Barium is natively implemented in Common Lisp, directly accessing the X client library and other platform libraries. It is not a wrapper for an existing non-lisp GUI toolkit.
Barium provides first class OpenGL support, allowing the creation of rich user interfaces mixing ordinary toolkit-provided widgets and OpenGL rendering surfaces in the same toplevel window.
It has the ability to dynamically create a GUI containing "traditional" widgets, updated on-the-fly in the usual style of incremental Lisp development. Regular (non-GL) widgets are rendered using Cairo.