Image Processing Reference
In-Depth Information
8.2.3
Demosaicking
As only one filter color is formed at each pixel in a one-sensor color camera system, only
one piece of color information is obtained at each pixel. On the other hand, the human eye
and brain perceive a color by a set of R, G, and B, as mentioned in Section 6.4. Therefore,
all R, G, and B information is necessary at each pixel to construct a color image for appli-
cations for humans to watch. Demosaicking creates color signals that are not captured
originally at each pixel.
As shown in Figure 8.6, pixel signals from sensor output are separated by every color, and
color signals lacking at each pixel represented as R', G', or B' in Figure 8.6c are generated in
the demosaicking operation for color signal interpolation. Thus, every color signal at each
pixel is obtained. In other words, demosaicking creates a flawless color signal, as shown in
Figure 1.10a, from a color signal obtained by a single-chip color camera system, as shown
in Figure 1.10b.
Since many subtle aspects such as smoothness of boundary and color noise level are
determined by the architecture of the demosaicking process, each camera company exer-
cises their ingenuity to develop this. As a technique to create color signals that are lack-
ing, correlation relations between colors are often used such that red signal distribution
is assumed to be proportional to green distribution in low-frequency domains. But if this
is executed in a simple way, conspicuous color errors are likely to be generated at color
boundaries, so it is necessary to process adaptively by checking patterns in detail. As an
example, adaptively processed demosaicking is shown in Figure 8.7, and a definite effect is
seen in whether it is shaggy or not on the border of the color. Algorithms such as this are
another source of differentiation between companies.
R′
RR
R′
RR
R′
R′
R′
R′
R′
RR
R′
RR
R′
R′
R′
R′
R′
RR
R′
RR
R′
R′
R′
R′
G
RGR
G
G
GG
GG′ G
GG
GG
G′
G′
G′
B
GBG
G′
G′
G
RGR
G
G
GG
G′
GG
GG
G
G′
B
GBG
G′
G′
G
RGR
G
G
B
GBG
GG
G′
G′
G
B′
B′
B′
B′
B
B′
B
B′
B
B
B′
B′
B′
B′
B
B′
B
B′
B
B
B′
B′
B′
B′
B
B′
B
B′
B
B
(a)
(b)
(c)
(d)
FIGURE 8.6
Demosaicking (color interpolation): (a) color filter array and pixel signal; (b) separation by every color; (c) color
interpolation by color signal creation through demosaicking; (d) completion of RGB set at each pixel.
