Chapter 29. GIF Format

GIF (Graphic Interchange Format) was the first graphic file type to be displayed by early web browsers, and it remains one of the most popular and versatile formats for distributing color images on the Web to this day. The GIF format was originally developed by CompuServe in 1987 to distribute images over their network to a variety of platforms (this is why you sometimes see GIFs referred to as "CompuServe GIF").

GIF files have the following characteristics:

• They are indexed color images with a maximum of 8-bit color information (256 colors).

• They use LZW compression, which is a lossless compression algorithm.

• They may be interlaced, displaying in a number of passes on download.

• They may contain transparent areas.

• They may contain multiple images, allowing for simple animations.

Any image can be saved as a GIF, but the format is most appropriate for images with areas of flat, solid color, such as logos, icons, charts, and so on (see Figure 29-1). Even if the image contains some photographic elements, if the majority of the image is flat color, GIF is your best bet. GIF is also a good option if you want portions of your image to be transparent, and it's your only option if you want the graphic to contain animation.

The GIF format is not particularly good for photographic images, because quality suffers as a result of the reduction to 256 colors and its compression scheme cannot work efficiently, resulting in larger files. Use JPEG for photos instead.

This chapter begins with further explanation of each of the features listed at the beginning of this chapter (with the exception of animation, which is covered in detail in Chapter 32). The second half of the chapter provides tips for creating GIFs, minimizing file size, and working with the web palette.

Figure 29-1. Examples of images well suited for GIF format

GIF History

CompuServe developed the GIF format to distribute color images over its network. The first version, GIF87a, was introduced in 1987 and featured LZW compression and the option of being interlaced. CompuServe released the improved GIF89a in 1989, which added transparency and animation capabilities. Both formats use the suffix .gif and are supported by all graphical browsers, however, the later GIF89a has become the standard because of its improved capabilities.

LZW Patent Controversy

When CompuServe based GIF on the LZW compression algorithm, they were not aware that it was covered by a U.S. patent held by Unisys corporation. In 1994, Unisys caused quite a stir when they decided to enforce their patent and charge royalties to all software developers that supported the GIF format. In the face of fees and legal hassles, the Internet population rushed to find nonproprietary alternatives to the GIF format, leading to the development of PNG. The U.S. patent on LZW compression ended in 2003, and will end worldwide by mid-2006.