIE Lab Color Space

– another substantial uniform space* 1/3

* 1/3 1/3

* 1/3 1/3

116( ) 16

500[( ) ( ) ]

200[( ) ( ) ]

n

n n

n n

YL

Y

X Ya

X Y

Y Zb

Y Z

where , ,n n nX Y Z : the XYZ coordinates of a reference white patch

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◎ Color Images

13-24

13-25

◎ Pseucoloring

。 Intensity Slicing

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。 Transformation

Define colormap functions:

( ), ( ), ( ), : gray levelR G Bf x f x f x x

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◎ Processing of Color Images

Two methods:

(a) (b)

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○ Noise Reduction

R G B

Apply medianfilter to R,G,B

Apply medianfilter to Y

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○ Contrast Enhancement

Perform on the intensity component (1) RGB YIQ (2) Apply histogram equalization to Y Y’ (3) Y’IQ R’G’B’

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○ Spatial Filtering

Both low- and high- pass filters are better

off applying to the intensity component

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○ Edge Detection

Two ways:

(1) Apply edge detection to the intensity

component

(2) Apply edge detection to each RGB

component