![[Bytes Link logo]](../../../cgif/bytelnk2.gif)
Demystifying Graphic File Formats
by By Ben Luna Coastal Area Users Group, Corpus Christi, TX March 2001 http://www.ppcugnj.org , akosiben@netscape.net - March 26, 2001 at 20:04:36:
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 bitmapped formats and vector formats.Bitmapped 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 itmapped 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 bitmapped images are called paint programs. Following are descriptions of some commonly used bitmapped 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 objectoriented graphics.) With vector graphics, images are represented as mathematical formulas that define all the shapes in the image. Vector graphics are more flexible than bitmapped graphics because they look the same even when they are scaled to different sizes. In contrast, bitmapped 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 (bitmapped images always appear the same regardless of a device’s resolution.) Another advantage is that images in vector graphics often require less memory that bitmapped 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 graphics. It supports 24 bits of color information, and is most commonly used for photographs and similar continuoustone bitmap images. The JPEG file format stores all of the color information in an RGB image, then reduces the file size by compressing it, or saving only the color information that is essential to the image. Most imaging applications and plugins let you determine the amount of compression used when saving a graphic in the JPEG format. Unlike GIF, JPEG does not support transparency. Use JPEG for scanned photographs and naturalistic artwork with highlights, shaded areas, and shadows. The more complex and subtly rendered the image is, the more likely it is that the image should be converted to JPEG. JPEG, like GIF, uses a “lossy” compression technique, which changes the original image by removing color information during the conversion process. The JPEG file format supports millions of colors. In theory, JPEG was designed so that changes made to the original image during conversion to JPEG would not be visible to the human eye. Most imaging applications let the user control the amount of lossy compression performed on an image, so you can trade off image quality for smaller file size and vice versa. Be aware that the chances of image degradation when converting to JPEG increase proportionally with the amount of compression you use. When saving a file in the JPEG format, add the”*.jpg” file extension to the end of its file name. Use the JPEG file format for images with only a few distinct colors, such as illustrations, cartoons, and images with blocks of color, such as icons, buttons, and horizontal rules.
PICT: The Picture file format is for use primarily on the Macintosh platform; it is the default format for Macintosh image files. The PICT format is most commonly used for bitmap images, but can be used for vector images as well. Avoid using PICT images for print publishing. The PICT format is “lossless,” meaning it does not remove information from the original image during the file format conversion process. Because the PICT format supports only limited compression on Macintoshes with QuickTime installed, PICT files are usually large. When saving an image as a PICT, add the file extension “.pct” to the end of its file name. Use the PICT format for images used in video editing, animation, desktop computer presentations, and multimedia authoring.
PNG: The Portable Network Graphics format will likely be the successor to the GIF file format. PNG is not yet widely supported by most Web browsers. Netscape versions 4.04 and later, and Internet Explorer version 4.01 and later, currently support this file format. However, PNG is expected to become a mainstream format for Web images and could replace GIF entirely. It is platform independent and should be used for single images only (not animation.) Compared with GIF, PNG offers greater color support and better compression, gamma correction for brightness control across platforms, better support for transparency, and a better method for displaying progressive images. When saving an image to the PNG format, add the file extension “.png” to the end of its file name.
CGM: The Computer Graphics Metafile is a format developed by several standards organizations. CGM is supported by many PC Software products. When saving an image to the CGM format, add the file extension “.cgm” to the end of its file name.
DXF: The Data Exchange File is a format developed by Autodesk. Almost all PCbased CAD systems support DXF.
GEM: The graphics file format used by GEMbased applications. GEM is a graphical user interface (GUI) developed by Digital Research.
HPGL: HewlettPackard Graphics Language is one of the oldest file formats. Although it is not very sophisticated, it is supported by many PCbased graphics products.
IGES: Initial Graphics Exchange Specification is an ANSI Standard for threedimensional wire frame models. IGES is supported by most PCbased CAD systems.
PIC This is a relatively simple file format developed by Lotus for representing graphs generated by Lotus 123. PIC is supported by a wide variety of PC applications.
PICT: Developed by Apple Computer in 1984 as the standard format for storing and exchanging graphics files. It is supported by all graphics programs that run on a Macintosh. WMF: The Windows file format is used for exchanging graphics between Microsoft Windows applications.
WMF files can also hold bitmapped images. The debate over which format is better still surfaces once in a while. My opinion is that each format has its place, just as the native format of the various paintanddraw programs. One format may be better than others in terms of quality; at other times, another format may be better in terms of size. Best way is to take the time to view and store your images in different formats and select the one optimum for your needs, storage capability, and ease of use.
Ben Luna of the Coastal Area Users Group, Corpus Christi, TX experience includes writing manuals for computers and software.
This article is brought to you by the Editorial Committee of the Association of Personal Computer User Groups (APCUG), an International organization to which this user group belongs.
![[------STRIPE-----]](../../../cgif/str1.gif){kind=small}
Site Disclaimer Suggestions? E-Mail to webmaster@noccc.org