I’ve recently come across an open-source rendering engine written in Java, which uses Global Illumination as its major selling point and which – to my surprise – looks really promising.
It goes by the name of Sunflow and is made available as a plugin for the main 3d packages (yes, including blender..), but slightly more important, this chap has converted it into a library (P5Sunflow) which can be used with Processing to create all sorts of pretty stuff.
It’s not really well documented at the moment and it isn’t compatible with the latest release of Processing (plus the fact that any 3d scene for some reason looks terrible on my machine) but I will keep an eye on the development of this little gem.