CompuServe Messages

Brian, My business is electronic pre-press. We prepare images for reproduction on printing presses. (We also are a printing house, but that's another story). If you look at a magazine like Vogue, or Architectural Digest, or National Geographic you will see Four Color Process printing at its best. Pictures in those magazines are four-color halftones which have a 'resolution' of 150 to 200 lines per inch. That is, if you examine the fine dot pattern that makes up the pictures, you will count 150 dots to the inch in a 150 lpi picture. Those pictures result from original continuous-tone transparencies that have been scanned on a drum scanner. In general, the scanner must produce an electronic file containg a scan frequency which is twice the lpi figure of the desired final halftone. That is, if a halftone of 150 lpi is needed, the scanner must scan the original at 300 dots per inch (pixels per inch). Each scanner pixel contains one byte of data. That byte is able to specify one of 256 levels of gray at the position in the original that it represents. The file size for such a sampling of data is given by the following formula: horizontal scan resolution x vertical scan resolution x area scanned. One square inch scanned at 300 dpi therefore results in a file size of 300 x 300 x 1 or 90,000 bytes. a full page scan (8 x 10) is 300 x 300 x 80 or 7.2 megs. For a full color scan (24 bit RGB), that figure becomes 300 x 300 x 80 x 3 or 21.6 megs. So, for an 8 x 10 full color image at 300 dpi, I have to have _at least_ 22 megs of RAM for the image _alone_, to say nothing of the operating system or the image manipulation program. To handle such an image, that is, to allow the manipulations that one performs on such an image, the image manipulation program requires about five times as much RAM (or virtual memory) as the image itself. For the 22 meg image, over 100 megabytes of RAM is a _must_. Virtual memory (hard drive) is hopelessly slow for this kind of work. [ MORE ]