Page 243 - Digital Analysis of Remotely Sensed Imagery

Image Enhancement 207

Solution
DN = 132 DN = 48 DN = 112
max min in
8
DN = (DN − DN ) × (2 − 1) / (DN − DN )
out in min max min
= (112 − 48) × 255 / (132 − 48) = 194
Note: The value must be rounded up or down to the nearest inte-
ger as DN must be an integer.
In practice, linear contrast stretching may be implemented either
nontruncated or truncated. Nontruncated linear stretching is called
full stretching, in which DN and DN are the actual values from the
max min
input image. In full stretching all information in the input image is
completely preserved in the output image (Fig. 6.3b). Full stretching
is limited in that a small degree of stretching is achievable, especially
when the histogram has long and skinny tails (Fig. 6.3a). In this case
the contrast of the same image can be stretched much more through
truncated stretching. In truncated linear stretching, also called satu-
rated linear stretching, DN and DN are special break points deter-
min max
mined from the image’s histogram (Fig. 6.3c). Abrupt changes in the
histogram are proper break points whose DN is taken as DN or DN .
min max

Frequency (a)

40 78 150
162
0 DN 255
Frequency (b)

0 DN 255
Frequency (c)

0 DN 255
FIGURE 6.3 Rearrangement of pixel DN enumerated at 8 bits after linear
contrast stretching. (a) Histogram of the raw band. The DN and DN are
min max
40 and 162, respectively; (b) histogram of the same band after
nontruncation linear stretching; (c) histogram of the same band after
truncated linear stretching. All those pixels with a DN below 78 and above
150 have been amalgamated to achieve a larger stretching ratio.