3-D Revolution: The History of Modern Stereoscopic Cinema

Chapter 20: The Polar Express in IMAX 3-D

The Polar Express in IMAX 3-D 20 New Technology for Large-Format Stereo Film The IMAX 3-D version of The Polar Express was a landmark for large-format stereoscopic cinema. Working with the IMAX Corporation and their 3-D specialist, Hugh Murray, who previously worked on stereo repurposing of Santa vs. the Snowman (2002) and Cyberworld (2000) for IMAX 3-D, the technicians at Sony Pictures Imageworks were given guidelines to create the second-eye view of scenes in Polar Express so that a true stereoscopic version of the film was generated. The original digital files were created using an updated technology that Sony Pictures Imageworks calls performance capture. Its predecessor, motion capture, by 2004 had been around for fifteen years and uses actors in skintight suits that have about forty-eight reflective markers on them. More detail is required on the face, so 150 markers are used on the face alone. With performance capture, three or four actors go through their paces in a 10 × 10–foot blue-screen stage that is surrounded by seventy special motioncapture cameras that record stereoscopic imagery of the movement of the reflective markers. These digital files are rendered and composited with computer-generated backgrounds and props as they are repurposed to stereo. “Stereo-repurposing is based on the fact that computer animation is actually constructed three-dimensionally,” said Murray.1 In 3-D animation all of the characters, the sets, and the props are three-dimensional geometry that is created in modeling programs that build mathematical surfaces that are later textured, lit and colored. 236 3-D Revolution The underlying data, the geometry that everything consists of, is inherently three-dimensional in its formation. Most of that material eventually goes through what’s known as a rendering process where all the surfaces are calculated. What we did with The Polar Express was to go right back to the original raw data, the animation files. There are two things you have to change to make it work really well in 3-D. It’s not just a question of having a right and left eye image. It’s also quite important that you match the audience’s perspective with the perspective in the theater if you’re going to get realistic differences between things and looking normal. That means changing the focal length of the lens that was originally used to shoot the film to something that matches the perspective of an audience in an IMAX theater. That’s usually a big change. A lot of stuff for 3-D is done with lenses that have a field of view of 15 to 20 degrees. The guy sitting in the middle seat in the middle row of an IMAX theater has a field of view of the screen of about 80 degrees. So, step one to this is going back to the original animation file, back to the original camera or point-of-view. This is basically a mathematical entity that represents a camera. It’s a virtual focal length but it’s really just changing the field of view. Working from two computer-generated points of view provides the stereoscopic filmmaker numerous advantages. “CGI [computer-generated imagery] is almost the ideal medium for 3-D,” said Murray. “One of the huge advantages you have is no depth of field, unless you calculate for it. You actually want to turn off the depth of field calculations when you’re doing it for true 3-D so that everything is sharp. “There is also another big advantage. 3-D is driven by the space between the left and right eye cameras, which is the interocular or the interaxial distance. In CG you can actually have that as an animatable parameter. This is practically impossible in a live action 3-D film. For example, you can get very close to objects without hurting people’s eyes. The camera gets close to the subject and you just animate the spacing between the cameras down appropriately so the stereo distance, the apparent distance in the theater, remains comfortable. People aren’t converging their eyes and you can keep the 3-D quite comfortable. When you’re moving from something that’s very close to something that’s far away, you can animate the interocular to make that move very easy on the eyes.” The Polar Express in IMAX 3-D 237 After Sony Pictures...