Digital Image Processing, Computer Science

Boundary Representation and Description

1. Digital Image Processing BOUNDARY REPRESENTATION AND DESCRIPTION SUBMITTED BY: R. GIRISAKTHI II MSC(CS) NADAR SARASWATHI COLLEGE OF ARTS AND SCIENCE

2. BOUNDARY REPRESENTATION Models are a more explicit representation that CSG, the object is represented by a complicated data structure giving information about each of the object’s faces, edges and vertices and how they are joined together. Appears to be a more natural representation for vision since surface information is readily available. The description of the object can be into two parts.

3. Topology: It records the connectivity of the faces, edges and vertices by means of pointers in the data structure. Geometry: It describes the exact shape and position of each of the edges, faces and vertices, the geometry of a vertex is just its position in space as given by its( x,y,z ) coordinates, edges may be straight lines, circular arcs, etc. A face is represented by some description of its surfaces(algebraic or parametric forms used).

4. CHAIN CODES They are used to represent a boundary by a connected sequence of straight line segments of specified length and direction. Typically this representation is based on 4 – or 8 connectivity of segments. The direction of each segments is coded by using a numbering scheme.

5. Diagram for chain code:

6. A boundary code formed as a sequence of such directional number is referred as a freeman chain code. Digital images are acquired & processed in a grid format with equal spacing in X and Y directions. So a chain code can be generated by following a boundary and assigning a direction to the segments connections every pair of pixels.

7. Unacceptable Method Resulting chain tends to be quite long. Any small disturbances along the boundary due to noise or imperfect segmentation caan changes in code. A solution to this problem is to resample the boundary by selecting a larger grid spacing Then, as the boundary is traversed, a boundary point is assigned to each node of the large grid, depending upon the proximity of original boundary to that node. The re-sampled boundary can now be represented by a 4 – or 8 code.

8. The accurancy of the resulting code representation depends on the spacing of the sampling grid. The chain code of a boundary depends upon the starting point. However the code can be normalized: Assume the chain is a circular sequence(given a chain of 1 to N codes; N+1=I). Redefine the starting point such that we generate an integer of smallest magnitude.

9. Chain code depends on orientation a rotation results in a different chain code one solution. Use the “first d ifference”of the chain code instead of the code itself. The difference is obtained by simply counting the number of directions that separate two adjacent elements.

10. THANK YOU!!!