coloring models
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Color Models
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Color models
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 nmRadiation can be expressed in wavelength ( ) or
frequency (f), c= f, where c=3.1010 cm/sec
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Physical properties of light
White light consists ofa spectrum of all
visible colors
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Physical properties of light
All kinds of light canbe described by theenergy of eachwavelength
The distributionshowing the relationbetween energy andwavelength (orfrequency) is calledenergy spectrum
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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 ), cp.ED
2) brightness (luminance), intensity of thelight (value ), cp. the area A
3) purity (saturation ), cp. ED - EW
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Physical properties of light
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 transparencyand reflection of the material
The spectral curves describe how light ofdifferent 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
the retina 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 by
subtraction 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 colormodels RGB and CMY)
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Additive/subtractive mixing
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Human color seeing
The retina of the human eye consists of cones (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 light
absorbing 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 modelsThe 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 are
generated 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 and
yellow 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. ifred 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:
cmy
111
r gb
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CMYK model
For printing and graphics art industry,CMY is not enough; a fourth primary, Kwhich stands for black, is added.
Conversions between RGB and CMYKare 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 ); the
color, cp. thedominant wave-length (128)
Saturation (0-1);the amount ofwhite (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 = 382720In Computer Graphics, usually enough with:
128*8*15 = 16384
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HLS model
Another model similarto HSV
L stands for Lightness
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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 inblue (and green?) than, for instance, inyellow