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2.3 The digital camera 71

1.0 1.0
0.8 0.8
0.6 0.6

0.4 0.4
0.2 0.2
0.0 0.0
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
(a) (b)
1.0 1.0

0.8 0.8
0.6 0.6
0.4 0.4
0.2 0.2
0.0 0.0
-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
(c) (d)

Figure 2.26 Sample point spread functions (PSF): The diameter of the blur disc (blue) in (a) is equal to half the
pixel spacing, while the diameter in (c) is twice the pixel spacing. The horizontal fill factor of the sensing chip
is 80% and is shown in brown. The convolution of these two kernels gives the point spread function, shown in
green. The Fourier response of the PSF (the MTF) is plotted in (b) and (d). The area above the Nyquist frequency
where aliasing occurs is shown in red.

ious image processing operations, such as resampling, upsampling, and downsampling. Sec-
tions 3.4 and 3.5.2 discuss these issues and show how careful selection of filters can reduce
the amount of aliasing that operations inject.

2.3.2 Color
In Section 2.2, we saw how lighting and surface reflections are functions of wavelength.
When the incoming light hits the imaging sensor, light from different parts of the spectrum is
somehow integrated into the discrete red, green, and blue (RGB) color values that we see in
a digital image. How does this process work and how can we analyze and manipulate color
values?
You probably recall from your childhood days the magical process of mixing paint colors
to obtain new ones. You may recall that blue+yellow makes green, red+blue makes purple,
and red+green makes brown. If you revisited this topic at a later age, you may have learned
that the proper subtractive primaries are actually cyan (a light blue-green), magenta (pink),
and yellow (Figure 2.27b), although black is also often used in four-color printing (CMYK).
(If you ever subsequently took any painting classes, you learned that colors can have even
more fanciful names, such as alizarin crimson, cerulean blue, and chartreuse.) The subtractive
colors are called subtractive because pigments in the paint absorb certain wavelengths in the
color spectrum.