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Colour Models and

its ApplicationHLS and YIQ

Anirban SahaThird yearComputer Science and TechnologyBengal College of Engineering and Technology

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Modules:

Prerequisite : Frequency, Light, Dominant frequency, Definitions.

YIQ model: Introduction, Application.

HLS model: Introduction, Discussion, Application.

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Requisites:

Visible Light: Red: 4.3 x 1014 hertzViolet: 7.5 x 1014 hertz

R-O-Y-G-B-I-VWavelength(700 nm)

Wavelength(400 nm)

Lowest Frequency

HighestFrequency

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HUEBRIGHTNESSSATURATION

COLOUR, DOMINENT FREQUENCY

PERCIEVED INTENSITY OF LIGHT

THE WASHING OUT EFFECT

ED = DOMINENT LIGHT. ENERGY DENSITYEW = ENERGY DENSITY OF WHITE LIGHT

AREA UNDER THE CURVE -> BRIGHTNESSMORE THE VALUE OF (ED-EW) PURER IS THE COLOUR

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Some more terms:

The dimensions of the HSV and HSL geometries, simple transformations of the not-perceptually-based RGB model, are not directly related to the photometric color-making attributes of the same names, as defined by scientists, for instance by the CIE or ASTM. Nonetheless, it is worth reviewing those definitions before leaping into the derivation of our models.

Hue is the “attribute of a visual sensation according to which an area appears to be similar to one of The perceived colors: red, yellow, green, and blue, or to a combination of two of them”.

Intensity  or radiance is the total amount of light passing through a particular area.Luminance, denoted Y and measured in cd/m2, is the radiance weighted by the effect of each

wavelength on a typical human observer. Often the term luminance is used for the relativeluminance, Y/Yn, where Yn is the luminance of the reference white point.

Luma is the weighted sum of gamma-corrected R′, G′, and B′ values, denoted Y′, and used in Y′CbCr, for JPEG compression and video transmission.

Brightness is the “attribute of a visual sensation according to which an area appears to emit more or less light”.

Lightness or value is the “brightness relative to the brightness of a similarly illuminated white”.Colorfulness is the “attribute of a visual sensation according to which the perceived color of an area

appears to be more or less chromatic”.Chroma is the “colorfulness relative to the brightness of a similarly illuminated white”.Saturation is the “colorfulness of a stimulus relative to its own brightness”.Brightness and colorfulness are absolute measures, which usually describe the spectral distribution 

of light entering the eye, while lightness and chroma are measured relative to some white point, and are thus often used for descriptions of surface colors, remaining roughly constant even as brightness and colorfulness change with different illumination.

Saturation = colorfulness/brightness = chroma/lightness.

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YIQmodel

Used in NTSC - TV monitors, using one composite signal.

RGB model requires 3 separate signals to form an image.It is based on the CIE and the XYZ colour models.

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National Television System Committee

NTSE: Analog television system used in most of North America,South America, Japan, South Korea, Taiwan, Burma, and some Pacific island nations and territories (see map). NTSC is also the name of the U.S. standardization body that developed the broadcast standard

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YIQ

Chromaticity Information (hue and purity)

Luminance (Brightness information)

Orange-Cyan hue

Green - Magenta

“Y” stands for Luminance“I” stands for inphase“Q” stands for quadrature

referring to the components of the quadrature amplitude modulation

RGB YIQ [TV signal using NTSC encoder]

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Y

I

Q

0.299 0.587 0.144

0.596 -0.275 -0.321

0.212 -0.528 0.311

R

G

B

=

YIQ RGB [TV signal using NTSC Decoder]

R

G

B

1 0.956 0.620

1 -0.272 -0.647

1 -1.108 1.705

Y

I

Q

=

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HSV and HSL models

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RGB :

Constitutes of Red Green and Blue.but it fails to match the

intuitions of human psychology.

Why?

It explores a wide range of colors Possible by mixing three colours…

Because, when we imagine a colour… our intuitions bring with it The aspects of lightness of the colour and the amount we use tocolour a specific region.

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HSV

HSL

ue

aturation

alue

ue

aturation

ightness

Result:

Mid 1970’s by pioneers of PARC and NYIT

Discussed in Aug 1978

Introduced in 1979By Tektronix Inc

… and has been used since then

Application:

>Aid selection>Colour Selection>Colour Comparision>Modification of Colour>Image analysis …in all picture editing softwares and in designing.

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HLS:

Represented by a double hexagonalCone, with WHITE at the top and BLACK at the bottom.

The HUE lies on the circle, LIGHTNESS gradually

increases from Black to White

SATURATION increases from centre to the periphery

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Hue specifies the angle about the vertical axis that locates the specific hue.

The colours are specified around the perimeter of the cone.

Complementary colours are 180 degrees apart

The vertical axis is called the “Lightness”

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Thank You.

Anirban Saha Ph: 9903055542Third year email: saha.a2006@gmail.comComputer Science and Technology Blog at: http://sahaanirban.wordpress.comBengal College of Engineering and Technology Twitter: @sahaanirban