colour unitiii

7/31/2019 Colour UnitIII

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Color Models


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Color models,contd

Different meanings of color:

painting

wavelength of visible light

human eye perception


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Physical properties of light

Visible light is part of the electromagnetic radiation (380-750 nm)

1 nm (nanometer) = 10-10 m (=10-7 cm)

1 (angstrom) = 10 nm

Radiation can be expressed in wavelength () orfrequency (f), c=f, where c=3.1010 cm/sec


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White light consists ofa spectrum of all

visible colors


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All kinds of light canbe described by theenergy of eachwavelength

The distributionshowing the relationbetween energy andwavelength (orfrequency) is calledenergy spectrum


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This distribution may indicate:

1) a dominant wavelength (or frequency)

which is the color of the light (hue), cp.ED

2) brightness (luminance), intensity of thelight (value), cp. the area A

3) purity (saturation), cp. ED - EW


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Energy spectrum for a light source with adominant frequency near the red color


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Material properties

The color of an object depends on the socalled spectral curves for transparency

and reflection of the materialThe spectral curves describe how light of

different wavelengths are refracted and

reflected (cp. the material coefficientsintroduced in the illumination models)


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Properties of reflected light

Incident white light upon an object is forsome wavelengths absorbed, for others

reflectedE.g. if all light is absorbed => blackIf all wavelengths but one are absorbed

=> the one color is observed as thecolor of the object by the reflection


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Color definitions

Complementary colors - two colorscombine to produce white light

Primary colors - (two or) three colors usedfor describing other colors

Two main principles for mixing colors:

additive mixing

subtractive mixing


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Additive mixing

pure colors are put close to each other => a mix on theretina of the human eye (cp. RGB)

overlapping gives yellow, cyan, magenta and white

the typical technique on color displays


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Subtractive mixing

color pigments are mixed directly in someliquid, e.g. ink

each color in the mixture absorbs its specificpart of the incident light

the color of the mixture is determined bysubtraction of colored light, e.g. yellowabsorbs blue => only red and green, i.e.yellow, will reach the eye (yellow because ofaddition)


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Subtractive mixing,contd

primary colors: cyan, magenta andyellow, i.e. CMY

the typical technique in printers/plotters connection between additive and

subtractive primary colors (cp. the color

models RGB and CMY)


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Additive/subtractive mixing


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Human color seeing

The retina of the human eye consists ofcones(7-8M),tappar, and rods (100-120M),

stavar, which are connected with nervefibres to the brain


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Human color seeing,contd

Theory: the cones consist of various lightabsorbing material

The light sensitivity of the cones and rods varieswith the wavelength, and between personsThe sum of the energy spectrum of the light

the reflection spectrum of the object the response spectrum of the eyedecides the color perception for a person


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Overview of color models

The human eye can perceive about 382000(!)different colors

Necessary with some kind of classification sys-tem; all using three coordinates as a basis:1) CIE standard2) RGB color model

3) CMY color model (also, CMYK)4) HSV color model5) HLS color model


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CIE standard

CommissionInternationale de

LEclairage (1931) not a computermodel

each color = aweighted sum ofthree imaginaryprimary colors


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RGB model

all colors aregenerated from thethree primaries

various colors areobtained bychanging theamount of eachprimary

additive mixing(r,g,b), 0r,g,b1


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RGB model,contd

the RGB cube 1 bit/primary => 8 colors, 8 bits/primary => 16M colors


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CMY model

cyan, magenta andyellow are comple-mentary colors ofred,green and blue,respectively

subtractive mixing

the typical printertechnique


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CMY model,contd

almost the samecube as with RGB;

only black white the various colorsare obtained byreducing light, e.g. if

red is absorbed =>green and blue areadded, i.e cyan


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RGB vs CMY

If the intensities are represented as 0r,g,b1and 0c,m,y1 (also coordinates 0-255 can

be used), then the relation between RGB andCMY can be described as:

c

m

y

=

1

1

1

r

g

b


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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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HSV model

HSV stands for Hue-Saturation-Value

described by a hexcone derived from the

RGB cube


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HSV model,contd

Hue (0-360); thecolor, cp. thedominant wave-length (128)

Saturation (0-1); theamount of white(130)

Value (0-1); theamount of black(23)


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HSV model,contd

The numbers given after each primary areestimates of how many levels a human being

is capable to distinguish between, which (intheory) gives the total number of colornuances:

128*130*23 = 382720

In Computer Graphics, usually enough with:128*8*15 = 16384


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HLS model

Another model similarto HSV

L stands forLightness


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Color models

Some more facts about colors:

The distance between two colors in the

color cube is not a measure of how farapart the colors are perceptionally!

Humans are more sensitive to shifts in

blue (and green?) than, for instance, inyellow

colour unitiii

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