Page 127 - Digital Analysis of Remotely Sensed Imagery

98 Cha pte r T h ree

codes can be coded using variable-length encoding to further reduce
data volume.
Run-length encoding can be extended to two-dimensional (2D)
binary images using various coding procedures, one of which is
known as relative-address coding. In this coding the binary transitions
that begin and end each pixel are tracked. The simplest way of
achieving this is to note the beginning and end positions of a nonvoid
pixel in a row by row fashion. The binary image shown in Fig. 3.8 is
coded as below:
Row 4 4, 6
Row 5 2, 6
Row 6 1, 8
Row 7 1, 10
Row 8 3, 12
Row 9 3, 12
Row 10 5, 8
Row 11 5, 6
Row 12 5, 6

As shown in the above diagram, a large compression ratio is
achieved if the binary image extends far in a row. Instead of recoding
all the pixels, only a pair of coordinates is needed for the presentation
of each row of image. This compression ratio may be further improved
by combining different coding methods (e.g., Huffman and run-length)
in one compression.

1 2 3 4 5 6 7 8 9 101112
1
2
3
4
5
6
7
8
9
10
11
12

FIGURE 3.8 Two-dimensional run-length coding is a useful way of reducing
remote sensing data volume, even though its most common application is in
compressing fax fi les.