Post on 25-Apr-2018
Color Representation
• Visible Light Spectrum• Color Matching• Trichromatic Color Theory• Psychophysics• CIE standard• RGB and CMYK Color Spaces• HLS Color Model• YIQ Color Model
Visible Light Spectrum and ColorsLight is an electro-magnetic radiation
• Hue: distinguished among colors• Saturation: how far is color from a gray of equal intensity• Lightness: perceived intensity of a reflective surface• Brightness: perceived intensity of emitting surface
Gamma X rays Infrared Radar FM TV AMUltra-violet
10 -12 10 -8 10 -4 10 41 10 8
electricityACShort-
wave
400 nm 500 nm 600 nm 700 nmWavelength in nanometers (1nm=10-9 m)
Wavelength in meters (m)
Visible light
Spectral Power Distribution• The Spectral Power Distribution of a light is a function f(λ) defining the energy at each wavelength
Blue Skylight Tungsten bulb
Red monitor phosphor Monochromatic light
400 500 600 7000
0.5
1
400 500 600 7000
0.5
1
400 500 600 7000
0.5
1
400 500 600 7000
0.5
1
Color Matching Experiment• Three primary lights are set to match a test light• Metamer: two lights visually undistinguishable (they might have different spectral power distributions)
+ -
+ -
+ -
test match
=~Test light(tungsten light)
Match light(monitor emission)
400 500 600 7000400 500 600 7000
Pow
er
Trichromatic Color Theory• Trichromatic: “tri”=three “chroma”=color also tristimulus
color vision is based on three primaries (three dimensional)
•Thomas Young • A few different retinal receptors operating with differentwavelength sensitivities allow humans to perceive colors
• Suggested 3 receptors
• Helmholtz & Maxwell• Color matching with 3 primaries
The Human Eye
conesOptic NerveFovea
Vitreous
Optic Disc
Lens
Pupil
Cornea
Ocular MuscleRetina
HumorIris
light
rods
amacrinebipolar
ganglion
horizontal
Retinal Photoreceptors• Cones: Sensitive to high illumination levels (Photopic vision)
Less sensitive than rods5 million cones in each eyeOnly cones in fovea (approx. 50,000)Density decreases with distance from fovea3 types differing in their spectral sensitivity: L , M, and S
Wavelength (nm)
Rel
ativ
e se
nsiti
vity
400 500 600 7000
0.25
0.5
0.75
1 LMS
Linear Color Spaces• Colors in 3D color space can be described as linear
combinations of 3 basis colors called primaries
A • + B • + C •=
The representation of the color having spectrum:
Is given by (A, B, C)
Choosing The Primaries• Stiles & Burch (1959) used 3 monochromatic primaries of wavelengths 444.4, 525.3 and 645.2
Color Matching Diagram
Problem: Subtractive components
r(λ)
g(λ)b(λ)
400 500 600 700
0
1
2
3
Wavelength (nm)
Prim
ary
Inte
nsity
CIE Color Standard
Wavelength (nm)
Tris
timul
usva
lues
400 500 600 7000.2
0.6
1
1.4
1.8z(λ)
y(λ)x(λ)
• CIE: Commision Internationale d’Eclairage (1931) defined a standard system (CIE- XYZ) for color representation
• Weights are non negative over the visible wavelengths• The 3 primaries associated with x y z color matching
functions cannot be easily realized in hardware• y was chosen to equal luminance of monochromatic lights
CIE Color Standard
650610
590
550
570
600
580
560
540
505
500
510520 530
490
495
485480
470450
1.00.50.0
0.5
0.9
y
0.0x
X
Z
Y
If X, Y and Z are the weights used to define a color C, then the chromaticity values x, y, z (independent from the luminosity) are given by:x= X/(X+Y+Z) y=Y/(X+Y+Z) z=Z/(X+Y+Z)(x,y,z) is a point on the plane X+Y+Z=1
CIE Chromaticity Diagram
RGB Color RepresentationCathode
(electron gun)
deflection yoke
focusing anode
shadow mask and phosphor coated screen
Cathode(electron gun)
deflection yoke
focusing anode
shadow mask and phosphor coated screen
phosphors on glass screen
shadow mask
electron guns
phosphors on glass screen
shadow mask
electron guns
• In a CRT each color can be defined by the required powerof each electron gun:
• The intensity is defined as:
•The chroma(ticy) is defined as:
C r R g G b B= + +
bgrI ++=
r R g G b BCr g b+ +
=+ +
RGB Color Images126141113636 111
36111
36 1212 17
17 11114
12617
36
12111
3636200 1214
126 1736 36
1111214
3636200 12
14126 1736 36
11136200
2001214111
14126
12617
36
36
36
361214
111
111
361261736
36200
11172
7212
12 17
10 1283617
200 1111214
126
126126
126
17 1717
17
36
36
36
3636
200200
20012
12
12
1414
111
111
72
7272 106 155
10 128
3617
200 11112
14
126
126
126
126
17 17
17
17
36
36
36
36
36
200
200
200
12
12
12
14
14
111
111
72
72
72 106 155
14
126 17
36 36
111
36200
200
12
14111
14
126
126
17
36
36
36
36
12
14
111
111
36
12617
36
36200
111
72
72
12
12 17
12614111
3636 111
36
111
36 12
12 17
17 111
14
126
17
36
12
111
3636200 12
14
126 17
36 36
111
12
14
3636200 12
black
redgreen
blue
white
RGB to CIE-XYZ Conversion• RGB to CIE-XYZ is a linear transformation:
• R = monochromatic primary 700nm• G = monochromatic primary 546.1nm• B = monochromatic primary 435.8nm
2 .3 6 5 0 .5 1 5 0 .0 0 50 .8 9 7 1 .4 2 6 0 .0 1 40 .4 6 8 0 .0 8 9 1 .0 0 9
R XG YB Z
− − − = −
RGB vs. CMY(K) Color Scheme• RGB and CMYK (Cyan, Magenta, Yellow andblacK) are hardware-oriented representations
• CMY is used in color photography and (with K) inmost color printers
RGB is Additive CMY is Subtractive
111
C RM GY B
= −
The HLS Color Model
Hue (red, green, yellow, blue ...)Saturation (pink, bright red, ....)Lightness (black, grey, white ....)
Munsell Book of Colors
• HLS: Hue Lightness, Saturation similar toHSV: Hue Saturation Value
The YIQ Color Model• Based on the concept of opponent colors• Used in NTSC Television (National Television Systems Committee)
• Similar method (YCbCr) used in JPEG and MPEG
• Y = Luminance• I = Red-Green• Q = Blue-Yellow
+
+--
0 .2 9 9 0 .5 8 7 0 .1 1 40 .5 9 6 0 .2 7 5 0 .3 2 10 .2 1 2 0 .5 2 3 0 .3 1 1
R YG IB Q
− − = −
The YIQ Color Model• The human eye is more sensitive to luminosity than to colors, so it ispossible to save space by encoding colors more coarsely
• Preferred by the NTSC because of backward compatibility with B/W TV
Original Y - Blur
I - Blur Q - Blur
Summary• CIE-XYZ
• Tristimulus Coordinates• Device Independent• Universal standard
• CIE-Lab• Perceptual Space, used to assess image quality
• RGB and CMY• Hardware oriented• Additive spaces used for CRT, printers, photography
• YIQ and YCbCr• Opponent Space• Used for color television broadcast and image compression
• HLS• Perceptual Digitized Space• Used for Human Interactive Painting