Double Negative also developed a motion capture-based crowd solution built around Houdini. Courtesy of Double Negative.

New Crowd System
One of the biggest issues for these scenes was the immense crowds. For the majority of shots, where no specific action was required, the crowd was created via live action footage being projected on cards. The team shot extras in groups of varying sizes, using as much variation as possible. For the more complicated crowd shots, Double Negative developed a motion capture-based crowd solution built around Houdini. First, the team captured a large variety of animation clips, which were then gathered in a motion library. Then, the R&D team wrote a solution inside of Houdini that placed crowd's agents as particles and layered animation from the motion capture clips on top of that initial placement and/or motion. Through a series of rules and based on the actions needed for the shot, the team ran the particles through a simulation that drove the application of the required motion-capture clips. Based on the speed and direction of a given particle, they were then able to apply the correctly blended motion onto the particle, so that the motion of the agent matched the motion of the particle. Houdini CHOPS network was used to blend between multiple motion capture clips to apply animation to each particle, which resulted in a smooth blend of separate animation clips.

In addition to the locomotion system, the crowds system was also capable of triggering specific actions within a simulation. "The key difference with most off-the-shelf crowd software packages," Jack continues, "is that the particles in our crowds simulation drive the application of the animation data, whereas in other systems, the animation motion drives the gross crowd motion."

Double Negative also tackled the climactic anti-matter explosion above the Italian capital. The explosion concept progressively changed from being a traditional nuclear explosion to more of a nebula effect. Recalls Cook, "We needed to create something that looked like the astronomical photographs and satellite images, but it needed to explode and evolve though time. In-house concept artist Christoph Unger worked on some concepts of what the explosion would look like at various stages."

When the bomb goes off at first, it starts like a traditional fireball explosion, for which practical elements were used. The fireball then contracts into a singularity, so all of the flames and smoke start to swirl back into the centre core before it explodes again into blue light and forms the nebula explosion. Proprietary fluid dynamics solver Squirt played a crucial part in the sequence. It gave effects artists the ability to have the simulation contract using negative expansion, where the density gets taken out of the simulation.

Double Negative also tackled the climactic anti-matter explosion above the Italian capital. The explosion concept progressively changed from being a traditional nuclear explosion to more of a nebula effect. Courtesy of Double Negative.

The explosion was rendered using in house renderer DNB. A custom shader Inferno made it possible to use the fluid simulation as a light source to light the surrounding clouds and smoke. Says Cook, "Because of how creative the brief was, a lot of it ended up in the 2D realm, with matte painting, etc. The team had to experiment with the way that it moved and the timing of it, involving more work on top of that to integrate all the elements, because it was constantly evolving." Squirt was also used to create simulations for ground-based effects such as exploding fountains, dust elements, swirling paper and debris.

Adds Bickerton, "As a final texture, we photographed laser textures created by firing a laser through a clear plastic skull. These animating ribbon textures were added to explosion imagery giving a random sub-atomic texture."

Digital Churches Interiors
Also working on Rome environments was CIS Vancouver. Led by VFX Supervisor Mark Breakspear, the team created three CG church environments: Santa Maria della Vittoria, Santa Maria del Popolo and St Peter's Basilica. All three churches were off the shooting grid, so visual effects were the only way they could appear in the film.

The approach was to recreate them digitally, stitching together thousands of digital photographs taken at the real locations, and projecting them onto accurate computer models of the buildings. "We used three Canon EOS 1DS MK2 cameras, with gave fantastic quality in the low light levels of the churches" Breakspear says. "We were not able to get HDR in the truest sense, but we managed to get around that by also shooting with a stills film camera. We used Kodak 5218, which was the same film stock from the main unit shoot. By taking image on film, we were able to make use of the wider range from those images for windows, highlights and areas where we wanted to add detail into the shadow areas. We scanned our film images at 5040 × 3684 @ 240 dpi, while our digital images were captured at 4064 x 2074 @ 72 dpi."