Here's how ILM used fluid dynamics to achieve three of the most exciting CG explosions in recent movie memory.
We can wax philosophical about how Terminator is a prophetic look at our technological capabilities in the face of our ethical accords. We can picture John Conner as a picture of Greek tragedy; he is the Oedipus who struggles against his destiny only to reinforce and cause it to happen in the first place. We can ponder the temporal existentialism of beings coming in from the future to prevent themselves from ever happening at all. We can wonder how we ourselves are headed into the technological paradox where we create machines to overcome us and take dominance. Or we can sit and watch the pretty pictures, our fingertips glistening with popcorn juice, our eyes wide, our imaginations stirred.
Terminator 3 doesnt dive deeply into philosophy, nor does it offer a truthful glimpse into the human condition. It does, however, blow a lot of stuff up, and thats what were here to talk about. Now Im not going to offer a recount of the film. Im not even going to tell you if I liked it or not. Id like to cut to the heart of the matter and speak about what I thought were some of the finest CG explosions I've ever seen in a film.
In particular, there is a nuclear explosion during a dream sequence in the beginning, that sets off the beginning of the end of the world as we know it Judgment Day. It is with this explosion that I perked up and peeled open my eyes for more, even momentarily forsaking my bag of Sour Patch Kids as I looked on.
When I walked into the theater, I was curious to see what Industrial Light & Magic would do with the effects in this film, having come away disappointed with The Hulk (not ILMs fault; I personally didnt agree with Ang Lee's direction). This single effects shot allayed any hesitance I may have had about seeing T3, and set a great tone for the rest of the film for me, because I knew that some great folks were on the job. Twice more I would see a stunning explosion like this one, and every one of them grabbed my devout attention, much to the chagrin of my Sour Patch Kids.
Now, what grabbed my attention with the first explosion in the dream was the level of detail inherent in the shot, and how it all came together to work as one. And I was struck by the nagging question, Howd they do that? Was that fluids? Was it custom code? And that to me is a mark of a good effect; along with the ever important question, That was an effect? Where? Really? Having been in effects animation a number of years now, Ive had my boyish wonder about effects yanked out from under me, replaced with an analytical process, so now I tend to watch effects laden films with a critical, if not deconstructivist eye. When I see something that makes me wonder how it was done, Im giddy. Not to say I know how everythings done far from it; but I tend to discount effects that I see clearly standing out as effects. Yet this explosion just looked natural, as if it belonged in the frame; well, as natural as a dramatic movie explosion should look at least.
The next day I was on the phone with ILMs Willi Geiger, the lead artist responsible for these explosions. He confirmed for me that a fluid dynamics system was used to create the motion for the explosion and laid out basically how they accomplished the shot.
First, understanding that these two explosion effects needed a high level of detail, which would need to generate a huge amount of particles, Geiger began a couple weeks of research and development with professor Ron Fedkiw of Stanford University. They worked toward generating a fluid solver to avoid the memory usage problems inherent with using massive amounts of particles to hammer out the motion of the explosion itself. They came up with a 2D fluid simulation method that provided them with almost realtime feedback for the explosions motion. This enabled the small effects team to design the flow of the explosion with a hands-on approach to get their desired result.
Geiger exported the fluid motion data in slices into Mayas particle engine to use as velocity fields with which to move Maya particles for better visualization. The slices of fluid data made it much easier to create the particle motion that they used to visualize the explosion. The explosion was then rendered through a custom volumetric raytracing renderer written by Geiger for T3 explodes on the big screen with some of the most stellar CG explosions ever made.
Interestingly enough, all the renders where output into different lighting passes using the three inherent color channels of the image files. The key light used on the explosions was output through the red channel, the bounce light through the green channel and the self-illumination glow of the explosion through the blue channel. This gave the compositor greater color and image control over the composite all from within a single output.
Two additional animators were brought in to add some ancillary details to both scenes in the last two weeks of the production period. But for the most part, Geiger and a single compositor worked through the short 10-week process from R&D to completion to produce these two shots. Quite a nice accomplishment, since the effect was right on the money. And the rest of the films effects clearly keep up the same standard set by these initial effects shots, though theey were clearly my personal favorites. Props to the peeps.
Dariush Derakhshani is an early 31 and makes for an interesting Googlism. Nicely bald and slowly going insane, he has a fear of commitment and of having to cook. He has written a bunch of articles littered about the Web, wrote for Maya: Secrets of the Pros and Maya 4.5 Savvy, is writing his own Maya book due out this SIGGRAPH, and co-moderates the 3d Hardware List e-mail forum (www.reiss-studio.com/3dhardware to join). A senior animator at Sight Effects in Venice, California, he can be found at www.painfulurination.com or you can send him viruses at firstname.lastname@example.org. He's got flat feet.