FLTK

The Fast, Light Toolkit (FLTK, pronounced fulltick) is a cross-platform graphical control element (GUI) library developed by Bill Spitzak and others. Made to accommodate 3D graphics programming, it has an interface to OpenGL, but it is also suitable for general GUI programming.

Using its own widget, drawing and event systems (though FLTK2 has gained experimental support for optionally using the cairo graphics library) abstracted from the underlying system-dependent code, it allows for writing programs which look the same on all supported operating systems.

FLTK is free and open-source software, licensed under GNU Lesser General Public License (LGPL) with an additional clause permitting static linking from applications with incompatible licenses.

In contrast to user interface libraries like GTK+, Qt, and wxWidgets, FLTK uses a more lightweight design and restricts itself to GUI functionality. Because of this, the library is very small (the FLTK "Hello World" program is around 100 KiB), and is usually statically linked. It also avoids complicated macros and separate code preprocessors, and does not use the following advanced C++ features: templates, exceptions, RTTI or, for FLTK 1.x, namespaces. Combined with the modest size of the package, this leads to a relatively short learning curve for new users.