Pixar embraced ray tracing in Cars for its vast landscapes and highly reflective vehicles. It used a hybrid rendering approach, combining REYES with ray tracing. 

"In any given year, one or the other of the big effects studios would be doing something that nobody has done before," says Batali. "And it's not always the same people." He cites the examples of ILM's work on Pearl Harbor and Sony's on The Polar Express. "We hear customers say things like 'We're randomly accessing 3 terabytes of data in order to get subsurface scattering information from real light sources in the universe.' And we say 'What? You can actually do that?' The fact that our software operates in that environment is a wonderful surprise."

By comparison, recalls Batali, "Way, way back, the idea of a complex frame was measured in small numbers of megabytes. Now it's measured in gigabytes. So complexity is growing at a Moore's Law pace, and it's all driven by further appreciation of the kind of visual complexity that we can deliver on computers. And it's not just delivering more complex lighting. More and more geometry -- for grass, fur and trees -- kicked in less than 15 years ago. The other thing that was dramatic was that, 20 years ago, the idea of a shader was a 10-line program. And 10 years ago it had gotten to the point of tens of thousands of lines on every single object in the scene. So there's complexity in every direction."

With WALL•E, director Andrew Stanton wanted to simulate an anamorphic look to evoke a widescreen, sci-fi film feeling. RenderMan introduced a new feature that simulated the effects of an anamorphic lens.

With respect to realistic lighting, Batali remarks, "The idea of trying to mimic reality has been fundamental to rendering from the very beginning. The film community has always wanted effects like caustics, but had to fake them. When ray tracing came within arm's reach -- in a computational sense -- it made things simpler. ILM's ambient occlusion work on Pearl Harbor was a landmark use of ray tracing."

The RenderMan Group originally may have been urged to add on-demand ray tracing to meet the needs of its visual effects customers, but Pixar's own embrace of ray tracing -- evident in director John Lasseter's 2006 film Cars -- marked a notable evolution. The film's vast landscapes and scenes with highly reflective vehicles prompted Pixar to use a hybrid rendering approach, combining REYES with ray tracing.

Batali thinks that people will continue to weigh the advantages of one approach vs. the other. "A RenderMan TD who's been in the trenches for 20 years knows how to get an effect without having to pay for it in a physical simulation sense. But the downside is the cost of a more brittle pipeline that may involve more multiple passes. Then you have folks who are bound entirely to physical simulation. And if, all of a sudden, a director says, 'I don't really like that,' they have no way to make a change. If you're going for pure physical simulation, you can't change physics, even though the director would like you to do so!

"In some sense, tweakability means that you can program the local laws of physics to your own requirements. That's always been the basis of a RenderMan-centric pipeline. We have to be able to program everything. You don't want to program everything, but you have to have that option, and that's really why I think we're on pretty firm ground. We have to provide 'back doors.' The art of illusion is the art of programmability."