Web Design in a Nutshell, 3rd Edition

LZW Compression

The GIF format uses LZW (Lempel-Zev-Welch) compression, which takes advantage of repetition in data streams. Translated into graphic terms, this means that LZW compression is extremely efficient at condensing strings of pixels of identical color. To use an extremely simplified example, when the compression scheme encounters a row of 15 identical blue pixels, it makes up a shorthand notation that means “15 blue pixels.” The next time it encounters 15 blue pixels, it uses only the code shorthand. By contrast, when it encounters a row that has a gentle gradation from blue to black, it needs to store a description for every pixel along the way, requiring more data. This is why GIFs are efficient at storing simple graphical images; the areas of flat color take advantage of the LZW compression.

One of the advantages of LZW compression is that it is “lossless,” meaning no image information is lost in the compression process, and the decompressed image is identical to the original. While some information may be lost in the conversion process from RGB to indexed color format, once it is converted, the compression itself is lossless.

Common Palettes

All indexed color images (such as GIF or PNG-8) use a palette of colors to define the colors in the image. The standard available palettes along with explanations are listed here. Some, like Adaptive, are methods for producing a custom palette based on the colors in the image. Others, like Grayscale or Web216 apply a preexisting palette ...