Primus is a shared library that provides OpenGL and GLX APIs and implements low-overhead local-only client-side OpenGL offloading via GLX forking, similar to VirtualGL. It intercepts GLX calls and redirects GL rendering to a secondary X display, presumably driven by a faster GPU. On swapping buffers, rendered contents are read back using a PBO and copied onto the drawable it was supposed to be rendered on in the first place.