digital image processing, color models

8/12/2019 digital image processing, color models

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Color Models


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What is Color

- Color is a powerful descriptor that often simplifies

object identification and extraction from a scene.

- Color is a fundamental attribute of our viewing

experience


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Preview


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Preview


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Light

Light is fundamental forcolor vision

Unless there is a source of light, there is nothing to see!

What do we see?

We do not see objects,but the light that has been

reflected by or transmitted through the objects


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Light and EM waves

Light is an electromagnetic wave

If its wavelength is comprised between 400 and 700 nm

(visible spectrum), the wave can be detected by the human

eye and is calledmonochromatic light


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Physical properties of light

This distribution may indicate:

1) a dominant wavelength (or frequency) which is

the color of the light (hue),

2) brightness (luminance), intensity of the light

(value),

3) purity (saturation), which describes the degree

of vividness.


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CIE standard

Commission Internationalede LEclairage (1931)

not a computer model

each color = a weighted

sum of three imaginaryprimary colors


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RGB model Color monitor, color video cameras

CMY model

Color printer

HSI model

Color image manipulation

XYZ (CIE standard, Y directly measures the luminance)

YUV (used in PAL color TV)

YIQ (used in NTSC color TV)

YCbCr (used in digital color TV standard BT.601)

Types of Color Model


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Basic quantities to describe the quality of light

source:

Radiance:Total amount of energy that flows from the

light source (in W). Luminance: A measure of the amount of energy an

observer perceives from the light source (in lm)

Brightness: A subjective descriptor that embodies the

achromatic notion of intensity and is practical

impossible to measure.

Color Fundamentals (cont)c


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Color Fundamentals (cont )

Standard wavelength values for

the primary colors


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Color Fundamentals (cont )

The characteristics generally used to distinguish one color

from another are Brightness,Hue, and Saturation.

Hue: Represents dominant color as perceive by an observer.

Saturation: Relative purity or the amount of white light mixed

with a hue

Hue and saturation taken together are called Chromatici ty, and

therefore, a color may be characterized by its Brightness and

Chromaticity.


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Color Fundamentals (cont)

Tri-stimulusvalues: The amount of Red, Green andBlue needed to form any particular color

Denoted by: X, Y and Z

ZYX

Xx

ZYX

Yy

ZYX

Zz

1 zyx

Tri-chromatic coefficient:


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Color Models

The purpose of a color model (also called

color space or color system) is to facilitate the

specification of colors in some standard,

generally accept way.

RGB(red, green, blue) : monitor, video camera.

CMY(cyan, magenta, yellow),CMYK(CMY, black)

model for color printing.

and HSImodel, which corresponds closely with the

way humans describe and interpret color.


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RGB(red, green, blue)The RGB colour space is related to human vision through the

tristimulus theory of colour vision.

The RGB is an additive colour model. The primary colours red,

green andblueare combined to reproduce other colours.

In the RGB colour space, a colour is represented by a triplet (r,g,b)

r gives the intensity of the red component

ggives the intensity of the green component

bgives the intensity of the blue component

Here we assume that r,g,b are real numbers in the interval [0,1].

You will often see the values of r,g,b as integers in the interval

[0,255].


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RGBColor model

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Active displays, such as computer monitors and television sets, emit

combinations of red, green and blue light. This is an additivecolor model

Source: www.mitsubishi.com


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The RGBColor Spaces


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CMYColor model

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Passive displays, such as color inkjet printers, absorblight instead of

emitting it. Combinations of cyan,magentaand yellowinks are used. This

is a subtractivecolor model.

Source: www.hp.com


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CMY

CMY cartridges for colour printers.


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The CMYand CMYKColor Spaces


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The CMYand CMYKColor Models

B

G

R

Y

M

C

1

1

1

Cyan, Magenta and Yellow are the secondary colors

of light

Most devices that deposit colored pigments on

paper, such as color printers and copiers, requireCMY data input.


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Example 11.3: The magenta is written in CMY as (0,1,0). In RGB it is

written as

giving,

that is, red and blue.

b

g

r

1

1

1

0

1

0

1

0

1

0

1

0

1

1

1

b

g

r


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CMYK model

For printing and graphics art industry, CMY is

not enough; a fourth primary, K which stands

for black, is added.

Conversions between RGB and CMYK are

possible, although they require some extra

processing.


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HSI Color Model

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Hdominant

wavelength

Spurity

% white

IIntensity

Source: http://www.cs.cornell.edu/courses/cs631/1999sp/


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HSI Color

Model


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HSI Color Model

Hueis defined as an angle 0 degrees is RED

120 degrees is GREEN

240 degrees is BLUE

Saturation is defined as the percentage of distance from

the center of the HSI triangle to the pyramid surface.

Values range from 0 to 1.

Intensity is denoted as the distance up the axis from

black.

Values range from 0 to 1

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HSVThe components of the HSV colour space are Hue, Saturationand Value.

Colour is the result of the perception of light at different wavelengths. Usually, we do not

experience light at a single wavelength but a blend of waves at different wavelengths. The

huecorresponds to the dominant wavelength and determines the type of the colour, for

example red, yellow, or blue.

The saturation determines the purity of the colour. High saturation gives pure colours

(narrow wavelength band), while low saturation means colours mixed with a lot of white

(white light combines all the visible wavelengths).

The valuedetermines the brightness. A value equal to zero represents absence of light,

while a high value gives bright colours.


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H.R. Pourreza

The HSI Colo r Models


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H.R. Pourreza


RG

B

H

S I

H S

I RG

B


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Converting colors from RGB to HSI

GBif360

GBif

H

2/12

1

)])(()[(

)]()[(2

1

cosBGBRGR

BRGR

)],,[min()(

31 BGRBGR

S

)(3

1BGRI



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Converting colors from

HIS to RGB

RG sector :

)1( SIB

)60cos(

cos1

H

HSIR

)(3 BRIG


1200 H


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HIS to RGB

GB sector :

)1( SIR

)60cos(

cos1

H

HSIG

)(3 GRIB


120HH

240120 H


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HIS to RGB

BR sector :

)1( SIG

)60cos(

cos1

H

HSIB

)(3 BGIR


240HH

360240 H


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YIQ Color Coordinate System

YIQ is defined by the National Television System

Committee (NTSC)

Y describes the luminance, I and Q describes the

chrominance.

A more compact representation of the color.

YUV plays similar role in PAL and SECAM.


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YUV/YCbCr Coordinate

YUV is the color coordinate used in color TV in

PAL system, somewhat different from YIQ.

YCbCr is the digital equivalent of YUV, used for

digital TV, with 8 bit for each component, in

range of 0-255


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YCbCr Color Space is used in MPEG video compression

standards

Y is luminance Cb is blue chromaticity

Cr is red chromaticity

Y = 0.257*R + 0.504*G + 0.098*B + 16

Cr = 0.439*R - 0.368*G - 0.071*B + 128

Cb = - 0.148*R - 0.291*G + 0.439*B + 128

YIQ color space (Matlab conversion function: rgb2ntsc):

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