Compact representation of contours using directional grid chain code

Abstract

An efficient contour-based method for the coding of binary shape information is described. Conventional chain coding techniques show high coding efficiency for lossless compression, but they exploit the coherence of the contour in only a restricted manner. Higher coding efficiency can be achieved by realising the neighbourhood relation as a Markov chain, and this is exploited in a new coding scheme, the directional grid chain coding (DGCC). The method is computationally efficient and the coding process adapts to the inherent changes in the contour. Two schemes are proposed, a lossless and a quasi-lossless method. The lossless scheme achieves 32% saving in bit rate compared with the conventional differential chain code (DCC). The second, quasi-lossless technique achieves 44% bit reduction compared with the DCC and the distortions present in the reconstructed contour are hardly noticeable to the human eye.