Page 246 - Digital Analysis of Remotely Sensed Imagery

210 Cha pte r S i x

f
DN
out

DN
in
FIGURE 6.5 The relationship between a pixel value in the input band and
the output band in a piecewise linear contrast stretching. The transformation
function is made up of a few line segments, each having its own slope.

Temperature
(degrees Celsius)
15
37
16
Cold Plume
17 DN Blue Green Red
15 120 0 136
18
38 16 256 0 0
17 210 46 0
19
18 180 76 0
20 19 90 166 0
20 0 256 0
39S
21
21 0 125 131
22 0 0 256
22
177 178 179 180E
FIGURE 6.6 An example of visualizing sea surface temperature using a look-up
table. (Source: Modifi ed from Gao and Lythe, 1996.) See also color insert.

6.1.5 Nonlinear Stretching
The function f in Eq. (6.1) can be nonlinear. Similar to piecewise linear
stretching, nonlinear stretching allows some part of the input image to
have a stretched contrast while contrast in some other DN ranges is
suppressed in the output image. Unlike linear contrast stretching in
which the contrast of an image is either enlarged or reduced, both
contrast stretching and contrast compression can be achieved in one
nonlinear stretching. Whether the contrast is stretched or suppressed
depends on the input DN value and the nonlinear function (Fig. 6.7).