What is it all about?
FLTK(Fast, Light Toolki) is a free cross-platform GUI library, suitable for 3D graphics programming via OpenGL. also optimized for IGU programming. developed on the abstract code dependent system, which allows you to write programs that look identical when compiled in any of the supported operating systems.
FLTK is designed to be small and modular enough to be statically linked, but works fine as a shared library. FLTK also includes an excellent UI builder called FLUID that can be used to create applications in minutes. FLTK is provided under the terms of the GNU Library Public License, Version 2 with exceptions that allow for static linking.