Conditional DCT event coding without side information in video compression

Abstract

We address a limitation of video compression standards that affects their coding efficiency. The standard methods use one or two Huffman codes for all DCT run-length events. This implicitly assumes that the statistics of DCT run-length events are nearly stationary, but they are not. There is room for significant improvement in coding efficiency by the use of conditional Huffman codes. We propose to condition the Huffman codes for runlength events on the values of previous events in the block, the quantization parameter, and other information available to the decoder. We give a systematic way of designing optimal sets of Huffman codes, to give the maximum gain in coding efficiency for a given number of codes. We achieve significant improvements in coding efficiency over the standard methods with a reasonable number of codes, in low bit rate compression. Our method is non-adaptive and has little effect on the running time of the encoder or decoder.