INTRODUCTION TO INTRODUCTION TO

COLOR MODELLINGCOLOR MODELLING

CONTENTS

• INTRODUCTION

• COLOR IMAGE PROCESS• COLOR IMAGE PROCESS

• COLOR MODELS

• COLOR MODELING –RGB – CMY – YIQ –

HSL – HSV

COLOR IMAGE PROCESSING

COLOR MODELS

• Color Model• A mathematical system for representing color

• The human eye combines 3 primary colors (using the 3 different types of cones) to discern all possible colors.types of cones) to discern all possible colors.

• Colors are just different light frequencies

• red – 700nm wavelength

• green – 546.1 nm wavelength

• blue – 435.8 nm wavelength

• Higher frequencies are cooler colors

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Color Models

Primary Colors

• Primary colors of light are additive

• Primary colors are red, green, and blue

• Combining red + green + blue yields white

• Primary colors of pigment are subtractive

• Primary colors are cyan, magenta, and yellow

• Combining cyan + magenta + yellow yields

black6

CMY YIQ HSL HSV

RGB

INTRODUCTION TO COLOR MODELLINGINTRODUCTION TO COLOR MODELLING

RGB

Gamma correction

Color lookup table

values

RGB Color 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 additive color model

Source: www.mitsubishi.com

CMY Color model

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Passive displays, such as 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

RGB Vs CMY

RGB color cube

RGB 24-bit color cube

RGB Example

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Original Green Band Blue BandRed Band

RGB and CMY Color Cubes

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RGB AND CMY COLOR CUBES

Light Intensity

• Note that intensity is a weighted function of the r, g, b

values.

• The human eye doesn’t weight each component

identically!

• intensity = 0.299*Red + 0.587*Green +

0.144*Blue

• Assume three light sources have the same actual intensity

but are colored red, green, and blue

• The green light will appear brightest followed by red and blue

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YIQ MODEL

YIQ is the color space used by the NTSC color TV system,

employed mainly in North and Central America, and Japan

The Y component represents the luma information, and is the

only component used by black-and-white television receivers.

YIQ(color modelling)

only component used by black-and-white television receivers.

I and Q represent the chrominance information. In YUV, the U

and V components can be thought of as X and Y coordinates

within the color space. I and Q can be thought of as a second

pair of axes on the same graph, rotated 33°; therefore IQ and

UV represent different coordinate systems on the same plane.

Color images are encoded as triplets of values.

RGB is an additive color model that is used for light-emitting devices,

e.g., CRT displays

CMY is a subtractive model that is used often for printers

Two common color models in imaging are RGB and CMY, two common

YIQ(color modelling)

Two common color models in imaging are RGB and CMY, two common

color models in video are YUV and YIQ.

YUV uses properties of the human eye to prioritize information. Y is the

black and white (luminance) image, U and V are the color difference

(chrominance) images.

YIQ uses similar idea.

Besides the hardware-oriented color models (i.e., RGB, CMY, YUV,

YIQ), HSB (Hue, Saturation, and Brightness) and HLS (Hue, Lightness,

and Saturation) are also commonly used.

Yiq colour descripton.

Example for yiq photo which is used in

televisions

HSV MODEL

HSV is another way to describe color with 3 parameters. RGB is the way

computer screens work, but not very intuitive. HSV is more intuitive, but

you need to convert it to RGB before you can draw a pixel with it. The

nicest application of this color model is that you can easily create

rainbow gradients or change the color, lightness or saturation of an

image with this color model.

HSV color obviously has the parameters H, S and V, or Hue, Saturation HSV color obviously has the parameters H, S and V, or Hue, Saturation

and Lightness Value.

Hue indicates the color sensation of the light, in other words if the color

is red, yellow, green, cyan, blue, magenta, ... This representation looks

almost the same as the visible spectrum of light, except on the right is

now the color magenta (the combination of red and blue), instead of

violet (light with a frequency higher than blue):

Hue works circular, so it can be represented on a circle instead. A hue of

360° looks the same again as a hue of 0°.

Saturation indicates the degree to which the hue differs from a neutral gray. The values run from 0%, which is no color, to 100%, which is the fullest saturation of a given hue at a given percentage of illumination. The more the spectrum of the light is concentrated around one wavelength, the more saturated the color will be.Lightness indicates the illumination of the color, at 0% the color is completely black, at 50% the color is pure, and at 100% it becomes white. In HSL color, a color with maximum lightness (L=255) is always white, no matter maximum lightness (L=255) is always white, no matter what the hue or saturation components are. Lightness is defined as (maxColor+minColor)/2 where maxColoris the R, G or B component with the maximum value, and minColor the one with the minimum value

HSL MODEL

HSL is another way to describe color with 3 parameters. RGB is the way

computer screens work, but not very intuitive. HSL is more intuitive, but

you need to convert it to RGB before you can draw a pixel with it. The

nicest application of this color model is that you can easily create rainbow

gradients or change the color, lightness or saturation of an image with this

color model.

HSL color obviously has the parameters H, S and L, or Hue, Saturation

and Lightness.

Hue indicates the color sensation of the light, in other words if the color is

red, yellow, green, cyan, blue, magenta, ... This representation looks almost

the same as the visible spectrum of light, except on the right is now the

color magenta (the combination of red and blue), instead of violet (light

with a frequency higher than blue):

Hue works circular, so it can be represented on a circle instead. A hue of

360° looks the same again as a hue of 0°.

Hue colour patterns

Graph showing relation between

(saturation,lightness,hue)

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