Information Technology Division

Visual Media Formats

To save computer memory and minimize the time required for file transfer, computer graphics are "compressed" into a file format. Most file formats were first used exclusively by the corporation that invented them before gaining widespread acceptance. Each format has its own strengths and weaknesses.

The most common file formats are GIF, JPEG, and BMP.

Compuserve originally developed the "Graphics Interchange Format" (GIF) for transferring images across their network. Later, Compuserve published the specifications, making the format available to third-party programmers in other applications.

In 1995, the owners of the LZW algorithm learned of its use in GIF compression. Now all software developers using the GIF format for profit must obtain a licensing agreement from the Unisys Corporation, who holds the LZW patent. With relatively low resolution but enduring popularity, the GIF format sacrifices color distinction for reduced file size. Consequently, GIF's are very good for graphics rather than pictures. A neat feature of GIF's is "interlacing": as the graphic is read in, the basic shape of the picture becomes visible and then the details are sketched in gradually.

Sanctioned by the International Standards Organization (ISO) , the JPEG format was developed by the Joint Photographic Experts Group. Simplifying the more complex tagged image file format (TIFF), the goal of JPEG was to establish a platform-independent file format. That goal was accomplished; now both Macintosh and IBM-compatible computers can interpret the JPEG format, which uses the JPEG algorithm to do its encoding. JPEG's are good with photographs because a full-color palette of 16 million colors is available.

Microsoft developed the bitmap (BMP) format for its Windows environment. A very powerful format, BMP offers multiple degrees of compression while maintaining color vibrancy. Unfortunately, specifications for the format are hard to come by, and programmers must scrape together third-party information.

Both the GIF and JPEG formats use "lossy compression."

"Lossy" refers to the "loss" of details during compression. Theoretically, the human eye couldn't see the details to begin with. Lossy compression eliminates fine degrees of color, stray pixels, and other fine details. Unfortunately, the removal of all these invisible details sometimes has a visible effect on image quality.

Furthermore, converting between GIF and JPEG files (in either direction) has unsightly results because the compression schemes are not complimentary. It's best to keep a copy of important image files in a large, "library" format that preserves details but is too large for everyday use (such as the TIFF format). Many software programs can convert from one graphics file format to another. These programs -- called "filters" -- are available free of charge from various sites on the Internet.

Video Formats

Apple developed the Quicktime format for the Macinosh. Now also available for Windows, Quicktime is an extremely flexible format capable of handling everything from stills to movies to broadcast-quality sound. In fact, Quicktime was designed to be compatible with any type of media that a programmer might specify, so it will probably be around -- in one version or another -- for a long time to come. More than just a media player, Quicktime can also be used for editing. It's available for free over the Internet. ( http://www.apple.com/quicktime/)

The MPEG format was developed by the Motion Picture Experts Group. When displaying a video clip, MPEG takes advantage of the fact that differences between frames are minute, and supplies information for updating only the portions of the image which change ("block-based motion-compensated prediction"). Although playback is very smooth, encoding a video clip in the MPEG format is a time-consuming task; digitizing a five minute video can tie up a computer for several hours. Recognized as an industry standard by the International Standards Organization, MPEG has several different version, the most popular being MPEG-1.

Microsoft invented the AVI format. AVI stands for "Audio Video Interleave." In an AVI file, video information is alternated with audio information; this technique provides good synchronization between motion and sound. The AVI format was designed for use with 24 bit applications; consequently, picture quality can be poor and motion jerky unless the host computer has a 16 bit "True Color" card that supports millions of colors. Furthermore, since AVI video clips can be compressed using several methods, it may be necessary to find a more recent version of AVI software before viewing the newer AVI video clips. The AVI format is common in Windows applications, but once again useful information about Microsoft's format is difficult to obtain.