ITU-T T.851: an enhanced entropy coding design for JPEG baseline images

Abstract

The Joint Photographic Experts Group (JPEG) baseline standard remains a popular and pervasive standard for continuous tone, still image coding. The J in JPEG acknowledges its two main parent organizations, ISO (International Organization for Standardization) and the ITU-T (International Telecommunications Union – Telecommunication). Notwithstanding their joint efforts, both groups have subsequently (and separately) standardized many improvements for still image coding. Recently, the ITU-T Study Group 16 completed the standardization for a new entropy coder - called the Q15-coder, whose statistical model is from the original JPEG-1 standard. This new standard, ITU-T Rec. T.851, can be used in lieu of the traditional Huffman (a form of variable length coding) entropy coder, and complements the QM arithmetic coder, both originally standardized in JPEG as ITU-T T.81 | ISO/IEC 10918:1. In contrast to Huffman entropy coding, arithmetic coding makes no assumptions about an image's statistics, but rather responds in real time. This paper will present a tutorial on arithmetic coding, provide a history of arithmetic coding in JPEG, share the motivation for T.851, outline its changes, and provide comparison results with both the baseline Huffman and the original QM-coder entropy coders. It will conclude with suggestions for future work.