Chapter 9 - Goodheart-Willcox

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problems. However, if the files have not been properly prepared, they may need to be returned to their point of origination. Using laser technology, the imagesetter outputs the page or color separations on the selected medium. To ensure high-quality output, especially with color separations, special attention must be given to calibration and maintenance of the imagesetter. Imagesetters are manufactured by a number of companies and they differ in speed, precision, resolution, screening technology, and media capability. Page description languages (PDLs) A page description language (PDL) serves as the interface between the page composition workstation and the RIP. PDLs are used in electronic publishing as a format by which all the elements to be placed on the page, their respective positions on the page, and the page’s position within the larger document are identified in a manner that the output device can understand. Several common PDLs are Adobe PostScript, Adobe portable document format (PDF), and Hewlett-Packard PCL (Printer Control Language). PDLs enable imagesetters developed by different companies to interpret electronic files from any number of personal computers and software programs. PostScript, PDF, and PCL are objectoriented, meaning that they describe a page in terms of geometrical objects such as lines, arcs, and circles. <strong>Chapter</strong> 9 Electronic Prepress and Digital Printing 179 Imagesetter Laser printer Ink-jet printer Figure 9-12. Compare reproduction of the same original image by three different output devices. An imagesetter will normally produce the most accurate reproduction. Adobe PostScript The most common use of Adobe PostScript is to describe the appearance of a page to an output device. Output devices use an interpreter to interpret PostScript files. An interpreter is a computer program that resides on a controller board in the printer. The interpreter receives the PostScript page descriptions and translates them into patterns of dots for a printer or pixels for a display. The interpreter allows PostScript files to be used on various output devices. After receiving a page description, the interpreter constructs a representation of the page to suit the output device. For example, the interpreter can determine whether the output device is a black-and-white or color printer, an RGB video monitor, or a 2000 dpi imagesetter. Once these parameters have been defined, the interpreter modifies its instructions accordingly. Since the introduction of the original Adobe PostScript, PostScript Levels 2 and 3 have been added. These new levels integrate the original Post- Script language, all previous language extensions, and new language features into the core PostScript language imaging model. The newer interpreters are not fully compatible with the original. Portable document format (PDF) Adobe Portable Document Format (PDF) has become the standard for electronic document distribution throughout the world. PDF is a universal file format that preserves all aspects of a native file 180 Graphic Communications regardless of the application or platform that uses the Adobe Acrobat application to create the PDF file. Anyone using the Adobe Acrobat Reader can view, navigate, and print the file exactly as the author intended. The PDF file is the leading electronic document workflow element in the paradigm "Create, Distribute, and Print as needed." In the PDF workflow, the PDF file is used to create the film or plates needed by the print production facility. Within that file, all of the necessary information is contained. The fonts, graphics, images, text, and document layout are already present — no further prepress steps remain to be completed by the production department. The Acrobat program consists of the applications Acrobat Distiller, Acrobat Exchange, and Acrobat Catalog, along with a distributable reader. See Figure 9-13. Figure 9-13. Adobe Acrobat simplifies the creation of PDF files. The Adobe Reader screen capture above illustrates a pull-down menu, thumbnails of pages in the document, and a completed page. (Adobe Acrobat) The Acrobat Distiller is the engine that creates PDF files. Acrobat Exchange is the segment of PDF that allows minimal editing, linking, and final formatting, as well as the ability to create hypertext links to other portions of the document, to other PDF files, or to Web sites. The Acrobat Catalog provides extensive indexing and searching capabilities. The Catalog searches through the text of PDF, indexing, referencing, and cataloging information. This application can be performed with one PDF, an entire directory, or CD full of PDF files. The Adobe Acrobat Reader has now been extended to the hand-held Palm device (a PDA, or Personal Digital Assistant), allowing mobile professionals the flexibility to easily view PDF files. This gives the PDA user the ability to view visually rich documents that were previously accessible only on a desktop or laptop computer. Additionally, these documents can be secured, so only users with passwords can view them. This makes possible greater flexibility in viewing complex documents for "on the go" professionals, streamlining their business processes. Unlike the complex, continuous stream of data in PostScript files, PDF files are simple, compact, object-oriented files. They process quickly and are also page-independent so single pages can be replaced or altered without reprocessing the other pages. Page independence also allows printing pages in any order from a single file. PDF files are also self-contained. This means the file has all the fonts and other resources needed to image it. PostScript was designed with the print industry in mind. PDF, on the other hand, has been created to support CD-ROM, Internet and intranet page production, digital printing technologies, and the 100% digital workflow requirements of computer-to-plate technology. An intranet is an internal network within an organization or institution that is based on the same network protocols as the Internet. Rather than redesigning materials, companies often recycle or repurpose the content of their printed media. The PDF format was created with repurposing in mind. PDF files are also much smaller than comparable PostScript files so they can be sent quickly across the Internet or a network for remote proofing or printing. To save mailing and printing costs, many government forms are available as PDF files. The files may be downloaded off the Internet or copied from compact disks. The PDF file format allows incorporation of an extended job ticket. An extended job ticket is an electronic document that contains all of the instructions required for processing a job, Figure 9-14. It includes customer information, proofing directives, trapping, imposition and ripping parameters, and even finishing and shipping instructions. The job ticket specifications can be easily viewed and modified by everyone who has access to the file. PDF is almost the ideal preflighting tool. If all the elements are not present at the time of the cre-
ation of the file, the user is warned. Other key features and benefits of PDF are the provision of a single file for viewing, distributing, archiving, editing and printing small file sizes and the provision of built-in preview. PDF also has the ability to access many types of files including EPS, TIFF, PICT, QuarkXPress, PageMaker, and PostScript from applications in both Macintosh and PC platforms. Portable document software typically will save a document bitmap, the ASCII text, and the font data. Even with compression, most PDF files are many times larger than the original file. <strong>Chapter</strong> 9 Electronic Prepress and Digital Printing 181 Figure 9-14. A job ticket contains several files allowing the operator access to customer information, specific page data, the type of processes involved, and the type of output. (AGFA) File compression Before sending digital data to a service bureau or printer, most publishers will compress, or reduce, the size of the files. Some programs automatically compress when the file is converted and then decompress when it is viewed. File compression requires less storage space, and enables easier data management and quicker transmission by eliminating the redundancies and other unnecessary elements from the original. Two common compression types are lossless compression and lossy compression. 182 Graphic Communications Lossless compression refers to data compression techniques in which no data is lost. The PKZIP compression technology is an example of lossless compression. The files are often referred to as zip files. Decompressing them is called unzipping. Decompression, simply put, is a reversal of the compression process. Files that have been compressed with PKZIP usually end with a .ZIP extension. ZIP files that end with an .EXE extension are selfextracting files which can be unzipped simply by opening the file. For most types of data, lossless compression techniques can reduce the space needed by about 50%. Lossless algorithms used for image compression assume that the likely value of a pixel can be inferred from the values of surrounding pixels. Because lossless compression does not discard any of the data, the decompressed image is identical to the original. Other common lossless compression methods are the Huffman method, Lempel-Ziv-Welch (LZW), and run-length encoding (RLE). Both the Huffman and LZW methods of compression are data compression techniques in which adjacent bits are replaced with codes of varying lengths. For example, this technique would encode the fact that zero occurs 20 times, rather than using 20 zeros. This information would use 4 bytes instead of 20. Run-length encoding (RLE) encodes digital data to reduce the amount of storage needed to hold the data without any loss of information. Each coded item consists of a data value and the number of adjacent pixels with the same data value. (In other words, strings of the same character are encoded as a single number.) This is a very efficient way of encoding large areas of flat color used in linework and text. Lossy compression refers to data compression techniques in which some data is lost. Lossy compression technologies attempt to eliminate redundant or unnecessary information. Most video compression technologies use a lossy compression. This improves the speed of data transfer but causes a slight degradation when the image is decompressed so it can be used. Lossless compression is preferred for images that are to be printed because each time a lossy compression is applied, more information is lost. The loss of data may not be noticeable on screen but it will be very noticeable in high-resolution printed output. Lossy compression techniques include quantization, Delta Pulse Code Modification (DPCM), and JPEG. Quantization is a filtering process that determines the amount and selection of data to eliminate so the data can be encoded with fewer bits. DPCM measures one set of bits and then measures differences from that set. The differences are then encoded into fewer bits. The JPEG file format was created by the Joint Photographic Experts Group in collaboration with the International Standards Organization (ISO) and the Consultative Committee for International Telegraphy and Telephony (CCITT). The JPEG format was designed to establish an international data compression standard for continuous-tone digital still images. JPEG compression is an open-system, cross-platform, cross-device standard that can reduce files to about 5% of their normal size. JPEG is based on the discrete cosine transform (DCT) algorithm. The DCT is a lossy compression algorithm that analyzes each pixel block, identifies color frequencies, and removes data redundancy. The JPEG algorithm requires the same processing amount to compress or decompress an image. JPEG compression can incorporate other algorithms, including quantization algorithms and one-dimensional modified Huffman encoding. JPEG is a popular standard on the Internet due to extreme compression and the ability to support 24-bit color. The JPEG file format allows the user to control the compression ratio and reproduction quality at the point of compression, Figure 9-15 The JPEG file format contains bitmap information only and supports grayscale, RGB, and CMYK color models. A major goal of JPEG is to maintain the appearance of an image rather than the actual data that constituted the original. This works because we are visually less sensitive to high-frequency color. JPEG is a lossy compression and therefore deletes some image information, but the expanded image remains visually whole. JPEG functions best for color and gray-scale continuous-tone images. Compressing images with high-contrast edges (line graphics or text) enough to reduce the file size significantly will adversely affect the portion containing the text. Selective compression enables users to specify different compression levels for different elements within a single image. EPS color image files embedded in a
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Chapter 9 - Goodheart-Willcox

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