A modification to the Huffman coding of JPEG´s baseline compression algorithm

Summary form only given. In a DCT block of a compressed image, the density of non-zero AC coefficients and their size decrease as the block is scanned in the zigzag order. This paper presents a minor modification to the Huffman coding phase of the JPEG baseline algorithm to exploit this redundancy for further compression. Our encoder divides each block into two roughly equal halves (bands) and codes each band using a separate code table. It moves the EOB (end-of-block) marker to the middle of the sequence of non-zero AC coefficients and then replaces it by one of two split markers, whose selection depends on whether the number of non-zero AC coefficients is even or odd. No marker is needed at the end of a block. The major difference between this approach and the spectral selection method of the JPEG progressive mode is that the band boundary is flexible in our approach (i.e., it can vary from block to block)