6 . 4 Standards and Formats
The variety and different capabilities of graphics formats present users with a number of problems. Which one to use? Making matters worse, often each format has many variations. Specifications change over time, and implementations don't necessarily keep up with the new specs. In addition, the implementations are often incomplete or use one or more optional portions of a specification.
Of course, some formats are more popular and widely used than others. For example, TIFF files on PCs and PICT files on Macs are common formats on those platforms. The longevity of your documents will be placed at risk if you choose an obscure graphics format. Inevitably, you will have to move the document to another system or document processor. At that time, you will be faced with graphics format conversion problems.
Within a project, the selection of a particular format as an internal standard is a good way to reduce problems in format conversion. For example, the computer graphics metafile (CGM) standard is used in the CALS arena to represent vector graphic images. (See Section 7 . 1 . 3 CALS and Electronic Publishing in Chapter 7 Applying Standards for more information on CALS.) Another graphics standard used to represent raster (bitmapped) images is the CCITT Group 4 format. This is otherwise know as FAX.
IGES, the Initial Graphics Exchange Specification, is another graphics standard. However, it was developed for a specific technical community Computer Aided Design (CAD). It, too, has achieved a moderate level of use in the CAD domain as the digital equivalent of blueprints. Far and away the champion of CAD interchange is AutoCAD's DXF. Because of the large installed base of AutoCad systems, it is the predominant exchange format for microcomputer based CAD systems. The main advantages of IGES are that it is not a proprietary specification and that it is often the only reasonable way to exchange information between the higherend "industrial strength" systems.
Graphics formats can be classified into three major categories: those dealing with bitmaps, those with geometrically defined objects, and those dealing with both (also known as metafiles). A graphic object means such items as lines, circles, curves, and other manipulable objects (these are often called vector graphics). The PICT format, widely used on the Macintosh, represents graphical objects. However, one of the objects is itself a bitmap, so PICT files are mixtures of bitmaps and objects.
The accompanying table, Graphics Format Characteristics, illustrates some of the characteristics of the more popular graphics formats. The column titled Native Platform indicates the original hardware platform for which the format was developeda good indication of reliable implementations. The "na" stands for not applicable, which is true for all the "real" standards that are developed in an open, platformindependent manner.(11)
Graphics Format Characteristics
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Name Color or Black & Bitmaps Objects Native MIME Notes
White or Both Platform type
The Tagged Image File Format (TIFF) is a widely used format for representing bitmap images. The TIFF format description of an image includes the resolution, size, gray level or color or bi-level choices, whether dithering was performed, and what compression scheme was used. A TIFF file may also keep a directory of images, which may correspond to all the page images of a document.
Many graphics formats use some form of data compression. Image files are large. Therefore, reliable compression is not just nice; it's a necessity. WYSIWYG publishing systems encourage the use of more and more images, which take up more and more space. Some common compression schemes are run length encoding (RLE), vertical replication, keyed compression (which include Huffman codes used by FAX), and Lempel-Ziv Welch (LZW).
The Joint Photographic Experts Group (JPEG) standard is the most significant image compression standard in a long time. JPEG has caught on as a widely used image compression technology, and Web browsers are starting to support this format natively.
The company C-Cube Microsystems, a leading developer of compression hardware, has defined a standard interface to compression algorithms. Called the Image Compression Interface (ICI), this specification allows conforming software products to work with each other. Using this technique, users may pick and choose from among any number of compression algorithms.(12)
Multimedia systems, the next stage in the evolution of desktop publishing, were the motivation for the creation of these new standards (JPEG and MPEG). Coupled with hardware, these standards will provide the performance required for real-time video on the desktop. Desktop video and multimedia publishing are the next wave of new publishing technologies. Interoperability requires the development and implementation of these standards. The creation of new contentrich documents is expensive and much too valuable to be bound to any single platform.
