Lauren Barton Martin Falck Nelson Kile, Jr. Carolyn O'Hare Robert Ryan Article Summary of The JPEG Still Picture Compression Standard-G. Wallace The sheer amounts of data necessary to represent a digital picture requires the use of compression technology to make the data stream manageable. Compress- ion rations of 1/10-1/50 are needed. The Joint Photographic Experts Group standard is working to develop the first international digital image compression standard. Their goal is one general purpose standard to meet the needs of almost all continuous-ton still-image applications. JPEG's standard must: a. have highest technical compression ratio b. be applicaable to all continuous-tne images c. have tractable computational complexity d. sequential encoding e. progressive encoding f. lossless encoding g. hierarchical encoding Process steps for DCT-Based Coding The concept is to compress a stream of 8x8 blocks of grayscale image samples. Each 8x8 block is input, makes its way through each processing step and yields output in compressed form into the data stream. This is inputed into the Forward DCT and the output from the decoder is the Inverse DCT. The FDCT takes the signal and decomposes it into the 64 unique 2-D spatial frequencies which comprise the input signal's spectrum. The DCT coefficients with freq= 0 in both dimensions is called the DC coefficients and the other 63 are called the AC coefficients. These two coefficients are summed together in a later step to reconstruct the image. The DC coefficients are expressed as a delta from the previous coefficients. All of the quantized coefficients are ordered into the zig-zag sequence to facilitate entropy coding by placing low-frequency coefficients in front of high-frequency coefficients. The JPEG proposal calls for using wither Huffman codig or arithmetic coding t o encode the quantized DCT coefficients as compactly as possible. JPEG uses a predictive method which is wholly independent of the DCT processing method. Although the JPEG standard is not a panacea. IF images use incompatible color spaces,for example, the JPEG will not make them interchangeable. The author hopes that JPEG will fade into common usage and become just another data type. ============================================================= MM Article Summaries by Group I: Fitzgerald, Kalafut, Klein, and Muhlenburg. "The JPEG Still Picture Compression Standard" by Gregory K. Wallace While digital images are seldom used in computer applications, a compression standard is definitely needed, like the facsimile standard which is only bi-level. The Joint Photographic Experts Group, or JPEG for short, is trying to develop a multi-level, continuous-tone general purpose image compression standard. This article presents that attempt. Their image-compression standard should: a) be near or at the state of the art with respect to compression rates and image quality with trade-offs between the 2 parameterizable; b) be applicable to any kind of continuous-tone digital source image; c) have tractable computerized complexity; and d) have the following encoding modes: sequential, progressive, lossless, and hierarchical. In January 1988, JPEG selected a Discrete Cosine Transform (DCT) based method. The architecture of the proposed standard included each of the 4 operating modes having 1 or more distinct codecs. A simple but comprehensive "toolkit" was also involved, and the Baseline sequential codec will handle most basic applications. DCT-based coding involved the Forward DCT (FDCT), which was a harmonic analyzer, and the Inverse DCT (IDCT), or the harmonic synthesizer. Picture quality levels varied over different ranges of compression. JPEG chose a simple predictive method for its predictive lossless coding. As well as handling a variety of source image formats, JPEG was concerned with multiple-component images, which were implemented with interleaved "data units". Baseline and other DCT sequential codecs used intermediate entropy coding representatives which were comprised of 2 symbols combining run length, size, and amplitude. JPEG also included an interchange format syntax in their proposal. Part 1 of JPEG's standardization schedule includes the 4 modes of operating, their respective codecs, the interchange format syntax, and implementation guidelines. Part 2 will contain compliance tests for the implementations of the encoders and decoders. In conclusion, JPEG's standard will not solve every digital image concern in computing, only the compression issues.