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The purpose of this article is to briefly describe some of the most common graphic file formats for image files, as well as how to determine which file format to use for Web graphics and print publications. When an image is saved to a specific file format, you are telling your applications how to write the image's information to disk. The specific file format you choose depends on the graphics software application you are using (e.g., Adobe Illustrator, Freehand, Adobe Photoshop) and how and where you will use your image (e.g., the Web or a print publication.)
Graphic file formats can be broadly categorized into bit-mapped formats and vector formats.
Bit-mapped formats
This format is a representation, consisting of rows and columns of dots, of a graphics image in computer memory. This is sometimes called raster graphics. The value of each dot, whether it is filled or not, is stored in one or more bits of data. The density of the dots, known as the resolution, determines how sharply the image is represented. This is often expressed in dots per inch (dpi), or simply by the number of rows and columns, such as 640 by 480. To display a bit-mapped image on a monitor or to print in a printer, the computer translates the bit map into pixels for monitor screens, or ink dots for printer. Programs that manipulate bit-mapped images are called paint programs.
Following are descriptions of some commonly used bit-mapped file formats:
BMP: The Bitmap file format is used for bitmap graphics on the Windows platform only. Unlike other file formats, which store image data from top to bottom and pixels in red/green/blue order, the BMP format stores image data from bottom to top and pixels in blue/green/red order. This means that if memory is tight, BMP graphics will sometimes appear drawn from bottom to top. Compression of BMP files is not supported, so they are usually very large. When saving a file to the BMP format, add the ".bmp " file extension to the end of its file name.
GIF: The Graphics Interchange Format was originally developed by CompuServe in 1987. It is one of the most popular file formats for Web graphics and for exchanging graphics files between computers. It is most commonly used for bitmap images composed of line drawings or blocks of a few distinct colors. The GIF format supports 8 bits of color information or less. In addition, the GIF89 file format supports transparency, allowing you to make a color in your image transparent. (Please note: CompuServe GIF87 does not support transparency.) This feature makes GIF a particularly popular format for Web images. GIF is a "lossy" file format. It reduces an image's file size by removing bits of color information during the conversion process. The GIF format supports 256 colors or less. When creating images for the Web, be aware that only 216 colors are shared between Macintosh and Windows monitors. These colors, called the "Web palette," should be used when creating GIFs for the Web because colors that are not in this palette display differently on Macintosh and Windows monitors. When saving an image to the GIF format, add the ".gif" file extension to the end of its file name.
PCX: Originally developed by ZSOFT for its PC Paintbrush program, PCX is a common graphics file format supported by many graphics programs, as well as most optical scanners and fax modems. When saving an image to the PCX format, add the ".pcx" file extension to the end of its file name.
TIFF: Tagged Image File Format is a standard file format for storing images as bit maps. It is used especially for scanned images because it can support any size, resolution, and color depth. When saving an image to the TIFF format, add the ".tif" file extension to the end of its file name.
Vector Graphics
The other method for representing images is known as vector graphics (or object-oriented graphics.) With vector graphics, images are represented as mathematical formulas that define all the shapes in the image. Vector graphics are more flexible than bit-mapped graphics because they look the same even when they are scaled to different sizes. In contrast, bit-mapped graphics become ragged when they are shrunk or enlarged. Programs that enable the user to create and manipulate vector graphics are called draw programs. Images stored as vectors look better on monitors and printers with higher resolution (bit-mapped images always appear the same regardless of a device's resolution.) Another advantage is that images in vector graphics often require less memory that bit-mapped images. Almost all sophisticated graphics systems, including CADD systems and animation software, use vector graphics.
Following are descriptions of some commonly used vector graphics file formats:
EPS: The Encapsulated PostScript file format is a metafile format; it can be used for vector images or bitmap images. The EPS file format can be used on a variety of platforms, including Macintosh and Windows. When you place an EPS image into a document, you can scale it up or down without information loss. This format contains PostScript information and should be used when printing to a PostScript output device. The PostScript language, which was developed by Adobe, is the industry standard for desktop publishing software and hardware. EPS files can be graphics or images of whole pages that include text, font, graphics, and page layout information.
JPEG: Like GIF, the Joint Photographic Experts Group format is one of the most popular formats for Web (Continued on page 12)
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