Prince of Persia Rewind with 'Dagger Time'
The open shutter look was discussed with Wood, who provided photographic reference, but they needed something different from a long smear effect with the camera moving around a frozen object. "We wanted the camera moving around a moving human form that had a frozen long exposure," Ellis explains. "This, as far as we knew, had not been done before so we needed a new technique in order to achieve it.
"We'd done some work previously on 'event capture.' The Quantum of Solace freefall sequence used the technique; then we developed it further for Prince of Persia. It allowed us to achieve something that couldn't be done any other way. This is a technique which records a live scene using multiple cameras, then reconstructs the entire scene in 3D, allowing us to create new camera moves, slip timing of the actors, change lighting, reconstruct the environment and pretty much mess around with whatever we wanted.
"One problem we found with this technique is that as our photographic textures are derived from locked camera positions; specular highlights tend to jump over an image rather than smoothly roll over a surface as they do in real photography. [We] had to correct this by manually painting out such problems. The great advantage of this technique was that it answered all of our technical requirements while giving us great creative freedom. With some restrictions based on texture coverage, we could essentially redesign live-action shots after they'd been shot. The camera is independent from the action. A camera move can be created after the shot has been filmed, actors' timing can be slipped and they can be manipulated to break them apart or change them as if they were conventional 3D."
Initial previs for the rewind scenes was done by Nvizage, which blocked out the choreography of each scene with forward action, freezes, reverse action and the 'ghost' character but without rewind or ghost effects. "We knew that the additional effects work would take the form of long extended motion blur and particle and that it would alter the composition and dynamics in each shot," Ellis continues. "We worked closely with Tom Wood to re-work the previs, creating a shoot template not only for which shots were needed to tell the story but also where we'd need to build our camera arrays around the action. We knew, however, that even with all of the planning we'd done, things were likely to change when it came to the edit and with the addition of our effects, we therefore needed the flexibility that our 'event capture' technique would give us."
According to Ellis, the event capture required clean, crisp photography with a minimum of motion blur but a maximum depth of field. This provided a better result when projecting the nine cameras onto 3D geometry and was valuable in creating convincing new camera moves as it meant that they could apply their own motion blur and depth of field. The process involved volume carving, shrink wrapping and dense stereo displacement.