In a short scene in Super 8, amateur filmmakers Joe (Joel Courtney) and Charles (Riley Griffiths) discover that their only surviving bit of Super 8mm footage of a massive train wreck is fogged and damaged, and they get into a heated argument, stepping in front of the projector while it’s still running.
The simplicity of the scene is deceptive. Because the projected footage would have to incorporate CGI — it offers a glimpse of a mysterious creature escaping from the wreckage — there was actually no footage to project when the scene was shot, so the filmmakers had to find a way to shoot the scene and add the projected image in post. Visual-effects artists would also need to apply an appropriate level of distortion to the image while it’s projected onto the boys.
To achieve the objective, Industrial Light & Magic visual-effects supervisor Kim Libreri proposed what he calls a “diabolically complex” post technique. He and his team borrowed an old Super 8 projector from the production’s props department and modified it to use a high-powered LED bulb instead of a standard tungsten projection bulb. Motion-control cameraman Steve Switaj built a box that generated a 48Hz signal to sync the production’s main cameras, Panaflex Millennium XL2s, with the bulb in the projector. The shots that required compositing would be captured at 48 fps, and on every other exposed frame of film the LED projector lamp would go off.
The result was two separate shots from the same camera. “Imagine playing back this footage at full speed,” explains Libreri. “On the odd frames, you only see the ambient light in the room, without anything coming from the projector. On the even frames, the projector light is on in addition to the ambient light.
“Because we were only interested in the light coming from the projector, we took these two sequences of images and subtracted one image from the other, like ‘A minus B.’ ‘A’ is the image that has the projector light plus ambient light, and ‘B’ is the image with only ambient light. When you subtract B from A, you’re left with the light coming from the projector.”
The trick with shooting at 48 fps is that the two sets of images didn’t line up perfectly. Libreri compares the offset to video interlacing. Also, shooting at twice the normal frame rate meant the XL2s’ shutters were spinning twice as fast, reducing the motion blur caused by the actors’ movements. In post, Libreri’s team used The Foundry’s Kronos plug-in for Nuke to line up and add the proper amount of motion blur to each clip so they could be matched and keyed.
Distorting the image when it played on the actors’ faces posed another challenge. “The solution was stereo triangulation,” says Libreri. “If you know the relative position and orientation of two cameras and draw two lines [to them] from a single point in space, one through the center of each lens to a projected point on the image plane, you can calculate where it will be in three-dimensional space.”
Two Sony PMW-EX3 cameras triangulated the actors’ positions and features in 3-D virtual space for the purposes of distorting and compositing the finished projector images back into the scene.
With a chuckle, Libreri recalls that shooting the scene with the strobe light going off raised a few eyebrows on set. “I was trying to convince Larry [Fong] and J.J. [Abrams] that it would all work, and footage would magically be projected on the kids’ faces in the end.
“That’s why J.J.’s movies are so great,” he muses. “He is a true believer in the magic of visual effects, and he was happy to let us develop a custom technique for his film.”