AFAIK, Gallium is the open source driver for ATI/AMD graphics card.Gallium 0.4 on AMD JUNIPER
I wasn't able to use OPENGL in Linux while using Nouveau,
which is the nVidia's open source counterpart driver.
This was true for both Processing 1.5.1 & Processing 2.0!
OPENGL worked here after I've installed the closed source driver!
After that, even WebGL was fixed!