With visual effects extravaganzas like Sky Captain and the World of Tomorrow getting more and more complex, previs is becoming a must in determining if a shot can or cannot be accomplished. Images courtesy of Engine Room. ™ & © 2004 Paramount Pictures. All rights reserved.

However, Schmit cautions against using RTPV in every case. “Motion control is really cool,” he says. But it costs money, so you have to weigh the value you get out of it — if there’s no CG in the scene, or interaction with virtual characters, you may want to save the extra costs.” He also warns of depending on previs too completely. “Something will always come up that you hadn’t planned for when you created the previs. It can be the weather, or perhaps someone mis-measured the set or perhaps the performance of one of the actors doesn’t come through. You should always be ready to adapt to local circumstances, and be ready to make necessary changes.”

Previs Pros and Cons
Visual effects pioneer Doug Trumbull (2001: A Space Odyssey, Close Encounters of the Third Kind, Star Trek: The Motion Picture and Silent Running) agrees that you have to use caution with previs: “The previs can be really useless if you run into something totally unexpected,” he says. “Like the Back to the Future ride at Universal Studios. We had planned that in great detail, but when we tried it out it didn’t look right, and we had to quickly create a small working model of the ride with an overhead dome. Much of the camera work turned out to be counter-intuitive, in conjunction with the motion platform — for instance, to give the feeling of acceleration, we wound up tilting the seat back, and the camera up to level the horizon.”

Still, Trumbull is a fan of realtime previs. “For Disney’s Book of Pooh, we shot all 52 episodes with puppets on virtual sets, all composited in realtime. The finished result was in the viewfinder. With only a three-frame lag, it was possible to make much higher quality decisions about each scene.” He also believes that previs should not be limited to using digital charaters. “Sometimes stand-ins or stunt doubles make more sense than CG 3D characters. In certain situations there’s a limit to what you can learn when you use CG characters to block out camera moves. In Close Encounters of the Third Kind, for instance, the lighting was an important element, which was easier to understand with real characters.”

Hoyt Yeatman understands the powerful future of previs. The visual effects supervisor landed his first production deal/ directing gig with a previs short of his G-Force project.

The Future of Previs
One clear trend for previs is that directors now expect more detail and faster turnaround times for changes or “what-if” explorations, which demand either some good computers on location or a fat pipeline to a render farm, so that scenes can be rendered and presented more quickly.

And look for more previs integration with motion control cameras: for planning the specific motion of cameras and cranes before the shoot, then for combining and compositing real and virtual elements during the shoot for immediate playback to the director and finally for tight integration with post- production effects work. Since motion control is still very labor intensive, major improvements in tracking-related hardware and software should lead to quantum gains in this area.

Finally, we can expect previs to be used more and more by producers and others as a sales tool to get projects greenlit. For example, Vfx supervisor Hoyt Yeatman recently prevised a mock trailer that helped land his first directing gig on G-Force. “Presentation details in a previs, with…textures and lighting as well as aural content to `punch up’ certain scenes, are invaluable,” notes PLF’s Cushing. This demand for whole segments of a film long before it is shot will also reinforce another trend: that previs is increasingly being used for whole sequences and even for the entire film.

Christopher Harz is an executive consultant for new media. He has produced video games for films such as Spawn, The Fifth Element, Titanic and Lost in Space. As Perceptronics svp of program development, Harz helped build the first massively multiplayer online game worlds, including the $240 million 3-D SIMNET. He worked on C3I, combat robots and war gaming at the RAND Corp., the military think tank.