efore bulky electronic image files can be transmitted over networks via phone or satellite lines, they must be compressed so they will fit. Compression technology, via hardware or software, enables images to be transmitted quickly and stored inexpensively on optical disks or magnetic tape. Files must then be decompressed before users can read them.
Compression condenses the number of bits in imaging files by eliminating empty data fields and redundant or unnecessary data. White space in documents, for instance, is saved as chunks instead of individual pixels. And repeated phrases are replaced with computer symbols known as tokens. Compression ratios range anywhere from 10-to-1 to 100-to-1, depending on the file type .
Two types of compression methodologies exist: lossy and loss-less. Lossy technology deletes certain image data during compression. This scheme is ideal for applications where small details can be lost-a few pixels in a non-mission-critical photo, for example. Several federal agencies recently have adopted the JPEG (Joint Photographic Experts Group) standard for lossy compression. This relatively new standard has helped bring down the cost of compression technology and has made it easier for users to exchange imaging files.
Loss-less compression techniques are used in applications where no details can be lost-such as fingerprint reproductions, reconnaissance photos or medical images. This scheme produces the highest compression ratios but also takes the most time. Photos from NASA space shuttle missions, for instance, can take days to compress.
Most loss-less compression technology, such as the wavelet scalar quantization algorithm, is proprietary-meaning it is not interoperable with other products. As part of its effort to move to commercial, off-the-shelf products, the Defense Department has adopted the MPEG-2 (Moving Pictures Experts Group) standard for digital audio and video compression. MPEG-2 produces high-resolution images and is significantly less expensive than customized solutions.
Firms such as Analytic Sciences, Compression Labs, Stac and TMS still search for new schemes that conform to standards. Aquidneck Systems, for example, has hardware that does real-time compression. And Iterated Systems offers fractal-image compression, which takes up 1/100 the storage space needed by other methods.