9. Explain about the basic relationships and distance measures between pixels in a digital image.

Tuesday, 30 April 2013

9. Explain about the basic relationships and distance measures between pixels in a digital image.



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Neighbors of a Pixel:

A pixel p at coordinates (x, y) has four horizontal and vertical neighbors whose coordinates are
given by (x+1, y), (x-1, y), (x, y+1), (x, y-1). This set of pixels, called the 4-neighbors of p, is
denoted by N4 (p). Each pixel is a unit distance from (x, y), and some of the neighbors of p lie
outside the digital image if (x, y) is on the border of the image.

The four diagonal neighbors of p have coordinates (x+1, y+1), (x+1, y-1), (x-1, y+1), (x-1, y-1)
and are denoted by ND (p). These points, together with the 4-neighbors, are called the 8-
neighbors of p, denoted by N8 (p). As before, some of the points in ND (p) and N8 (p) fall outside
the image if (x, y) is on the border of the image.

Connectivity:

Connectivity between pixels is a fundamental concept that simplifies the definition of numerous
digital image concepts, such as regions and boundaries. To establish if two pixels are connected,
it must be determined if they are neighbors and if their gray levels satisfy a specified criterion of
similarity (say, if their gray levels are equal). For instance, in a binary image with values 0 and 1,
two pixels may be 4-neighbors, but they are said to be connected only if they have the same
value.

Let V be the set of gray-level values used to define adjacency. In a binary image, V={1} if we
are referring to adjacency of pixels with value 1. In a grayscale image, the idea is the same, but
set V typically contains more elements. For example, in the adjacency of pixels with a range of
possible gray-level values 0 to 255, set V could be any subset of these 256 values. We consider
three types of adjacency:

(a) 4-adjacency. Two pixels p and q with values from V are 4-adjacent if q is in the set N4 (p).

(b) 8-adjacency. Two pixels p and q with values from V are 8-adjacent if q is in the set N8 (p).

(c) m-adjacency (mixed adjacency).Two pixels p and q with values from V are m-adjacent if

(i) q is in N4 (p), or

(ii) q is in ND (p) and the set has no pixels whose values are from V.

Mixed adjacency is a modification of 8-adjacency. It is introduced to eliminate the ambiguities
that often arise when 8-adjacency is used. For example, consider the pixel arrangement shown in
Fig.9 (a) for V= {1}.The three pixels at the top of Fig.9 (b) show multiple (ambiguous) 8-
adjacency, as indicated by the dashed lines. This ambiguity is removed by using m-adjacency, as
shown in Fig. 9 (c).Two image subsets S1 and S2 are adjacent if some pixel in S1 is adjacent to
some pixel in S2. It is understood here and in the following definitions that adjacent means 4-, 8-
, or m-adjacent. A (digital) path (or curve) from pixel p with coordinates (x, y) to pixel q with
coordinates (s, t) is a sequence of distinct pixels with coordinates
                     (x0,y0),(x1,y1),.............................................................,(xn,yn)
where (x0,y0)=(x,y) and (xn,yn)=(s,t) and pixels (xiyi) and  (xi-1,yi-1) are adjacent for 1<=i<=n.
In this case, n is the length of the path. If (x0, y0) = (xn, yn) , the path is a closed path. We can define 4-, 8-, or m-paths depending on the type of adjacency specified. For example, the paths shown in Fig. 9 (b) between the northeast and southeast points are 8-paths, and the path in Fig. 9 (c) is an m-path. Note the absence of ambiguity in the m-path. Let S represent a subset of pixels in an image. Two pixels p and q are said to be connected in S if there exists a path between them consisting entirely of pixels in S. For any pixel p in S, the set of pixels that are connected to it in S is called a connected component of S. If it only has one connected component, then set S is called a connected set.

Let R be a subset of pixels in an image. We call R a region of the image if R is a connected set.
The boundary (also called border or contour) of a region R is the set of pixels in the region that
have one or more neighbors that are not in R. If R happens to be an entire image (which we
recall is a rectangular set of pixels), then its boundary is defined as the set of pixels in the first
and last rows and columns of the image. This extra definition is required because an image has
no neighbors beyond its border. Normally, when we refer to a region, we are referring to a subset
Fig.9 (a) Arrangement of pixels; (b) pixels that are 8-adjacent (shown dashed) to the center
pixel; (c) m-adjacency

of an image, and any pixels in the boundary of the region that happen to coincide with the border
of the image are included implicitly as part of the region boundary.

Distance Measures:

For pixels p, q, and z, with coordinates (x, y), (s, t), and (v, w), respectively, D is a distance
function or metric if

 The Euclidean distance between p and q is defined as
 For this distance measure, the pixels having a distance less than or equal to some value r from(x,
y) are the points contained in a disk of radius r centered at (x, y).

 The D4 distance (also called city-block distance) between p and q is defined as
 In this case, the pixels having a D4 distance from (x, y) less than or equal to some value r form a
diamond centered at (x, y). For example, the pixels with D4 distance . 2 from (x, y) (the center
point) form the following contours of constant distance:

The pixels with D4 =1 are the 4-neighbors of (x, y).

 The D8 distance (also called chessboard distance) between p and q is defined as
 In this case, the pixels with D8 distance from(x, y) less than or equal to some value r form a
square centered at (x, y). For example, the pixels with D8 distance ≤ 2 from(x, y) (the center
point) form the following contours of constant distance:

 The pixels with D8=1 are the 8-neighbors of (x, y). Note that the D4 and D8 distances between p
and q are independent of any paths that might exist between the points because these distances
involve only the coordinates of the points. If we elect to consider m-adjacency, however, the Dm
distance between two points is defined as the shortest m-path between the points. In this case, the
distance between two pixels will depend on the values of the pixels along the path, as well as the
values of their neighbors. For instance, consider the following arrangement of pixels and assume
that p, p2 , and p4 have value 1 and that p1 and p3 can have a value of 0 or 1:
 Suppose that we consider adjacency of pixels valued 1 (i.e. = {1}). If p1 and p3 are 0, the length
of the shortest m-path (the Dm distance) between p and p4 is 2. If p1 is 1, then p2 and p will no
longer be m-adjacent (see the definition of m-adjacency) and the length of the shortest m-path
becomes 3 (the path goes through the points pp1p2p4). Similar comments apply if p3 is 1 (and p1
is 0); in this case, the length of the shortest m-path also is 3. Finally, if both p1 and p3 are 1 the
length of the shortest m-path between p and p4 is 4. In this case, the path goes through the
sequence of points pp1p2p3p4.

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