Entropy-constrained gradient-match vector quantization for image coding

Side-match VQ (SMVQ) is a well-known class of FSVQ used for low-bit rate image/video coding. It exploits the spatial correlation between the neighboring blocks to select several codewords that are very close to the encoding block from the master codebook. But if the block boundary is in the region edge area, the spatial correlations are not high and the SMVQ can't select proper codewords to encode blocks. An entropy-constrained gradient-match VQ (ECGMVQ) is proposed. Instead of exploiting the spatial correlation, the ECGMVQ uses the gradient contiguity property to select the codewords. The state function of ECGMVQ can select better codevectors than the SMVQ. In addition, the entropy-constrained rule is applied to the encoding process to reduce the bit rate. Simulation results show that the improvement of ECGMVQ over the SMVQ is up to 4-5 dB at nearly the same bit rate. Further, the perceptual image quality is better than that of SMVQ, especially in the region edge area.