Index compression for vector quantisation using modified coding tree assignment scheme

Recently, a lossless vector quantisation (VQ) index compression algorithm, called the coding tree assignment scheme with improved search-order coding (CTAS-ISOC) algorithm, has been proposed to enhance the coding efficiency of the original SOC by exploiting the correlations of the neighbouring blocks using the left-pair and upper-pair patterns in the index domain. This study proposes a modified coding tree assignment scheme (MCTAS) to further improve the coding performance of CTAS-ISOC by the dynamic index table coding (DITC). The DITC technique exploits the correlation of neighbouring index pairs not in the original vector-quantised index map, but in the temporarily constructed index table that has been classified and updated for each current index. The searching matched index in a previously qualified index table results in low-time complexity. In addition, the identical index table can be regenerated in the reconstruction process of the index map at the decoder end. Experimental results show the time complexity of MCTAS is more efficient than that of CTAS-ISOC. Moreover, the proposed MCTAS apparently reduces the bit rate in comparison with conventional VQ and some popular lossless index coding schemes, such as index searching algorithm with index associated list, adaptive index coding scheme, SOC and so on.