For each such region, a point on the boundary is selected and its coordinates are transmitted. The encoder then moves along the boundary of the region and, at each step, transmits a symbol representing the direction of this movement.
This continues until the encoder returns to the starting position, at which point the blob has been completely described, and encoding continues with the next blob in the image.
This encoding method is particularly effective for images consisting of a reasonably small number of large connected components.
Some popular chain codes include:
- the Freeman Chain Code of Eight Directions (FCCE)
- Directional Freeman Chain Code of Eight Directions (DFCCE)
- Vertex Chain Code (VCC)
- Three OrThogonal symbol chain code (3OT)
- Unsigned Manhattan Chain Code (UMCC)
Recently, the combination of move-to-front transform and adaptive run-length encoding accomplished efficient compression of the popular chain codes. Chain codes also can be used to obtain high levels of compression for image documents, outperforming standards like DjVu and JBIG2.
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