Discuss different discontinuities in image?
1 Answer

The three basic types of discontinuities in a digital image are point, line and edge.

Point Detection:

  • The detection of isolated point different from constant background image can be done using the following mask:
-1 -1 -1
-1 8 -1
-1 -1 -1
  • A point has been detected at the location on which the mask is centered if |R|>T where T is non negative threshold and.
  • The idea is that the gray level of an isolated point will be quite different from the gray levels of its neighbors.
  • The mask operation measures the weighted differences between the centre point and its neighbors. The differences that are large enough are determined by T considered isolated points in the image of interest.

Line Detection:

  • The various masks present for line detection are :

a) Horizontal mask

-1 -1 -1
2 2 2
-1 -1 -1

b) -45 degrees

-1 -1 2
-1 2 -1
2 -1 -1

c) 45 degrees

2 -1 -1
-1 2 -1
-1 -1 2

d) Vertical mask

-1 2 -1
-1 2 -1
-1 2 -1
  • If the horizontal mask is moved around an image it would respond more strongly to lines oriented horizontally.
  • With constant background the maximum response would result when the line is passing through the middle row of the mask.

Edge Detection:

  • Edges characterize object boundaries are therefore useful for segmentation and identification of objects in scene.
  • Edge point can be thought of as pixel location of abrupt gray levels. It is the boundary between two regions with relatively distinct gray level properties.
  • There are two types of edges step and ramp edge.
  • The step edges are detected using first order derivative filters like Robert, Sobel, Frichen and Prewit.
  • The ramp edges can be detected using second order derivative line Laplacian filter.
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