For complicated cloudshapes, Double Negative tweaked its proprietary volumetric rendering toolset, DNB, which is a combination of Maya fluids and DN Squirt.

"David wanted to open the film with a huge, blockbuster sequence and really kick start the film," Burke adds. "In the original book, there was the idea of the wizards attacking and invading the Muggle world, and he wanted to illustrate that quite efficiently, almost as an act of vandalism. So we came up with the idea of The Dark Mark appearing above the city of London. From there, Death Eaters race down, fly over Trafalgar Square, then try to cross over to the entrance of The Squeaky Cauldron, where we know that Diagon Alley is secretly hidden, and that is where you enter the Wizarding world. And once there, they attack Ollivander's Wand Shop and kidnap him. So the whole thing was conceptualized as one continuous move. David wanted it to be a real roller coaster, breakneck race down through the streets. And the only way we could do that was completely digital and we turned to Paul and his team because of the recent tools they developed on The Dark Knight. Obviously we were able to photograph all of these locations so that we could recreate them as accurately as possible."

"Because we were in a true 3-D environment, we could create the correct stereo separation," suggests Paul Franklin, Double Negative's visual effects supervisor. "This was certainly my first go at doing true stereo 3-D. I really think it's going to be a significant force in the future of cinema."

For Half-Blood Prince, the 3-D focused on the titles and the opening. "We travel through this very dimly lit cloudscape and we created that with our proprietary volumetric rendering toolset, DNB, which is a combination of Maya fluids and DN Squirt, which is our in-house fluid toolset, which we actually used to form the shape of the clouds and then rendered them with DNB," Franklin explains. "And there were some other tricks to give us a sense of lighting within the clouds, first revealing the Warner Bros. logo and then the Harry Potter logo coming through. It's a very architectural set up and then as you begin to push through this, the lightning flashes are timed with the soundtrack, which sounds like a distant war, which relates to an underlying theme of all-out war. Eventually this pulls us through to modern-day London, which we actually shot on location at the building of the Great London Assembly. It's a very ultra-modernist building very much at odds with the Wizarding world of Harry Potter, which always has that wistful, Dickensian look to it. David Yates has liked using modernist architecture for depicting the Muggle world. An atrium of clouds builds up with a very strong architectural sense. The huge Dark Mark falls over London. That was a real achievement using DN Squirt. We wrote a few new tricks and tools for it that allowed us to work with the underlying geometry of the Dark Mark, particularly a huge scull with giant fangs, and basically fill it with clouds. But then as we're animating it, we're getting all of the edges tearing away and bleeding away. It's a balance between natural cloud forms and the stylistic shape of this skull. We wanted it to look unnatural."

For this entirely CG sequence, Double Negative spent a lot of time taking aerial stills of downtown London. "We worked with a lot of satellite photography that we got off the internet," Franklin offers. "We got a hold of street plans. And we created a very detailed 3D model of this section of London and then populated it with CG people and vehicles. We developed a new system using Houdini to procedurally model all of the trees. From my point of view, the route that the camera flies in is pretty much the same one that I walk from Charing Cross Station into Double Negative every morning. One of the things about Diagon Alley is that everything had to be carefully hand-made. We took the components of the set usually to extend it based on our imaginations.

"After the Death Eaters grab Ollivander, we cut to the Millennium Bridge. This is a fantastic horizontal suspension bridge outside of the Tate Modern Gallery on the banks of the Thames. We're back in the modern world of steel and glass. The ominous sky is a matte painting derived from a plate that was shot off the roof of Double Negative, with lightning flashes added to it. The three plumes of the Death Eaters descend on the Millennium Bridge and twist around it flying back and forth. They vanish but they cause the bridge to turn and twist and ripple. We switch to a fully CG bridge and replace the river with a CG river using the dnOcean toolset so that we can create waves and swirling tracks left by the Death Eaters. We shot some close-ups of people on greenscreen. We then created the London environment as a 3D reprojected painting behind that. The bridge begins to break up and we treated it effectively like a character animation and rigged it using our character toolset because the deformations were so extreme. But then the character animation of the bridge drives a secondary dynamic system, which is built using our dnDynamite toolset, and this causes things like the decking planks to pop away and eventually as it really breaks up, we see the suspension cables snaking and whipping away, also using dnDynamite. Eventually, the bridge snaps in two and they originally had animations of people falling into the river but that was deemed to be [too intense]. We were able to animate proper water surfaces using Squirt and some enhancements to Houdini on the top of it."