If glowing festive lights, wheezing steam locomotives and hordes of dancing elves are your bag, Warner Brothers Pictures and Sony Pictures Imageworks have a treat for you this Christmas with their animated fantasy, The Polar Express. Based on Chris Van Allsburg's children's book, the film marked director Robert Zemeckis' ninth collaboration with Ken Ralston, senior visual effects supervisor at Sony Pictures Imageworks. Sharing credit with Jerome Chen, Ralston brought Van Allsburg's book to life using Imagemotion, a new form of integrated face and body motion performance capture, resulting in the all-encompassing title 'imagery and animation by' in the film's credits.

The technological feat was made all the more impressive in the last months of production when the studio greenlit an idea that had been brewing for the previous year: to coincide the theatrical release of The Polar Express with a large-format Imax presentation -- expanding the film from standard 35-millimeter, four-perforation format to Imax's 70-millimeter, 15-perforation format, in stereoscopic 3D. "Bob Zemeckis and his producing partner, Steve Starkey, hadn't originally planned to use a 3D platform," remarked Greg Foster, chairman and president of Imax Corporation's filmed entertainment division. "But when Warner Brothers and Castle Rock approached them, they said, 'Oh, my God, of course!' and kismet hit. Bob has always loved to push the envelope, from a technology point of view; this was family themed, which is where a large part of our boxoffice comes from; it was a Warner Brothers film, their sixth digitally mastered release; and Imageworks' Imagemotion technology overlapped quite organically into our DMR and 3D technology." The studio greenlit the large-format initiative after producing a 3D film test using trailer imagery and, as with previous digitally mastered releases, modified the film's widescreen aspect ratio to format imagery to the squarer Imax screen. "We tested images cropped at 2.35:1 aspect ratio," said associate producer Debbie Denise, "and it didn't feel like a cheat because there was still so much to look at on an 80-foot-tall screen."

To create the stereo imagery effect -- separate strips of film for right and left eye, projected simultaneously and viewed through polarized gasses -- Imageworks set up a team of 30 animators, led by visual effects producer John Clinton and visual effects supervisor Jim Berney, to analyze stereo options, which proved particularly effective in CG. "There are reasons why 3D works better in CG than live-action," said Hugh Murray, Imax vice president for technical production. "Depth of field is one of them. In the real world, because of physical limitations of optics, cameras can't see everything in focus at the same time. In CG, we can; and in 3D we need everything to be sharp. Another reason 3D is more comfortable to watch in CG than live-action is that cameras don't really exist in CG. They're just mathematical entities. So we had complete freedom to do anything we liked with the distance between the right and left-eye cameras -- the interocular distance -- and that became an animate-able parameter that we controlled constantly through the film."

Zemeckis' cinematic style also enhanced depth effects. "CG animators often use the equivalent of long lenses to avoid CG intersections," said Murray. "Bob didn't do that on this film. That was very fortunate, because long lenses in 3D compress depth in the same way that they compress perspective, and characters end up with that 'cardboardy' look -- flatter than they should be. Instead, Bob chose to use wide-angle lenses, or appropriately-angled lenses, for all of his shots. That gave characters real depth."

The 3D process did not require 2D matte paintings to be altered for stereo effect. "The human visual system uses more than binocular information to judge depth," Murray explained. "We use both eyes to judge depth for objects in proximity; but we found that if we set up a convincing stereo environment, a two-dimensional background was still convincing if it had the correct perspective. The viewer's brain just built it into a scene."

After establishing ground rules for 3D, the Imax team worked in parallel with the main Imageworks production to deconstruct final approved shots as they rolled out of the production pipeline. "We were like digital archeologists," remarked Jerome Chen. "We had to bring each shot back online, unearth it, try to find out how we did it, and then re-render and re-composite each shot for left and right-eye perspectives, adjusting cameras to create the right sense of depth and convergence. Going into a digital artist's compositing script was like going through their bedroom diary -- every person does it differently and it's a mess because they never expect anybody to go back and look at it. But once the Imax team had reconstructed shots, the results were amazing. In some respects, this should be the way everyone sees the movie, since it all originated in 3D." Imageworks occasionally introduced new elements, giving steam and smoke additional volumetric layers. Artists also refined atmospheric effects by placing a spherical 'clipping zone' around the camera. "We clipped effects at about three feet apparent distance in the theater," said Hugh Murray. "When snowflakes or sparks or any other effects came out into the theater, we faded them before they came too close to viewers to become annoying."

Murray met periodically with Imageworks and Zemeckis to view gray-shaded animation for camera approval and interocular adjustments. Shots were then rendered with full lighting and projected on dual digital projectors for stereo approval. Imageworks supplied 2K Cineon outputs, which Imax uprezzed to 4K and then sharpened with pixel interpolation before scanning laterally to 70-millimeter film, remastering the film onto 800-pound reels for special venue theaters. Despite the immensity of the imagery, projected seven stories high, the spectacle of The Polar Express remained rooted in the charm of the source material. "The essence of the book is never lost," affirmed Ken Ralston. "Bob didn't let the scope of the film, or the size of his canvas, crush the heart of the project."