Transcript C O L R

COLOR
and the human response to light
Idit Haran
Contents

Introduction:



The nature of light
The physiology of human vision
Color Spaces:





Linear (RGB, CMYK)
Artistic View (Munsell, HSV, HLS)
Standard (CIE-XYZ)
Perceptual (Luv, Lab)
Opponent (YIQ, YUV) – used in TV
2
Introduction
3
Electromagnetic Radiation - Spectrum
Gamma
10
X rays
-12
Ultraviolet
10
Infrared
-8
10
Radar
-4
FM
ShortTV wave
AM
4
1
10
Wavelength in meters (m)
AC
electricity
10
Visible light
400 nm
500 nm
600 nm
700 nm
Wavelength in nanometers (nm)
4
8
Spectral Power Distribution
The Spectral Power Distribution (SPD) of a
light is a function P(l) which defines the
power in the light at each wavelength
Relative Power

1
0.5
0
400
500
600
700
Wavelength (l)
5
Examples
6
The Interaction of Light and Matter

Some or all of the light may be absorbed depending
on the pigmentation of the object.
7
The Physiology of Human Vision
8
The Human Eye
9
The Human Retina
cones
rods
horizontal
bipolar
amacrine
ganglion
light
10
The Human Retina
11
Retinal Photoreceptors
12
Cones




High illumination levels (Photopic vision)
Less sensitive than rods.
5 million cones in each eye.
Density decreases
with distance from
fovea.
13
3 Types of Cones



L-cones, most sensitive to red light (610 nm)
M-cones, most sensitive to green light (560 nm)
S-cones, most sensitive to blue light (430 nm)
14
Cones Spectral Sensitivity
L, M , S 
 L   Pl Ll dl
l
15
Metamers

Two lights that appear the same visually.
They might have different SPDs (spectral
power distributions)
16
History

Tomas Young (1773-1829)
“A few different retinal receptors operating with different
wavelength sensitivities will allow humans to perceive the
number of colors that they do. “

James Clerk Maxwell (1872)
“We are capable of feeling three different color sensations.
Light of different kinds excites three sensations in
different proportions, and it is by the different
combinations of these three primary sensations that all
the varieties of visible color are produced. “

Trichromatic: “Tri”=three “chroma”=color
17
3D Color Spaces

Three types of cones suggests color is a 3D
quantity. How to define 3D color space?
Cubic Color Spaces
Polar Color Spaces
Brightness
Hue
G
Opponent Color Spaces
black-white
blue-yellow
B
R
red-green
18
Contents

Introduction:



The nature of light
The physiology of human vision
Color Spaces:





Linear (RGB, CMYK)
Artistic View (Munsell, HSV, HLS)
Standard (CIE-XYZ)
Perceptual (Luv, Lab)
Opponent (YIQ, YUV) – used in TV
19
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 :
is then given by:
(a, b, c)
20
RGB Color Model

Primary Intensity

RGB = Red, Green, Blue
Choose 3 primaries as the basis SPDs (Spectral
Power Distribution.)
3
2
1
0
400
500
600
Wavelength (nm)
700
21
Color Matching Experiment
test

match
-
+
-
+
-
Three primary lights are set to match a test light
Test light
Match light
1
1
~
=
0.75
0.5
0.25
0
+
400
500
600
700
0.75
0.5
0.25
0
400
500
600
700
22
CIE-RGB



Stiles & Burch (1959) Color matching Experiment.
Primaries are: 444.4 525.3 645.2
Given the 3 primaries, we can describe any light with
3 values (CIE-RGB):
(85, 38, 10)
(21, 45, 72)
(65, 54, 73)
23
RGB Image
111
36
12
17
3636 12
36111 14
111
36
111
36
14
126
36 17
126
36111 12
17 200
126 3617 12111 36200 12
126
200 7236 1212 17126 11117 14
36
200 111 14
36
72
36
12
17
12
126
36
12
10
14
36 111 36
200 36 1712 11136 200
14
200 111 1414 36126 1217
128 36126 36200
1711112
11136111
1414
36 1736 126
14127236
72200
17
126126
17
72
106
155
36
10
200
111
17
200
36
12
36
17
14
17
14
126
200
17
36 72
12
128
36
14
36
111
111
17
36
111
200
126
36
200
36
111
12 12
126
126
12
126
36
14
36
126
111
200
36
72
12
111
12
14
17 17
200
200
36
36
14
126
12
17
36
36
12
126
36
14
36
126
72 111
36
12
111
14
36
12
36
36
72
17
111
17
111
111
200
14
36
36
12
126
17
17
111
14
36
36
72
12
126
17
111
106 14 155 36
36
12
24
transmit
CMYK Color Model
Cyan – removes Red
B
G
CMYK = Cyan, Magenta, Yellow, blacK
R
Magenta – removes Green
B
G
R
Yellow – removes Blue
B
G
R
Black – removes all
25
Combining Colors
Additive (RGB)
Subtractive (CMYK)
26
Example: red = magenta + yellow
B
G
R
magenta
+
B
G
R
B
G
R
yellow
=
red
B
G
R
R
27
CMY + Black
C + M + Y = K (black)



Using three inks for black is expensive
C+M+Y = dark brown not black
Black instead of C+M+Y is crisper with more contrast
=
100
C
50
M
70
Y
+
50
K
50
C
0
M
20
Y
28
Example
29
Example
50
100
150
200
50
100
150
200
250
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Example
50
100
150
200
50
100
150
200
250
31
Example
50
100
150
200
50
100
150
200
250
32
Example
50
100
150
200
50
100
150
200
250
33
From RGB to CMY
 C  1  R 
     
 M   1   G 
 Y  1  B 
     
 R  1  C 
     
 G   1   M 
 B  1  Y 
     
34
Color Spaces





Linear (RGB, CMYK)
Artistic View (Munsell, HSV, HLS)
Standard (CIE-XYZ)
Perceptual (LUV, Lab)
Opponent (YIQ, YUV) – used in TV
35
The Artist Point of View



Hue - The color we see (red, green, purple)
Saturation - How far is the color from gray
(pink is less saturated than red, sky blue is
less saturated than royal blue)
Brightness/Lightness (Luminance) - How
bright is the color
white
36
Munsell Color System
Equal perceptual steps in Hue Saturation Value.
Hue:
R, YR, Y, GY, G, BG, B, PB, P, RP
(each subdivided into 10)
Value: 0 ... 10
(dark ... pure white)
Chroma: 0 ... 20
(neutral ... saturated)
Example:
5YR 8/4
37
Munsell Book of Colors
38
Munsell Book of Colors
39
HSV/HSB Color Space
HSV = Hue Saturation Value
HSB = Hue Saturation Brightness
Saturation Scale
Brightness Scale
40
HSV
Saturation
Value
Hue
41
HLS Color Space
HLS = Hue Lightness Saturation
V
green
120°
cyan
yellow
0.5
red
0°
Blue
240°
magenta
H
0.0
black
S
42
Color Spaces





Linear (RGB, CMYK)
Artistic View (Munsell, HSV, HLS)
Standard (CIE-XYZ)
Perceptual (Luv, Lab)
Opponent (YIQ, YUV) – used in TV
43
CIE Color Standard

Why do we need a standard ?

RGB differ from one device to another
44
CIE Color Standard

Why do we need a standard ?


RGB differ from one device to another
RGB cannot represent all colors
RGB Color Matching Functions
45
CIE Color Standard - 1931



CIE - Commision Internationale d’Eclairage
1931 - defined a standard system for color
representation.
XYZ tristimulus coordinate system.
X
Y
Z
46
XYZ Spectral Power Distribution


Non negative over the
visible wavelengths.
The 3 primaries associated
with x y z spectral power
distribution are unrealizable
(negative power in some of
the wavelengths).
y was chosen to equal
luminance of monochromatic
lights.
1.8
Tristimulus values

1.4
z(l)
y(l)
1
x(l)
0.6
0.2
400
500
600
700
Wavelength (nm)
47
RGB to XYZ

X
Y
Z
RGB to XYZ is a linear transformation
=
0.490 0.310 0.200
0.177 0.813 0.011
0.000 0.010 0.990
R
G
B
48
CIE Chromaticity Diagram
0.9
520
X
530
540
550
510
y
505
Y
560
570
580
500
0.5
Z
590
600
610
650
495
490
X =x
X+Y+Z
Y =y
X+Y+Z
Z =z
X+Y+Z
x+y+z = 1
485
480
0.0
0.0
470
450
0.5
1.0x
49
Color Naming
0.9
520
530
540
550
510
y
505
green
560
yellow- 570
green
580
yellow
500
0.5
495
490 cyan
485
blue
480
purple
white
pink
590
orange 600
610
red
650
magenta
470
450
0.0
x
0.5
1.0
50
Blackbody Radiators and
CIE Standard Illuminants
CIE Standard Illuminants:
2500 - tungsten light (A)
4800 - Sunset
10K - blue sky
6500 - Average daylight (D65)
51
Chromaticity Defined in Polar Coordinates
Given a reference white.
0.8
Dominant Wavelength –
wavelength of the spectral
color which added to the 0.6
reference white, produces
the given color.
0.4
reference white
0.2
0
0
0.2
0.4
0.6
0.8
52
Chromaticity Defined in Polar Coordinates
Given a reference white.
Dominant Wavelength
0.8
0.6
Complementary
Wavelength - wavelength
of the spectral color which
added to the given color, 0.4
produces the reference
white.
reference white
0.2
0
0
0.2
0.4
0.6
0.8
53
Chromaticity Defined in Polar Coordinates
Given a reference white.
Dominant Wavelength
0.8
Complementary
Wavelength
0.6
0.4
Excitation Purity –
the ratio of the lengths
between the given color
and reference white and 0.2
between the dominant
wavelength light and
reference white.
0
Ranges between 0 .. 1.
purity
reference white
0
0.2
0.4
0.6
0.8
54
Device Color Gamut


We can use the CIE chromaticity diagram to
compare the gamut of various devices:
Note, for example,
that a color printer
cannot reproduce
all shades available
on a color monitor
55
Color Spaces





Linear (RGB, CMYK)
Artistic View (Munsell, HSV, HLS)
Standard (CIE-XYZ)
Perceptual (Luv, Lab)
Opponent (YIQ, YUV) – used in TV
56
Luminance v.s. Brightness
Brightness
(Lightness)
V in HSV
Equal intensity steps:
Equal brightness steps:
Luminance
Luminance
(intensity) vs
Y in XYZ
DI2
DI1
I1
I1 < I2, DI1 = DI2
57
I2
Weber’s Law
DI = constant
I
(I is intensity, DI is change in intensity)
Weber’s Law:
Perceived Brightness = log (I)
Perceived Brightness
In general, DI needed for just noticeable difference
(JND) over background I was found to satisfy:
Intensity
58
Munsell lines of constant Hue and Chroma
0.5
0.4
0.3
y
0.2
0.1
Value =1/
0
0
0.1
0.2
0.3
x
0.4
0.5
0.6
59
MacAdam Ellipses of JND
(Just Noticeable Difference
0.8
0.6
y
(Ellipses
scaled by 10)
0.4
0.2
0
0
0.2
0.4
x
0.6
60
Perceptual Color Spaces




An improvement over CIE-XYZ that represents better
uniform color spaces
The transformation from XYZ space to perceptual
space is Non Linear.
Two standard adopted by CIE are
L*u’v’ and L*a*b*
The L* line in both spaces is a replacement of the Y
lightness scale in the XYZ model, but it is more
indicative of the actual visual differences.
61
Munsell Lines and MacAdam Ellipses
plotted in CIE-L*u’v’ coordinates
100
Value =5/
100
50
v*
50
0
v*
-50
-50
-100
-100
-150
-150 -100
0
-150
-150 -100
-50
0
u*
50
100
150 200
-50
0
50
100
150 200
u*
62
Distance should be measured in
perceptual color spaces
63
Color Spaces





Linear (RGB, CMYK)
Artistic View (Munsell, HSV, HLS)
Standard (CIE-XYZ)
Perceptual (Luv, Lab)
Opponent (YIQ, YUV) – used in TV
64
Opponent Color Spaces
+
black-white
+
blue-yellow
-
+
red-green
65
YIQ Color Model


YIQ is the color model used for color TV in America
(NTSC= National Television Systems Committee)
Y is luminance, I & Q are color (I=red/green,Q=blue/yellow)




Note: Y is the same as CIE’s Y
Result: backwards compatibility with B/W TV!
Convert from RGB to YIQ:
0.11   R 
Y  0.30 0.59
 I   0.60  0.28  0.32 G 
  
 
Q  0.21  0.52 0.31   B 
The YIQ model exploits properties of our visual system, which
allows to assign different bandwidth for each of the primaries
(4 MHz to Y, 1.5 to I and 0.6 to Q)
66
YUV Color Model




YUV is the color model used for color TV in
Israel (PAL), and in video. Also called YCbCr.
Y is luminance as in YIQ.
U and V are blue and red (Cb and Cr).
The YUV uses the same benefits as YIQ,
(5.5 MHz for Y, 1.3 for U and V).

Converting from RGB to YUV:



Y = 0.299R + 0.587G + 0.114B
U = 0.492(B – Y)
V = 0.877(R – Y)
67
YUV - Example
Y
U
V
68
Summary

Light  Eye (Cones,Rods)  [l,m,s]  Color

Many 3D color models:




Reproducing Metamers to Colors
Different reproduction Gamut
More / Less intuitive
CIE standards
69