What earlier CompuServe image encoding did GIF replace?

GIF replaced CompuServe’s earlier run-length encoding format. That earlier format was designed for black-and-white images, while GIF was introduced to provide a more capable color-image format.

CompuServe released the Graphics Interchange Format on June 15, 1987. GIF used Lempel–Ziv–Welch, or LZW, compression, a lossless method that could reduce file sizes without changing the image data. This made color images more practical to download over the slow modem connections common at the time.

Run-length encoding, by contrast, stores repeated consecutive values as a value-and-count pair. It works especially well for simple images containing long runs of identical pixels, but it was less flexible as a general image format. The older CompuServe format is sometimes described as RLE rather than spelled out as “run-length encoding.”

A common mix-up is to say that GIF replaced LZW. The opposite is true: LZW is the compression technique used inside GIF files. GIF later gained animation and transparency-related features in the 1989 GIF89a revision, helping it remain important on the early Web.