Metisse is constituted by a virtual X server called Xwnc, a special version of FVWM and an FVWM module FvwmAmetista (after Ametista).
Xwnc is a mix of Xvnc and XDarwin. It draws nothing on your screen, every things is drawn into pixmaps. Similarly as Xvnc, but with a different protocol, Xwnc can send these pixmaps (and others information) to a "viewer". FvwmAmetista is such a viewer, it uses OpenGL (via nucleo) for rendering the X desktop into a window of a "regular" 3D accelerated X server.