A contour-based approach to binary shape coding using a multiple grid chain code

Abstract

This paper presents a contour-based approach to e$ciently code binary shape information in the context of object-based video coding. This approach meets some of the most important requirements identi"ed for the MPEG-4 standard, notably e$cient coding and low delay. The proposed methods support both object-based lossless and quasi-lossless coding modes. For the cases where low delay is a primary requirement, a macroblock-based coding mode is proposed which can take advantage of inter-frame coding to improve the coding e$ciency. The approach presented here relies on a grid di!erent from that used for the pixels to represent the shape } the hexagonal grid } which simpli"es the task of contour coding. Using this grid, an appraoch based on a di!erential chain code (DCC) is proposed for the lossless mode while, for the quasi-lossless case, an approach based on the multiple grid chain code (MGCC) principle is proposed. The MGCC combines both contour simpli"cation and contour prediction to reduce the number of bits needed to code the shapes. Results for alpha plane coding of MPEG-4 video test sequences are presented in order to illustrate the performance of the several modes of operation, and a comparison is made with the shape-coding tool chosen by MPEG-4.

Compact representation of contours using directional grid chain code

“…As representation approaches the lossless case, coding of the increased number of vertices shows a close similarity to the chain code. A variant of the DCC, called the multiple grid chain code (MGCC), was proposed by Nunes and Marques [21,22]. MGCC is based on a hexagonal grid and six-connected inter-pixel edge links, as shown in Fig.…”

Two-Dimensional Shape Coding

“…In [27], the performance of different quantization schemes is compared, whereas in [32], the rate-distortion characteristics of certain chain codes are studied. Some chain codes also include simplifications of the contour in order to increase coding efficiency [33,37]. This is similar to filtering the object shape with morphological filters and then coding with a chain code [35].…”

“…The publication of the MPEG-4 standard and its work on shape coding [3,26,33] inspired the invention of many shape coding algorithms that are able to outperform MPEG-4 shape coding in terms of coding efficiency while being more computationally demanding. Mainly due to rate distortion optimization, the vertex-based method described in [24] provides an average rate reduction of 7.8% with respect to the content-based arithmetic coder in MPEG-4.…”

Context based coding of binary shapes by object boundary straightness analysis

“…The skeleton-based method proposed in [15] gives bitrates 8 − 18% smaller than CAE. The differential chain code method proposed in [12] gives reductions on CAE up to 10%, but also increments up to 13%, for an average reduction of less than 1%. All these results where obtained using different MPEG-4 test sequences, at QCIF, CIF and SIF resolutions.…”