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October 2010

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As the trend toward 3D stereoscopic projects continues to grow, so does the need for information about working in this new world. Even if job titles aren’t changing, the intricacies of the jobs are.You might be experienced in compositing in 2D, but that doesn’t mean the transition to 3D stereo will be seamless.The same rules no longer apply.Things that weren’t an issue before need to be addressed in order to make sure the audience is literally comfortable with their viewing experience. Below, a handful of compositors who have worked on recent 3D stereo projects share their wisdom. POST: How different is compositing in 3D stereo than compositing in 2D? What are those differences and can you point to a recent stereo project as an example? DAVID COX: “The addition of the ‘3rd dimension’ is a surprisingly pow- erful issue that needs to be understood so good composites can be made without giving people headaches.Although we know that the world is 3D in reality, in fact our perception of it is the result of a number of mental processes. One if these is ‘stereopsis,’ which is where the brain takes the two 2D images from our eyes and draws some understanding of distance from them. But it’s not the only method it uses — there are quite a few. “For example, those horses in the distance aren’t miniature horses — they are just further away. Holding our hand out at arm’s length proves that our hand is closer than the rest of the world because we can’t see the world through our hand.The brain adds up all these ‘depth cues’ to decide where objects really are.The thing with 3D compositing is that we can accidentally mix these up and that causes a nasty headache. A classic one is to place a title on the screen, which is optically in front of an object (we can’t see the object where the title is over it), but stereoscopically be- hind it — i.e., the title is further away from the viewer than the object it is covering.This is a great example to show how instantly a viewer can be given a big headache.The brain is being given two conflicting ideas about where the title is. It is both in front of and behind the background object at the same time, and the human reaction against this is powerful. “I did a recent commercial for Samsung where I was compositing CGI butterflies onto a real-world background. In the 2D version, it was easy to suggest the placement of the butterfly by casting a few fake shadows over objects that I wanted the butterfly to feel near. However, in the 3D ver- sion is was clear that the butterfly was six feet closer to the camera than where it should have been. No amount of fake shadows tricked the viewer, and the only answer was to make sure the effect of the parallax was identical between the shots.” ILM’s Jon Alexander on Avatar: “Even in the best-case scenario, there were numerous technical fixes that needed to happen to the original left and right eye views to make them acceptable to composite into.” TIM CREAN: “We like to tell clients that stereoscopic, live-action com- positing take 2.5 times as long as a traditional 2D compositing job.You’ve got to execute all of the compositing and prep work that goes into it twice. Once that’s done you’ve got to spend time combining the two eyes properly, not only on a technical level but I believe on an artistic level as well, so you are utilizing the 3D environment to its fullest. On a recent job for fashion design house Armani Exchange, we tried to live up to every expectation of what the stereoscopic medium can be: a very dynamic and engaging experience.” JON ALEXANDER: “The big difference is that you can’t get away with as much compositing in stereo. If you look at pretty much any effects shot frame for frame you can probably find some minute technical or artistic error.That’s just the nature of what we do. It’s not that we are trying to get away with a less-than-perfect shot but the idea is to not blow the budget chasing things that in context no one will ever see.The goal is always to make sure that the effects play a supporting role in service to the story and not pull the audience out of the moment by obvious technical flaws. “When working on a stereo production, even a slight difference or minor flaw in one eye or the other can be disastrous.The discrepancy gets magni- fied as your brain tries to resolve what it thinks should be just slightly skewed views of the same image. Irregularities that last for an instant in a shot with a lot of motion are not so much the problem. It’s the long lingering scenes where you have lots of time to notice that something is just not right. “Live-action stereo is the most challenging because you have so little control over the background plates. Before you can even begin to compos- ite elements into the plates you need to focus on correcting the differences simply caused by shooting with two cameras. I’m ignoring, for the moment, attempts to convert movies shot traditionally, with a single camera and ‘converted’ to stereo after the fact. Obviously, the most successful recent stereo movie that ILM contributed to was Avatar. Even with the most tech- nically savvy director — James Cameron — a healthy budget and state-of- the-art cameras, you can’t absolutely control optics and physics.There are going to be slight to major differences in the original photography mainly based on illumination positions relative to the two cameras. So even in this best-case scenario, there were numerous technical fixes that needed to happen to the original left and right eye views to make them acceptable to composite into.That preparatory stage is the biggest difference when you October 2010 • Post 29

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