The three basic types of discontinuities in a digital image are point, line and edge.
- The detection of isolated point different from constant background image can be done using the following mask:
- 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.
- The various masks present for line detection are :
a) Horizontal mask
b) -45 degrees
c) 45 degrees
d) Vertical mask
- 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.
- 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.