Colour UnitIII
Transcript of Colour UnitIII
-
7/31/2019 Colour UnitIII
1/29
Color Models
-
7/31/2019 Colour UnitIII
2/29
Color models,contd
Different meanings of color:
painting
wavelength of visible light
human eye perception
-
7/31/2019 Colour UnitIII
3/29
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
-
7/31/2019 Colour UnitIII
4/29
Physical properties of light
White light consists ofa spectrum of all
visible colors
-
7/31/2019 Colour UnitIII
5/29
Physical properties of light
All kinds of light canbe described by theenergy of eachwavelength
The distributionshowing the relationbetween energy andwavelength (orfrequency) is calledenergy spectrum
-
7/31/2019 Colour UnitIII
6/29
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
-
7/31/2019 Colour UnitIII
7/29
Physical properties of light
Energy spectrum for a light source with adominant frequency near the red color
-
7/31/2019 Colour UnitIII
8/29
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)
-
7/31/2019 Colour UnitIII
9/29
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
-
7/31/2019 Colour UnitIII
10/29
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
-
7/31/2019 Colour UnitIII
11/29
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
-
7/31/2019 Colour UnitIII
12/29
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)
-
7/31/2019 Colour UnitIII
13/29
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)
-
7/31/2019 Colour UnitIII
14/29
Additive/subtractive mixing
-
7/31/2019 Colour UnitIII
15/29
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
-
7/31/2019 Colour UnitIII
16/29
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
-
7/31/2019 Colour UnitIII
17/29
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
-
7/31/2019 Colour UnitIII
18/29
CIE standard
CommissionInternationale de
LEclairage (1931) not a computermodel
each color = aweighted sum ofthree imaginaryprimary colors
-
7/31/2019 Colour UnitIII
19/29
RGB model
all colors aregenerated from thethree primaries
various colors areobtained bychanging theamount of eachprimary
additive mixing(r,g,b), 0r,g,b1
-
7/31/2019 Colour UnitIII
20/29
RGB model,contd
the RGB cube 1 bit/primary => 8 colors, 8 bits/primary => 16M colors
-
7/31/2019 Colour UnitIII
21/29
CMY model
cyan, magenta andyellow are comple-mentary colors ofred,green and blue,respectively
subtractive mixing
the typical printertechnique
-
7/31/2019 Colour UnitIII
22/29
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
-
7/31/2019 Colour UnitIII
23/29
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
-
7/31/2019 Colour UnitIII
24/29
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.
-
7/31/2019 Colour UnitIII
25/29
HSV model
HSV stands for Hue-Saturation-Value
described by a hexcone derived from the
RGB cube
-
7/31/2019 Colour UnitIII
26/29
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)
-
7/31/2019 Colour UnitIII
27/29
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
-
7/31/2019 Colour UnitIII
28/29
HLS model
Another model similarto HSV
L stands forLightness
-
7/31/2019 Colour UnitIII
29/29
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