For the past few years, the Joint Photographic Experts Group (JPEG) has undertaken the task of developing the first international general-purpose digital image compression standard for continuous-tone (multilevel) still images, both grayscale and color. This paper gives an overview of JPEG’s proposed image compression standard.

Both individual continuous-tone image applications (photovideotex, desktop publishing, graphic arts, color facsimile, newspaper wirephoto transmission, medical imaging, and so on) and the exchange of images across application boundaries require a compression standard in order to develop significantly beyond their present state.

JPEG’s proposed ISO standard specifies a comprehensive toolkit (containing many codecs for four modes of operation: sequential, progressive, lossless, and hierarchical encoding) that can span a wide range of continuous-tone image applications. Codecs differ according to the precision of the source image samples they can handle or the entropy coding method they use.

First, Wallace describes the key processing steps of both the 8×8 DCT-based and predictive lossless codecs for single-component source images. He then discusses multiple-component control on images containing from 2 to 255 components (spectral bands, channels, or colors). Brief presentations of the baseline and other DCT sequential codecs, the DCT progressive codecs, and the hierarchical mode of encoding follow. The interchange format syntax specified by the JPEG proposal to ensure that a compressed image can be exchanged between different application environments is only mentioned.

Specialists involved in digital imaging applications should read this paper to get acquainted with the architecture and basic processing of the JPEG proposal. The ISO draft standard should be used for implementation purposes, however.