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Dennis Chileshe MWANSA Satoshi MIZUNO Makoto FUJIMURA Hideo KURODA
In DCT transform coding it is usually necessary to discard some of the ac coefficients obtained after the transform operation for data compression reasons. Although most of the energy is usually compacted in the few coefficients that are transmitted, there are many instances where the discarded coefficients contain significant information. The absence of these coefficients at the decoder can lead to visible degradation of the reconstructed image especially around slow moving objects. We propose a simple but effective method which uses an indirect form of vector quantization to supplement scalar quantization in the transform domain. The distribution pattern of coefficients that fall below a fixed threshold is vector quantized and an index of the pattern chosen from a codebook is transmitted together with two averages; one for the positive coefficients and the other for negative coefficients. In the reconstruction, the average values are used instead of setting the corresponding coefficients to zero. This is tantamount to quantizing the mid range frequency coefficients with 1 bit but the resulting bit-rate is much less. We aim to propose an alternative to using traditional vector quantization which entails computational complexities and large time and memory requirements.