GSLib (The GrayScale Library) is a streaming, DCT-based, lossy image compression library. GSLib is mostly oriented on 8-bit grayscale images. Truecolor 24-bit images also can be compressed, but before they will be converted into grayscale format.

The GSLib compression library provides in-memory compression and decompression functions. Compression (decompression) can be done in a single step if the buffers are large enough, or can be done by repeated calls of the compression (decompression) function. In the latter case, the application must provide more input and/or consume the output (providing more output space) before each call.

In other words, GSLib is analogous to JPEG but used for grayscale images and have much more simple and convinient interface.