THEORY AND PRACTICE

Where the Compression Is

by Lowell Rapaport

With the coming wave of color-capable production scanners, users will have more options when it comes to compressing images. As always, users can rely on accelerated scanner interface boards or the computer-based capture and image processing software, but they'll also see more options for scanner-based compression. Each choice has its merits and pitfalls.

"Compressing images in the computer gives you maximum flexibility," asserts Jack Berlin, president of Pegasus Imaging, an imaging technology and toolkit developer in Tampa, FL. "You can switch image compression codecs to suit your application or, within a codec, you can adjust the quality of the compressed images."

There's also an advantage in sending raw scanned data to image processing tools because they are generally designed to work with uncompressed images. When this software encounters compressed images, it usually has to decompress the image, apply the processes, such as deskew, and then recompress the image.

The downside of computer-based compression is the demand for bandwidth and processing power. While uncompressed bitonal scans of an 8.5-inch x 11-inch page range from 500K to 1000 K, color images can consume up to 12MBs each. With such large scans, a color scanner can easily saturate a 160 Mbyte/sec Ultra3 SCSI interface.

"Color scans take up to four seconds to compress on a computer," according to Jean-Dominique Moons, director of product marketing at capture software and scanner board manufacturer Kofax, Irvine, CA. "Compressing color images on a computer leaves few CPU cycles for other scan and capture activities like deskew, despeckle and OCR."

Turning the job of color image compression over to a scanner or an accelerated scanner board unloads the job of compressing images from the main CPU to more specialized processors. It also saves bandwidth on the scanner interface, increasing potential scanning speed.

While some high-speed color scanners now reaching the market offer onboard compression, this choice, too, has disadvantages. "Color scanners generally only compress to JPEG," Moons points out. As improved color compression codecs, like JPEG 2000, reach the market, this onboard compression may become obsolete.

With the high speed of the latest computers, it no longer matters where you compress bitonal scans. Bitonal images can be compressed in a computer faster than a fully saturated SCSI interface can feed data. Color images, however, will likely benefit from compression in the scanner or on a scanner board. If you need to process color images before compressing them, be prepared to run several high-end computers in parallel in order to achieve production speeds.