Page 194 - Intelligent Communication Systems

CHAPTER 12/COMPUTER VISION 165

FIGURE 12.10 Centerline extraction.

12.4.1.3 Line Thinning
Line thinning is used to establish the characteristics of a binary image. In more con-
crete terms, a contour line with a width of 1 pixel is obtained from a contour line
with a larger width by progressively eliminating the contour line's pixels, from the
outside in. Figure 12.10 shows the image of centerline extraction.

12.4.1.4 Digital Image
With a digital image, a point has a domain of 1 pixel and a line is a connection of
such points, in which case, the points are called connected. Digital images have
either 4-connected images or 8-connected images.

4-neighbors and 8-neighbors; Figure 12.11 shows eight pixels surrounding
point P 0 . These pixels are called the 8-neighbors. The four pixels just above
P 0, just below P 0 , just left of P 0, and just right of P 0 are the called 4-
neighbors. An image is connected upward if P 2 is 1 and connected
downward if P 6 is 1. When pixels from P l to P 8 are 0, Pj is called an isolation
point. When some of the pixels from PI to P 7 are 1, P 0 is a boundary point.
When all pixels from Pj to P 8 are 1, then P 0 is an internal point.
4-connected and 8-connected: When a pixel M is connected to a pixel N with
4-neighbors, M and N are said to be 4-connected. Figure 12.12 presents an
example of being 4-connected from M to N, where a^ a 2, a 3, and a 4 are the
four paths. All of the pixels that occupy the four paths are called the
components of the 4-connected. When M and P are 8-connected, M is
connected to P through pixels with 8-neighbors. In Figure 12.12, M and P
are 8-connected, where a\, a 2, a^, a 4, and a s are the eight paths.
Connectivity: The number of connected components that are separated from
each other is called the connectivity. In Figure 12.12, the connectivity is 1
in the case of the 8-connected and the connectivity is 3 in the case of the
4-connected.