The Ghostscript source code distribution requires a library written by the Independent JPEG Group (IJG), the zlib compression library, and optionally the libpng image format library. These libraries do NOT fall under the GNU License; each comes with its own license, all of which are more permissive than and compatible with the GPL. For more information, please see the README files in the respective library sources. The executable versions of Ghostscript are based in part on the work of these groups.