Color Models

Arka Prokash Mazumdar

Color

• It’s all electromagnetic (EM) radiation – Different colors correspond to radiation of

different wavelengths

– Intensity of each wavelength specified by amplitude

• We perceive EM radiation with in the 400-700 nm range

– Between infra-red and ultraviolet

Visible Light

Wavelength

• Most light we see is not just a single wavelength

• A combination of many wavelengths

• Referred to as a spectrum

Wavelength

• Representation of color on screen display is RGB. (additive color)

• Printers use CMY(K), (subtractive color)

Human Eye

Human Eye

• 3 different types of cone receptors in the fovea of retina

– S, M, and L

– blue, green, and red sensors

• Peak sensitivities are located at approximately:

– 445nm, 535nm, and 575nm

– for the "average" observer

Human Eye : Absorption of Light

Physical properties of light

• This distribution may indicate:

1. Dominant wavelength (frequency): the color of the light (hue)

2. Brightness (luminance): intensity of the light (value)

3. Purity (saturation): the degree of vividness (mix with white)

CIE Standard

International Commission on Illumination (Commission Internationale de L’Eclairage) (1931)

• Color matching functions

• Tested with 440,545 and 580nm

CIE: Experiment

CIE space

CIE space• Irregular 3D volume

shape is difficult to understand

• Chromaticity diagram – the same color of the

varying intensity (Y) should all end up at the same point

Color Models

• 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)

RGB

• Standard

RGB color space

CMY color model

• Passive displays (e.g. color inkjet printers) absorb light instead of emitting it

• Combinations of cyan, magenta and yellow inks are used– This is a subtractive color model.

Source: www.hp.com

CMY

CMY and CMYK

RGB to CMY model

The conversion from RGB to CMY is given by the formula

Example 11.2: The red colour is written in RGB as (1,0,0). In CMY it is written as

that is, magenta and yellow.

1

1

0

0

0

1

1

1

1

1

1

1

b

g

r

y

m

c

b

g

r

y

m

c

1

1

1

HSV/HIS model

• HSV is a non-linearly transformed (skewed) version of RGB cube

– Hue: quantity that distinguishes color family, say red from yellow, green from blue

– Saturation (Chroma): color intensity (strong to weak). Intensity of distinctive hue, or degree of color sensation from that of white or grey

– Value (luminance)/I(intensity): light color or dark color

HSV/HSI model

H dominant

wavelength

Spurity

% white

IIntensity

HSV/HSI model

• Hue is 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 thecenter 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

HSV/HIS model

HSV/HIS model

RGB to HSI

GB if 360

GB if

H

2/12

1

)])(()[(

)]()[(2

1

cosBGBRGR

BRGR

)],,[min()(

31 BGR

BGRS

)(3

1BGRI

RG sector :

)1( SIB

)60cos(

cos1

H

HSIR

)(3 BRIG

HSI to RGB

1200 H

GB sector :

)1( SIR

)60cos(

cos1

H

HSIG

)(3 GRIB

HSI to RGB

120 HH

240120 H

BR sector :

)1( SIG

)60cos(

cos1

H

HSIB

)(3 BGIR

HSI to RGB

240 HH

360240 H

The HSI Color Models

Comparisons: RGB, CMY, HSI

YUV/YIQ

• YUV: for PAL and SECAM analog video– Also used in CCIR 601 standard for digital video

• Y (luminance) is the CIE Y primaryY = 0.299R + 0.587G + 0.114B

• Chrominance:– defined as the difference between a color and a

reference white at the same luminance

YUV/YIQ

• It is be represented by U and V

• The color differences:U = B – Y; V = R – Y

• YIQ is defined by the National Television System Committee (NTSC)

– Similar to YUV

YCbCr

• Y: luminance

• Cb: Chrominance of Blue

• Cr: Chrominance of Red

• Digital equivalent of YUV, used for digital TV

– 8 bit for each component

– in range of 0-255

YCbCr

• Used in MPEG video compression standards

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