Chain code is a lossless compression technique that represents the coordinates of a continuous object boundary in an image as a string of numbers. Each number represents the direction of the next point along the connected line segment. Chain codes work best for binary images, representing them as a connected sequence of straight line segments based on 4 or 8-connectivity. The chain code provides a concise representation of a shape contour by describing each edge as a sequence of direction codes from its starting point.

1. Chain Code in DIP
Rishav Bhurtel 19MTRDS008

2. • Chain code is a lossless compression technique used for
representing an object in images. The co-ordinates of any
continuous boundary of an object can be represented as a string
of numbers where each number represents a particular direction
in which the next point on the connected line is present.
• Chain codes are used to represent the binary by a connected
sequence of straight –line segments. This represented is based
on 4-connectivity and 8-connectivity of the segments.
• The chain code works best with binary images and is a concise
way of representing a shape contour.
What is Chain Code?

3. The chain code direction convention is given
below:

4. • As an edge is traced from its beginning point to the end point
the direction that must be taken to move from one pixel to the
next is given by the number represented in either the 4-chain
code or the 8-chain code.
• As an edge can be completely described in terms of its starting
coordinate and its sequence of chain codes descriptors. Of the
two chain codes, the 4-chain is easier requiring only four
different code values.

5. For e.g.: Given an image, for 4-chain code for
the same.

6. • Let us take a starting point which is arbitrary chosen (p). From
this point on, the cursor follows the contour, usually in the
clockwise direction. For every pixel the direction is noted.

8. The 4-chain code is given edge is
0 0 3 0 3 3 2 2 2 1 1 1

9. •Hence instead of storing the coordinates of each pixel, we can store the chain code and
starting coordinates. Remember 8-connectivity is closer to the human eye than 4-connectivity.
•The chain code allows an edge to be described using vertical, horizontal as well as diagonal
descriptors at 45 0 , 135 0 , 225 0 and 315 0 .
•The problem with above chain code is, if we change starting point we will get different chain
code, even if the object is same.
•Therefore the chain code is normalized with respect to the starting point.
•We treat the chain code as a circular sequence of numbers and redefine the starting point so
that the resulting sequence of numbers, forms an integer of minimum magnitude.
•Circular codes can be also normalized for rotation by taking the first difference of chain code.

10. First Difference is:
• Circular difference = last – first = 0 – 0 = 0
• Therefore circular chain code =
• 0 0 0 3 0 3 3 2 2 2 1 1 1
• Shape Number: Shape no of a boundary obtained from a chain
code is defined as the smallest magnitude of the circular first
difference.

11. Chain-code example. a) coding the 8 directions; b) initial point and edge following
For 8-Chain code the process is same as 4 but
we will have extra directions for diagonal pixels.

12. Thank you!