Page 260 - Digital Analysis of Remotely Sensed Imagery

222 Cha pte r S i x

(a) (b)

(c) (d)
FIGURE 6.13 Effect of various convolutions within a 3 × 3 kernel on an image.
(a) Raw band; (b) low-pass fi ltered output; (c) high-pass fi ltered image; (d) median
fi ltered image.
consideration. In comparison with the raw image (Fig. 6.13a), the
smoothed image has reduced contrast and a blurred appearance (Fig.
6.13b). The degree of blurring is related to the kernel size. A large
kernel causes more smoothing than a small one. It also results in a
greater loss in high-frequency details.
It is possible to treat pixels in the kernel as having a differential
significance by assigning nonuniform weights to the elements. Below
are two examples of a 3 × 3 kernel with different weights:

⎛ 2 2 ⎞
⎛ 111⎞ ⎜ 1 ⎟
1 ⎜ 000 ⎟ 1 ⎜ 2 2 ⎟
6 ⎜ ⎟ and + ⎜ 1 2 1 ⎟
⎝ 111 ⎠ 62 2 ⎜ 2 ⎟
⎜ ⎝ 2 2 1 2 ⎟ ⎠