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.
A new technique is described for color conversions of JPEG images. For each input block of each
component, the conversion for the 63 AC coefficients is processed in the transform domain instead of the
spatial domain. Only the DC coefficients for each input block of the color components are transformed to
the spatial domain and then processed through the traditional lookup table to create color-converted output
DC coefficients for each block. Given each converted DC value for each block, the remaining 63 AC
coefficients are then converted directly in the transform domain via scaling functions that are accessed via a
table as a function of only the DC term. For n-dimensional color space to m-dimensional color space
conversion, n component blocks create m component blocks. An IDCT can then be applied to the m
component blocks to create spatial domain data or these output blocks can be quantized and entropy
encoded to create JPEG compressed data in the m-dimensional color space.
A high speed multilevel color printer required custom halftoning hardware. For this multitone environment we revised traditional halftone thresholding (i.e. turning the output from no intensity to full intensity). Unfortunately, intermediate values did not print large areas reliably. Legacy image data files existed that were already halftoned. To correct these problems, binary halftone methods were modified to produce multi-bit outputs. This was accomplished by using threshold matrices to determine when to allow printing. The input minus the threshhold value was used to index into a lookup table to select the output intensity. Design of the down-loadable threshold matrices solved the print consistency problem. The custom hardware ensured that a zero input value did not print and a maximum value printed as a saturated output. These solutions were implemented using custom high-speed logic capable of outputting 66 MegaPels/sec.
An international team of technical experts, the Joint Photographic Experts Group (JPEG), has been working for the past four years on a color image data compression standard for use worldwide in a variety of still image applications. This joint committee operated under ISO/IEC JTCl/SC2/Working Group 8 (Coded Representation of Picture and Audio Information) until it recently moved to a new working group, ISO/IEC JTC1/SC2/WG10 (Photographic Image Coding). The work is being done jointly with a special rapporteur group on image communication within CCITT Study Group VIII.<p> </p> Within the United States, work in support of this international activity is focused within the U.S. picture coding committee, Accredited Standards Committee X3 Technical Committee X3L3 (formerly Task Group X3L2.8). Other standards applications groups are eagerly awaiting the completion of JPEG in order to effectively incorporate color images into their application environments.<p> </p> JPEG's intensive technical research and refinement stages have been completed and an International Standards Organization (ISO) Committee Draft for the evolving JPEG data compression standard has been submitted for international balloting. Extensive testing of the applicability to various environments is in progress. Companies are beginning to announce JPEG software or hardware availability, even though they recognize that the JPEG algorithm may change.<p> </p> This paper will provide a critical review of the evolving JPEG color image data compression standard. A history of JPEG is given first, followed by the steps still needed for adoption of JPEG as an international image data compression standard. Then the JPEG "toolkit" is explored with additional detail on the entropy coders and data interleave conventions. In the results section, the compression performance numbers on the JPEG test images are given for the baseline Huffman sequential coding system both with and without resynchronization. The arithmetic coding results are given for the sequential DCT-based mode with and without resynchronization, several progressive DCT-based processes and the sequential lossless mode. The final section briefly notes some applications and implementations of JPEG.