Image Processing Reference
In-Depth Information
⎡
R
G
B
⎤
⎡
1
.
000
0
.
000
1
.
140
⎤
⎡
Y
U
V
⎤
R
∈[
0
,
255
]
⎣
⎦
=
⎣
⎦
·
⎣
⎦
1
.
000
−
0
.
395
−
0
.
581
G
∈[
0
,
255
]
(3.31)
1
.
000
2
.
032
0
.
000
B
∈[
0
,
255
]
Another well know color space using this principle is the YC
b
C
r
color space,
which is used in for example JPEG and MPEG. YC
b
C
r
uses the weights:
W
R
=
0
.
299,
W
G
=
0
.
587,
W
B
=
0
.
114,
W
X
1
=
0
.
5, and
W
X
2
=
0
.
5, and has the conver-
sions listed below. See Appendix F for details.
⎡
⎤
⎡
⎤
⎡
⎤
⎡
⎤
Y
C
b
C
r
0
.
299
0
.
587
0
.
114
R
G
B
0
128
128
Y
∈[
0
,
255
]
⎣
⎦
=
⎣
⎦
·
⎣
⎦
+
⎣
⎦
−
0
.
169
−
0
.
331
0
.
500
C
b
∈[
0
,
255
]
0
.
500
−
0
.
419
−
0
.
081
C
r
∈[
0
,
255
]
(3.32)
⎡
⎤
⎡
⎤
⎡
⎤
R
G
B
1
.
000
0
.
000
1
.
403
Y
C
b
−
R
∈[
0
,
255
]
⎣
⎦
=
⎣
⎦
·
⎣
⎦
1
.
000
−
0
.
344
−
0
.
714
128
G
∈[
0
,
255
]
(3.33)
1
.
000
1
.
773
0
.
000
C
r
−
128
B
∈[
0
,
255
]
Note that 128 is added/subtracted in order to bring the values into the range
.
Note also the simplicity of the conversions compared to those for HSI and HSV. In
Table
3.3
the YC
b
C
r
values of some RGB values are shown.
[
0
,
255
]
3.4
Further Information
When reading literature on color spaces and color processing it is important to re-
alize that a number of different terms are used.
6
Unfortunately, some of these terms
are used interchangeably even though they might have different physical/perceptu-
al/technical meanings. We therefore give a guideline to some of the terms you are
likely to encounter when reading literature on colors:
Chromatic Color
All colors in the RGB color cube except those lying on the gray-
line spanned by
(
0
,
0
,
0
)
and
(
255
,
255
,
255
)
.
Achromatic Color
The colorless values in the RGB cube, i.e., all those colors lying
on the gray-line. The opposite of chromatic color.
Shades of gray
The same as achromatic color.
Intensity
The average amount of energy, i.e.,
(R
+
G
+
B)/
3.
Brightness
The amount of light perceived by a human.
Lightness
The amount of light perceived by a human.
Luminance
The amount of light perceived by a human. Note that when you ven-
ture into the science of color understanding, the luminance defines the amount of
emitted light.
Luma
Gamma-corrected luminance.
6
When going into color perception and color understanding even more terms are added to the
vocabulary.
