Chain code

A chain code is a lossless compression algorithm for monochrome images. The basic principle of chain codes is to separately encode each connected component, or "blob", in the image.

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.

In use

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 documents in images, outperforming standards like DjVu and JBIG2.

See also

• Image compression