Object description - Feature Extraction and Image Processing - page 248

an arrangement of pixels within the area and the arrangement of pixels in the perimeter or
boundary , respectively. This region versus perimeter kind of representation is common in
image analysis. For example, edges can be located by region growing (to label area) or by
differentiation (to label perimeter), as covered in Chapter 4. There are actually many
techniques that can be used to obtain descriptors of an object's boundary. Here, we shall
just concentrate on three forms of descriptors: chain codes and two forms based on Fourier
characterisation . For region descriptors we shall distinguish between basic descriptors and
statistical descriptors defined by moments.

7.2 Boundary descriptions

7.2.1 Boundary and region

A region usually describes contents (or interior points) which are surrounded by a boundary
(or perimeter) which is often called the region's contour . The form of the contour is
generally referred to as its shape . A point can be defined to be on the boundary (contour)
if it is part of the region and there is at least one pixel in its neighbourhood that is not part
of the region. The boundary itself is usually found by contour following: we first find one
point on the contour and then progress round the contour either in a clockwise direction,
or anti-clockwise, finding the nearest (or next) contour point.
In order to define the interior points in a region and the points in the boundary, we need
to consider neighbouring relationships between pixels. These relationships are described
by means of connectivity rules. There are two common ways of defining connectivity: 4-
way (or 4-neighbourhood) where only immediate neighbours are analysed for connectivity;
or 8-way (or 8-neighbourhood) where all the eight pixels surrounding a chosen pixel are
analysed for connectivity. These two types of connectivity are illustrated in Figure 7.1 . In
this figure, the pixel is shown in light grey and its neighbours in dark grey. In 4-way
connectivity, Figure 7.1 (a), a pixel has four neighbours in the directions north, east, south
and west, its immediate neighbours. The four extra neighbours in 8-way connectivity,
Figure 7.1 (b), are those in the directions north east, south east, south west and north west,
the points at the corners .

(a) 4-way connectivity (b) 8-way connectivity

Figure 7.1 Main types of connectivity analysis