Chapter 3
In the following, we provide some color figures that could not be
reproduced in the paper version of the book.
1. Color Attributes
It was mentioned in section 3.3 that there are three color attributes:
brightness, hue, and saturation. Figure 3cd.1 demonstrates these three
attributes.
(a)
(b)
(c)
Figure 3cd.1. Example of perceptual attributes of color. a) different
brightness levels (dark to bright), b) different hues (red to violet), and
c) saturation (the dark blue at the left side is highly saturated whereas
the faded blue at the right side has low saturation).
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Multimedia Signals and Systems
2. Additive Color Mixing
It was mentioned in section 3.3.2 that a wide range of colors can be
obtained by mixing three primary colors R, G, and B. Figure 3cd.2 shows a
few composite colors by mixing the three primaries in equal amount. It is
observed that mixtures of {blue, red}, {blue, green} and {red, green}
produce magenta, cyan and yellow color, respectively. When all red, green
and blue are mixed in equal proportion, white color is obtained.
Red
Ligh
t
Blue
Light
Green
Light
Figure 3cd.2. Primary colors can be added to obtain different composite colors.
Chapter 3: Human Visual Systems and Perceptions
3
3. Subtractive Color Mixing
However, when paints of two different colors are mixed, we observe a
different composite color. Figure 3cd.3 shows an experiment of mixing
yellow and blue paint. It is observed that the composite color is green
instead of white as seen in Fig. 3cd.3(a). This phenomenon is explained in
Fig. 3cd.3(b). Yellow paint is seen yellow because it reflects lights of
wavelength in the range 500-600 nm, and absorbs all other wavelengths. On
the other hand, blue paint is blue because it reflects lights of wavelength in
the range 425-525 nm, and absorbs all other wavelengths. When yellow and
blue paints are mixed, only a narrow band of wavelengths (500-530 nm) are
reflected and all other wavelengths are absorbed. Hence, we see the green
color. We note that most display devices employ additive color mixing to
reproduce different colors while the printing industry employs subtractive
color mixing to reproduce colors. Fig. 3cd.3(c) shows some more colors
obtained by mixing different proportions of cyan, magenta and yellow dyes.
Absorbed by
blue pigments
Blue
Paint
Yellow
Paint
Reflectance
Absorbed by
yellow pigments
400
(a)
425
450
475 500 525 550
Wavelength (in nm)
575
(b)
(c)
Figure 3cd.3. Subtractive color mixing. a) mixture of yellow and blue
paint produces green color, b) composite color is the difference between
two added colors, c) mixture of cyan, magenta, and yellow colors.
600
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Multimedia Signals and Systems
4. Nonuniform Color Space
It was mentioned in section 3.3.5.3 that color spaces such as R, G, B
have a major disadvantage. The Euclidian distance between two color
points in these spaces may not corresponds to the perceptual distance
between the two colors. Figure 3cd.4 demonstrates this with a simple
example.
Figure 3cd.4(a) displays two concentric green circles. The color
corresponding to the inner circle is represented by r=0.2, g=0.6, b=0.2
(normalized values) in {R,G,B} color space. The corresponding pixel
values, with 8 bit representation, are r=51, g=153, b=51. The color
corresponding to the outer circle is represented by r=0.2, g=0.62, b=0.2 with
pixel values r=51, g=158, b=51. We note that the two colors have a
Euclidian distance of 0.2 (=0.62-0.6). The distance is very small, and hence
the colors are perceptually very similar, but can still be distinguished. On
the other hand, Fig. 3cd.4(b) displays two concentric blue circles. The color
corresponding to the inner circle is represented by r=0.2, g=0.2, b=0.6 in
{R,G,B} color space. The corresponding pixel values, with 8 bit
representation, are r=51, g=51, b=153. The color corresponding to the outer
circle is represented by r=0.2, g=0.2, b=0.62 with pixel values r=51, g=51,
b=158. We note that the two colors also have a Euclidian distance of 0.2
(=0.62-0.6). However, the colors are perceptually indistinguishable (the two
circles seem to have the same color).
(a)
(b)
Figure 3cd.4. Perceptual distance experiment using two concentric circles in
R,G,B color space. a) Inner circle: r=0.2, g=0.6, b=0.2; outer circle: r==0.2,
g=0.6, b=0.2, b) Inner circle: r=0.2, g=0.2, b=0.6; outer circle: r==0.2, g=0.2,
b=0.62. The values of r, g, and b are normalized (1 corresponds to a pixel value of
255). In both cases, the Euclidian distance is 0.02.