Thus, while Framestore created vipers and huge quantities of sand, MPC worked on the CG Alamut City and used ALICE  to simulate the Persian attack and Cinesite got the Royal City of Nasaf and Avrat bazaar chase, Double Negative focused on the rewind effect.
This included the final Sandglass Chamber that is effectively a mammoth rewind, incorporating flowing, magical sand, glowing figure and rewind images -- all in a very nearly fully CG environment of photoreal rocks cascading and crumbling from the walls, according to Tom Wood, the overall visual effects supervisor.
"The rewind sequences were Dneg's biggest challenge," Wood suggests. "Using their proprietary 'event capture' software to carve a 3D version of a scene, captured with up to nine Arri 435 cameras slaved together shooting at 48fps with a 90º shutter. We renamed the traditional 'bullet time' to 'dagger time,' as we were now able to place a camera anywhere along the path of the original camera layout, at any time. We could run forwards and/or backwards at any time: meaning we could see all time at any place. This meant we could create a full motion long exposure effect that we could travel past or through. The idea was for the rewinding character, visible within their extended long exposure, to reverse through their own path, dissipating it through simulated fluid turbulence created by their own body motion.
Overall, there are four rewind sequences consisting of 200 shots, which took Dneg 18 months to complete. According to Mike Ellis, Dneg's visual effects supervisor, there were three main objectives: detach the viewers so they could experience the rewind (the "ghost" effect), the need to see people reverse in a magical way (the "rewind" effect) and the need to change the whole environment to distinguish between rewinding and regular forward action. Not only that, but the same actor would need to appear twice in many shots moving both forward and in reverse simultaneously with two distinctly different looks, requiring the need to freeze and rewind some aspects of the same shot and a certain amount of relighting.
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.
"So it was really the transition in the shoot schedule from forward action to rewind action that took the longest time to set up because we had to accommodate this boost to the lighting. As soon as we had the first rewind set-up in the can, the others followed much more quickly. We carefully planned the position of each camera and marked up the set accordingly, so we were quickly able to set-up our cameras for each shot. "
As for the differences between the rewind sequences, Ellis says the first two involving Dastan (Jake Gyllenhaal), which come right after the other, were the most challenging. "We see time slow down to a freeze, we see the ghost very closely with magical sand particles flowing straight into the camera and we get a really good look at all the component parts of a rewind scene."
The third rewind happens with Dastan and his brother Tus (Richard Coyle). This presented its own challenges. "A very ornate 3D background had to be built and lit," Ellis continues. "We also had a difficult camera takeover as the dagger is pressed between a conventional stedicam shot and our full nine locked camera array. We had the stedicam spiral out from the dagger while running nine cameras at the same time, the operator had to cleverly squeeze between two cameras in order to exit the shot."
The fourth Rewind happens in the Oasis as the group is attacked by snakes. "It was a much faster paced action scene, which actually kept this scene simpler," Ellis observes. "Reduced length and travel of camera moves meant that we could use [fewer] cameras in the array. In this scene, we worked closely with Framestore, which created the snakes. Maya  scene files and rendered elements were passed backwards and forwards between the facilities."
Fortunately, there was no magical dagger required for that.
Bill Desowitz is senior editor of AWN & VFXWorld.