I don't want to be pushy. So this is the last time I talk about that but VTK *is* fast and already provide volume rendering among other things.
An opengl binding would be great though but if I'm given the choice between a fully featured, highly optimized, highly parallel lib and doing it all over again from opengl I would really prefer the first option. Even worse, I would rather not use something as low level as opengl to display my data, having to reimplement everything each time I want a new feature is just not sensible (and not fast). Even as a basic and "fast" package you'll end up being slower for even medium-sized datasets because it would require years of optimization to reach the speed of VTK.
