Ed #10 Prepress

Volumes have been written about each of these software programs, and summarizing them, even briefly, is too big a job for Ed. But it’s important to know a few terms that help explain how things work.

The most basic element in digitized images is the bitmap, also known as a pixel, short for picture element. Pixels are tiny black and white or colored squares that are arranged together like mosaic tiles to form images. Black and white pixels are formed with only one bit of information—zero or one. Images containing grays or colors need more bits of information, as many as 24 bits per pixel, to reproduce high quality color.

When an image is scanned or captured by a digital camera, the number of pixels that are saved (measured by pixels per inch, or ppi) determines the resolution of the image. The greater the number of pixels, the higher the resolution of the image, and the more computer storage space it requires.

While bitmaps are used for photos, vector graphics are usually used to reproduce illustrations, logotypes and other line art. Rather than telling the printer exactly where to place each pixel, vector graphics are mathematically generated and can scale up and down smoothly, with no loss of quality, and none of the saw-toothed edges that can be seen on enlarged bitmapped images.

Once an image is created there are a variety of ways to store, transfer, and print it. One of the most common is a JPEG, for Joint Photographic Experts Group. JPEGs have become the standard for compressing continuous tone color or black and white images. While JPEGs work well for sharing vacation photos online, they can cause problems for printers, because they are most often “lossy,” which means that when the image is compressed to make it easy to send, it discards data from the image that can never be retrieved. What’s more, each time a JPEG image is resaved, it is recompressed, which will cause further deterioration of the image.

Another way to store and transmit images is known as TIFF, for Tagged Image File Format. TIFF images are easily imported into a large number of applications including QuarkXPress® and Adobe® InDesign®. TIFF files promise lossless compression with no impact on the quality of the image. The amount of compression may be limited, however, and TIFF images can take a relatively long time to transfer online.

More than any other single technology, Adobe® PostScript® provides the foun-dation for modern prepress processes. PostScript is a computer language that describes how the pages of a document should look.

PostScript has largely given way to the Portable Document Format or PDF. A file format rather than a programming language, PDFs combine a special type of PostScript language with a system that allows fonts to be transferred accurately and a storage system that bundles everything together and allows it to be compressed.

Although PDF files can contain vector graphics, text and bitmapped images, they require only a fraction of the memory space of original or TIFF files. PDF files are also device independent, so they can be shared, viewed, navigated and printed by anyone with the right free software, on any computer. What’s more, the same file can be used for proofing, image setting and digital printing.