Matthias Hopf is X.org developer working for SUSE / Novell. As his main interests include using 3D graphics, working on Xgl is a natural thing for him.
Xgl - The Current Future of X
XAA has pretty much reached the end of its productive life. One promissing successor is OpenGL, an industry standard API that could help stabilize the internal rendering interface of the Xserver. This API has been used by David Reveman to create Xgl. It enables the use of modern features like pixel shaders, and it allows for accelerating Render and Composite independent of the graphics hardware. Additionally, Xgl is a technology enabler, allowing for these neat effects that compiz is now showing.
Slides: fosdem_2006_xgl.pdf