Transcript Color Imaging © 2002-2003 by Yu Hen Hu ECE533 Digital Image Processing.

Color Imaging
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
1
Describing Chromatic lights

Radiance (watt):
» Total amount of energy flow from the light source.

Luminance (lumens, lm):
» measure of amount of energy an observer
perceives from a light source. It varies based on
distance from the source, wavelength, etc.

Brightness:
» a subjective descriptor, describing color sensation.
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
2
Primary Colors

Primary colors of
light (additive):
» Red (700 nm), 65%
cones sensitive to
red light.
» Green (546.1nm),
33%
» Blue(435.8nm). 2%
cones sensitive to
blue light.

Mixing of R,G,B may
NOT generate ALL
visible colors.
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
3
Primary and Secondary Colors of
Lights and Pigments

Primary colors of
pigment
(subtractive):
» magenta,
» cyan, and
» yellow.
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
4
Characterization of Color


Colors are distinguished from one another based on
brightness, hue, and saturation.
Hue:
» an attribute associated with the dominant wavelength in a
mixture of light waves. It represents the dominant color as
perceived by an observer.

Saturation:
» specifies relative purity or the amount of white lights mixed
with a hue.

Hue and saturation together are called chromaticity.
Example: color palette
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
5
Chromaticity Diagram

Tri-chromatic coefficients:
» Let X, Y, Z: tri-stimulus values
representing the amounts of red,
green, and blue needed to form any
particular color.
X
Y
,y
,
X Y  Z
X Y  Z
Z
z
X Y  Z
x
» Since x + y + z = 1, x and y along
will make a chromaticity diagram

CIE Chromaticity diagram
» x-axis: red, y-axis: green
» Color on boundary are completely
saturated.
» Saturation @ pts of equal energy is
zero
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
6
Color Gamut

Any 3 points in the
chromaticity diagram
can produce all
colors within that
triangle. Due to the
tongue-shape
indicates that no
mixing of three
primary color can
produce ALL
possible colors
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
7
Color Models
RGB color model: monitor, video
 CMY (CMYK) color model: printing
 HIS: close to HVS

© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
8
RGB Color Model




R, G, B at 3 axis ranging
in [0 1] each
Gray scale along the
diagonal
If each component is
quantized into 256 levels
[0:255], the total number
of different colors that
can be produced is (28)3
= 224 = 16,777,216
colors.
RGB safe color:
» Quantize each
components into 6
levels from 0 to 255.
© 2002-2003 by Yu Hen Hu
24-bit RGB color cube
ECE533 Digital Image Processing
RGB safe color cube
9
HSI Color Model

Hue:
» an attribute describing
pure color

Saturation:
» The degree of which
a pure color is diluted
by white light.

HSI model
» Hue and saturation lie
in a plane
perpendicular to an
intensity axis.
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
10
Color Coordinate Transform

RGB  CYM
 C  1  R 
 Y   1  G 
    
 M  1  B 

HSI  RGB
0  H  120o
B  I (1  S )

S  cos H 
R  I  1 

o
cos(60

H
)


G  1  ( R  B)

RGB  HSI
  ( R  G )  ( R  B)  / 2 
  cos 

2
 ( R  G )  ( R  B)(G  B ) 
BG
 
H 
360   B  G
3  min( R, G, B)
S  1
RG  B
I   R  G  B / 3
1
» Others see text book
© 2002-2003 by Yu Hen Hu
ECE533 Digital Image Processing
11