Using multiple Huffman code tables for optimal coding of DCT blocks

Summary form only given. It is a well-known observation that when a DCT block is traversed in a zig-zag order, the AC coefficients generally decrease in size and the run-length of zero coefficients increase in number. Therefore, use of a single AC Huffman code table in the JPEG baseline algorithm leads to sub-optimal coding, and it is desirable to use multiple code tables, one for each DCT coefficient position, if necessary. It creates a problem, because a nonzero coefficient, X, and the run-length, Z, of zero coefficients that precede X, are coded as one element (Z,X), and therefore, the decoder may not know which table to use to decode the next X. To solve this problem, we made a minor modification to the JPEG Huffman coding algorithm. To evaluate reduction in the code size using our method, we compressed the luminancec component of ten well-known test images at default quality level and computed AC Huffman code size.